Astrobites RSS

Víctor M. Blanco 4-meter Telescope

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Dark Energy Survey: Cosmology Results with ~1500 New High-Redshift Type Ia Supernovae Using the Full 5-Year Dataset
Authors: Dark Energy Survey Collaboration
Status: Published in ApJL

The Dark Energy Survey (DES) Collaboration is an international team of scientists that aims to measure and understand the nature of an elusive energy density component in the universe, dark energy. The DES was conducted using the 4-metre Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile and took observations from 2013 to 2019. The survey used a special camera called the Dark Energy Camera (DECam). DECam has a wide field of view (about 14 times the size of the full Moon in the sky), allowing for detection of galaxies over a large sky area. It also allows for sensitive measurements of the redshifted light from these galaxies with a 570-megapixel camera with 74 CCDs with minimal readout noise in the measurements. This research article specifically focuses on the results of the DES supernova survey (more about their other survey data can be seen here), which was designed to test cosmology with a large sample of supernova observations.

In 1998, two teams of scientists measuring the brightness of supernovae unexpectedly discovered anomalously faint supernovae at specific times in the earlier universe, indicating that the universe is accelerating in its expansion (they got a Nobel Prize for this discovery). Before 1998, cosmologists believed in three possibilities for future expansion of the universe: it would either stop and reverse (resulting in a collapse), it would come to a halt (resulting in a static universe), or it would reach a constant expansion rate. These scenarios assumed the universe only consisted of matter being influenced by gravity and radiation.

The discovery of an accelerating expansion changed this. Type Ia supernovae have a standard brightness, which allows us to determine their distance based on how faint they look in a telescope. We can also measure the redshift of supernovae from spectra, which can be compared to predictions from cosmological models that relate the redshifting of the light to the universe’s expansion rate over time and how far the light has travelled. Thus, we can plot the observed redshifts of the supernovae against their distance (from the measurements of their brightness). This plot is known as a Hubble diagram and can be used to fit a cosmological model. In today’s article, the DES Collaboration has done exactly this to test cosmological models. This time, however, instead of only the 52 supernovae that the discoverers of dark energy had in 1998, there are 1,635 supernovae in the DES five-year dataset — more than 30 times more!

Lighting the Way with the Universe’s Candles

The 1,635 supernovae found and used by DES (after quality cuts) cover redshifts greater than z ~ 0.1, so 194 Type Ia supernovae from samples external to DES are included in the data analysis to cover low redshifts (see Figure 1). In total, this resulted in an analysis of 1,829 supernovae. Part of the cuts to the data involved removing contaminants — transients that look like Type Ia supernovae but might actually be something else. In order to distinguish between the Type Ia supernovae and the contaminants, two machine-learning classifiers were used; they were trained on simulated Type Ia supernova light curves or Type II (core-collapse) supernova light curves (see more about different supernova classification in this bite and Type Ia light curves here).

Hubble diagram of Dark Energy Survey supernovae

Figure 1: The Hubble diagram of the DES supernovae from the five-year sample (blue points) and the external data (orange points) used in the analysis. The lower panel shows the difference from the measured and theoretical distance moduli for the best fit to a time-varying dark energy model. [DES Collaboration 2024]

Supernovae can be classified using spectroscopy, but in the DES analysis the machine learning classifies them using multi-band photometry. This is akin to low-resolution spectroscopy, as the flux from the supernovae is measured in a few different filters, instead of many different wavelengths. This approach allowed DES to observe many more supernovae than before in their survey. The classifiers gave the supernovae a probability of being a Type Ia, as shown in Figure 1 above, and these probabilities were used as weights in the model fitting analysis. To remove human bias in the analysis, the pipelines used were tested on blinded data — that is, the data were made to look different deliberately. This allows for one to ensure the pipeline works well and that those completing the analysis do not introduce bias towards an expected result.

Hints of Time-Varying Dark Energy?

In the standard model of cosmology, ΛCDM, dark energy is assumed to have a constant energy density — i.e., a cosmological constant. This model has been favoured by DES data previously. Furthermore, the results from measurements of the cosmic microwave background by the Planck space mission have preferred this model, and a ΛCDM model with zero curvature — that is to say, the universe has a flat geometry meaning that two parallel beams of light will stay parallel as they propagate through spacetime. If the universe has a curved geometry, the beams can eventually diverge or cross over (see more description here). However, the DES collaboration tests the data with various models: standard ΛCDM, a “flat” ΛCDM (zero curvature is assumed), and two time-varying dark energy models (also with the flat assumption).

The tests on the DES data alone and with combinations of external data for standard ΛCDM and flat ΛCDM find results consistent with those found previously for the matter density of the universe and the curvature — the fitted values are equal to those found previously by Planck within ~95% confidence bounds.

However, the story changes slightly for the time-varying dark energy models. In the first, wCDM allows dark energy to vary over time, letting the equation-of-state parameter, w, vary as a free parameter instead of being fixed to w = −1. In the second model, w0waCDM, the equation of state is modelled with a redshift dependence. One should find in the first model that w = −1, or in the second model that w0 = −1 and wa = −1, to be consistent with a cosmological constant dark energy. These constraints are not exactly favoured by the DES data and combinations, as shown by Figure 2 below.

contours and likelihoods for modeled parameters

Figure 2: Contours (best fit-regions in the parameter space) and likelihoods (conditional probability distributions for the fits to the parameters) for the fits to the matter density, Ωm, and dark energy equation of state parameters, for the w0waCDM model. The different coloured contours and likelihoods represent the different data combinations indicated by the legend. [DES Collaboration 2024]

There is a marginal preference for a time-varying equation of state as shown by the results above — the data prefer this over a model with a cosmological constant with ~95% confidence — just over 2σ. The best fits from the combination of Planck, DES and eBOSS find w = −0.773 (+0.075/−0.067) and wa = −0.83 (+0.033/−0.042) for the w0waCDM. The best fit for wCDM is w = −0.941 ± 0.026.

While we can’t confidently state from these results that dark energy must be time varying, these results here could be a hint at new physics to be discovered by cosmology in the future — but only further analysis and data can tell.

Original astrobite edited by Kylee Carden.

About the author, Abbé Whitford:

I am a third-year PhD student at the University of Queensland, studying large-scale structure cosmology with galaxy clustering and peculiar velocities, and using large-scale structure to measure the properties of neutrinos.

Infrared images of the exoplanet HIP 65426 b from JWST

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: HIP 65426 Is a High-Frequency Delta Scuti Pulsator in Plausible Spin–Orbit Alignment with Its Directly Imaged Exoplanet
Authors: Aldo G. Sepulveda et al.
First Author’s Institution: University of Hawaiʻi at Mānoa
Status: Published in AJ

What Is HIP 65426?

HIP 65426 is a star relatively close to Earth, and it has a giant planet called HIP 65426b orbiting around it that has been directly imaged. You might recall HIP 65426b from a JWST early science release, as it was the first exoplanet directly imaged by JWST.

The star itself is part of a group of young stars called the Lower Centaurus–Crux (LCC) moving group, which is around 10–23 million years old, but scientists estimate HIP 65426 to be around 14 million years old using different methods. This star rotates very quickly and shows signs of potential pulsations in its brightness. Confirming these pulsations, known as δ Scuti pulsations, could help determine the star’s age more precisely. Determining ages of stars is actually surprisingly difficult, so any method that can accurately predict ages is very intriguing to astronomers.

Now switching gears briefly, planet HIP 65426b is located relatively far from the star, between 62 and 120 times the distance between Earth and the Sun. Its orbit is tilted at a significant angle relative to our line of sight. This is particularly interesting because the alignment between a star and its orbiting companions, like planets or brown dwarfs, can tell us more about how these systems formed and evolved.

A recent work revealed that misalignments are common with brown dwarfs, but the orbits of giant planets tended to be aligned or nearly aligned with the spins of their host stars. Understanding whether planets like HIP 65426b are aligned with their stars helps us understand planet formation and the history of these systems.

Observing with and Using Data from TESS

Time-series photometry from the Transiting Exoplanet Survey Satellite (TESS) has provided a lot of data about the rotation of stars and any variations in brightness caused by features on their surfaces or by orbiting objects passing in front of them. Time-series photometry also probes for other phenomena, including stellar pulsations and transit events. In this article, the authors use this data, along with some data from direct imaging of the HIP 65426 system, to investigate the orbital inclination of the exoplanet HIP 65426b. They aim to determine whether there is evidence for misalignment between the planet and its host star.

The star was observed by TESS in three different time periods called sectors (Figure 1). These sectors spanned from April 2019 to May 2019, April 2021 to May 2021, and April 2023 to May 2023. Data was collected from the star every 2 minutes during these time periods. The data was analyzed using a software called lightkurve, which helps process and analyze the light curves of stars. To ensure the data are clean and free from contamination, the authors first removed any unusual or outlier data points from the light curves. Then, they examined a region around the star within a radius of 80 arcseconds to see if any nearby objects were affecting the measurements. This is important because contamination from other sources can affect the accuracy of the analysis.

TESS time-series photometry of HIP 65426

Figure 1: TESS time-series photometry of HIP 65426 for Sectors (a) 11, (b) 38, and (c) 64. [Adapted from Sepulveda et al. 2024]

Identification of the δ Scuti pulsations for Mass and Age Estimations

Several pulsation modes, spanning 28–131 cycles per day, were identified in the star. This is consistent with a high-frequency Scuti pulsator. The presence of these high-frequency Scuti pulsations confirms the young age of HIP 65426 and may even provide an opportunity to estimate its age through detailed asteroseismic modeling, which is beyond the scope of the article.

The authors also investigated the possibility of pulsation timing variations caused by mutual gravitation with an orbital companion. This is typically measurable only for sufficiently massive planets with long enough periods. No such variations were detected, which places an upper limit of 12.8 Jupiter masses on the mass of HIP 65426b.

Stellar Inclination of the Host Star

Using a known relation between the star’s rotation period, its radius, and a measure of its rotational velocity, one can constrain the angle between the star’s rotational axis and our line of sight, also known as stellar inclination. This article uses a Bayesian framework that properly computes the inclination using these parameters. Based on their analysis using values of these parameters from literature (radius from isochrones and rotational velocity from spectroscopy) and TESS measurements (rotation period), the authors place statistical limits on the inclination difference between the star and the planet, the median value being 105 (+7/-9) degrees.

plot of sky-projected orbits for HIP 65426 b

Figure 2: A sample of 100 sky-projected orbits used for fitting the orbit of HIP 65426b. The cyan star represents the position of HIP 65426 and the orange dots represent the relative astrometry of HIP 65426b. [Adapted from Sepulveda et al. 2024]

Orbital Inclination of the Giant Planet

The orbit of the planet was measured out using astrometric measurements from various sources, including high-precision measurements from VLTI/GRAVITY.  From MCMC fitting of Keplerian orbits using the Python package orbitize, the median orbital inclination is estimated to be 108 (+6/-3) degrees, consistent with recent studies of the system although different input measurements were used in this work. Orbits drawn from the fitting process are shown in Figure 2.

Is There a Misalignment?

Figure 3 says no! Here the authors compared the inclination of HIP 65426b with the inclination of its host star. As the plot shows, the stellar and planetary orbital inclinations line up within their uncertainties, and hence there’s a lack of evidence for a misalignment, just a small star–planet obliquity as suggested by the roughly 3-degree difference in inclination.

plot of normalized probability density as a function of inclination angle

Figure 3: The normalized orbital and stellar inclination posteriors for the HIP 65426 system. The purple histogram corresponds to the orbital inclination of the planet and the gray plot represents the stellar inclination of the host star. [Sepulveda et al. 2024]

This seems to be in line with the general trend of alignment where directly imaged long-period giant planets appear aligned with their host stars, as shown by the plot in Figure 4, where the orbital and host-star inclinations for six directly imaged exoplanet systems are being compared. This type of perfect alignment also extends to debris disks, which are analogous to our solar system’s Kuiper Belt.

plot of host star inclination versus orbital inclination

Figure 4: Comparison of orbital inclinations and host-star inclinations for six directly imaged exoplanet systems comprising 11 total companions. [Sepulveda et al. 2024]

If the observed trend of relatively aligned orbits between stars and their imaged giant planets continues, it goes against recent understanding from a 2023 work that suggests misalignments are common in brown-dwarf systems. These differences between giant planets and brown dwarfs could extend to other key characteristics, like their orbital shapes, which might indicate that they form through different processes.

Now, What Can We Tell About the Formation of HIP 65426b?

There are two key models that explain how planets could form: core accretion and disk instability. Core accretion does not really explain how this planet is born because it is farther away from the host star than the region where core accretion would take place. The lack of evidence for misalignment also disfavors the core-accretion scenario. Given the large orbital eccentricity, planet–planet scattering could be a possible mechanism. This scenario suggests that the planet formed closer to its star via core accretion and was then scattered to its current position by the gravitational interactions with other planets in the system. However, planet–planet scattering typically results in orbits being tilted relative to each other, which isn’t the case here, so the lack of significant misalignment between the HIP 65426b’s orbit and its star’s rotation axis doesn’t strongly support this idea.

It is important to note that these theories are not conclusive. The current data don’t provide complete information about the system’s geometry, so it’s still possible that the star’s actual tilt might be larger than what’s currently estimated. Additionally, the orbital eccentricity is not yet concretely determined, so further astrometric measurements can change our current geometric understanding of the system.

The Big Picture

This article describes yet another work that combined space-based brightness data and direct imaging data to understand other planetary systems well after they have formed and understand the implications of their obliquity. With new missions and exoplanet surveys, new systems will be discovered that will also usher in more similar studies of inclinations and orbital architecture.

Original astrobite edited by Amaya Sinha.

About the author, Maria Vincent:

Maria is a PhD candidate in astronomy at the Institute for Astronomy, University of Hawai’i at Manoa. Her research focuses on adaptive optics and high-contrast imaging science and instrumentation with ground-based telescopes. Driven by a fascination with planet formation and the intricate processes shaping our solar system, she uses the Subaru Coronagraphic Extreme Adaptive Optics suite to observe and study morphological features of protoplanetary disks in near-infrared wavelengths, aiming to understand disk structure and processes governing planet formation. On the instrumentation side, she is working on designing and constructing an optical testbed to test and characterize a new deformable mirror as part of the upcoming High-order Advanced Keck Adaptive Optics upgrade. Outside of work, she enjoys blogging, mystery, historical and science fiction literature and cinemedia, photography, hiking, and travel.

star-forming molecular gas

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Blowing Star Formation Away in AGN Hosts (BAH) – I. First Observation of Warm Molecular Outflows with JWST MIRI
Authors: J.H. Costa-Souza et al.
First Author’s Institution: Federal University of Santa Maria
Status: Published in ApJ

One of the key questions in galaxy evolution is why big galaxies are so rare. We see lots of medium-sized galaxies (galaxies about the size of our Milky Way, with about 100 billion stars) but very few truly enormous ones (five or ten times bigger). We believe the answer to this question is feedback. As galaxies get bigger, physical processes occur that shut down further star formation. In the case of big galaxies, the most likely sources of feedback are active galactic nuclei: the supermassive black holes at the centers of galaxies. These objects can produce a truly astonishing amount of energy, and if some of that energy can act on the gas in the galaxy that is trying to form new stars (heating it up, moving it around, or expelling it from the galaxy entirely), it could interrupt star formation enough to explain why we don’t see many big galaxies.

In order to tell if this is happening, we need to study this star-forming gas. Stars are mainly formed from molecular hydrogen gas (H2). This gas comes in different phases: a cold (less than 100K) phase, a warm (100 to 1000K) phase, and a rare hot (more than 1000K) phase. It’s also not just enough to see that the gas is there — we need to see how it’s moving, in all three dimensions, because active galactic nucleus feedback can manifest itself as a large-scale outflow of gas. This is possible with specially designed instruments known as integral field units, which measure a full spectrum of light in each spatial pixel of an image. Provided the gas we’re trying to study emits some sort of spectral line, the integral field unit can measure its velocity towards or away from us using the Doppler shift. This type of analysis is known as galaxy kinematics.

In today’s article, the authors target specifically the warm phase of molecular gas, which thankfully emits a whole series of spectral lines. These lines are mostly rotational lines of the H2 molecule, and they emit in the mid-infrared, which is perfect for targeting with the Mid-Infrared Instrument (MIRI), an integral field unit on JWST. The authors are looking at one specific active galactic nucleus host: UGC 8782. The active galactic nucleus in this galaxy is a low-ionization nuclear emission region, or LINER (astrobites has a full guide on active galactic nucleus classification here), and it’s only about 200 megaparsecs (650 million light-years) away, which, by galaxy standards, is very close. From other optical and radio imaging, the authors figured that UGC 8782 had outflows in at least some phases of gas, which made it likely that it would have a warm molecular gas outflow as well.

What the authors found from their JWST observations was pretty much exactly what they expected: a large-scale outflow in the warm molecular gas of the galaxy. Figure 1 breaks down the emission into components that come from the main disk of the galaxy (which is behaving pretty normally — just doing regular rotation) and components that come from a large-scale outflow from the center of the galaxy. This is mostly apparent in the bulk velocity of the gas (the bottom-center panel), where there are negative velocities where the normal disk rotation has positive velocities, and in the velocity dispersion (the bottom-right panel), which is more than double the typical dispersion of the galaxy in the region where the outflow is occurring. This means that this gas has a lot more energy than the rest of the gas in the galaxy, another sign that it’s interacting with the active galactic nucleus. Not shown is a second outflow that is faster but smaller.

plots showing the gas kinematics of the galaxy UGC 8782

Figure 1: The kinematics of the warm molecular gas in UGC 8782. The gas is broken down into a disk component (top), tracing the regular rotation of the host galaxy, and an outflow component (bottom) of gas being pushed out of the center of the galaxy by the active galactic nucleus. The flux distribution (left), gas velocity (center), and velocity dispersion, or scatter in the velocity (right) are shown for each component. [Adapted from Costa-Souza et al. 2024]

This is great information — it’s good to know that these outflows are happening in the warm molecular gas phase! However, that’s not all that can be found from the H2 data from MIRI. The authors detect several different rotational H2 lines, which means that the different lines can be compared to determine the temperature of the gas and to get a more accurate measurement of the mass of the gas. This information can then be combined to measure how fast the active galactic nucleus is pushing mass out of the center of the galaxy (the “mass outflow rate”) and how much energy that requires (the “kinetic power”). The authors do this calculation for gas at different distances away from the active galactic nucleus. The results are shown in Figure 2.

plots of mass outflow rate and energy required to expel that amount of mass

Figure 2: The rate at which mass is being pushed out of the center of the galaxy by the active galactic nucleus (top) and the energy required to push that mass out (bottom), as a function of distance from the active galactic nucleus. Three phases of gas are shown: the warm molecular gas being studied in this article (light blue), the hotter molecular gas (orange), and the ionized gas (dark blue). [Costa-Souza et al. 2024]

What the authors find is that the warm molecular gas is dominating the outflow both in terms of its mass outflow rate and its kinetic power. It’s so strong, it could push all the warm molecular gas available in the center of UGC 8782 away in only about a million years. Between 2% and 5% of the energy the active galactic nucleus is outputting as light has to go into the molecular gas to create an outflow this powerful.

An outflow this strong and powerful is fantastic evidence for feedback from an active galactic nucleus acting strongly on the star-forming gas in a galaxy, which means we’re one step closer to understanding the mystery of giant galaxies in the universe — it could be active galactic nucleus feedback causing them not to get made! Still, this is only one galaxy. We’ll have to study many others in the future to see how common this is, but JWST’s MIRI is more than up to the task.

Original astrobite edited by Storm Colloms.

About the author, Delaney Dunne:

I’m a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency line-intensity mapping experiment based in California’s Owens Valley.

two simple line drawings of the sun with sunspots

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Analyses of Johannes Kepler’s Sunspot Drawings in 1607: A Revised Scenario for the Solar Cycles in the Early 17th Century
Authors: Hisashi Hayakawa et al.
First Author’s Institution: Nagoya University
Status: Published in ApJL

Solar Cycles and the Maunder Minimum

A typical solar cycle lasts around 11 years. Each cycle begins with a solar minimum, characterized by quiet solar activity and few visible sunspots. Around the middle of the cycle, the Sun’s magnetic field flips, causing a solar maximum — a spike in magnetic activity that manifests on the Sun’s surface as an increased number of sunspots and solar flares. Then the magnetic fields settle, ending the cycle with another solar minimum, and the cycle repeats. In general, sunspots appear at high absolute latitudes during the solar maximum and drift towards the Sun’s equator as the cycle winds down, making the latitude of these sunspots an approximate indicator of how far a solar cycle has progressed (see Figure 1).

Schematic of sunspot latitude and frequency during a solar maximum and a solar minimum

Figure 1: Schematic of sunspot latitude and frequency during a solar maximum and a solar minimum. [Annelia Anderson]

However, solar cycles are not always perfectly regular; we have around 400 years of telescopic sunspot observations, during which there have been several irregular periods. Most significantly, during the Maunder Minimum from 1645 to 1715, sunspots all but disappeared. Understanding and predicting solar cycles and such grand minima remains an open problem in astronomy. In the case of the Maunder Minimum, there is an ongoing debate around how and when solar cycles transitioned from regular cycles to a prolonged grand minimum, and specifically whether or not cycle durations changed.

Besides telescopic observations, past solar activity can be estimated from the amount of carbon-14 in tree rings. Some of the carbon-14 in Earth’s atmosphere is created by cosmic rays interacting with atmospheric nitrogen. When the Sun is especially active, its enhanced magnetic field shields Earth from cosmic rays, which results in less carbon-14 in the atmosphere for trees to absorb. Results can be hard to interpret, though, because other factors like weather have a greater effect on carbon-14 production than solar activity does. Some carbon-14 solar cycle reconstructions have suggested extremely short solar cycles before the Maunder Minimum, while others have found solar cycles with regular to slightly long durations.

Understanding the onset of the Maunder Minimum is further complicated by the fact that telescopic sunspot observations only began in 1610 (just after the invention of telescopes). Observations began sometime during Solar Cycle −13, making it difficult to determine exactly when the cycle began — and therefore, difficult to determine whether the duration of Solar Cycle −13 was anomalous. Luckily for the authors of today’s article, a bit of data exists from the time before telescopes — drawings made by Johannes Kepler, the highly influential German astronomer best known for his laws of planetary motion, as he observed sunspots via camera obscura.

Kepler’s 1607 Sunspot Drawings

On 28 May 1607, Kepler made two sunspot drawings and recorded his activities and observations throughout the day. According to his book Phaenomenon singulare seu Mercurius in Sole (Kepler originally interpreted these sunspots as a transit of Mercury), his afternoon went like this:

Around 4:00 PM, Kepler noticed the clouds were clearing. There was a sunspot group large enough to be seen by the naked eye, so he headed home to observe the Sun. His house was on the bank of the Vltava River in Prague, and inside he had converted a room into a camera obscura — by only allowing the Sun’s light to enter through a pinhole, he was able to project an image of the Sun onto a sheet of paper. He traced this image and the large visible sunspot group. Next, he headed to the workshop of his friend Jost Bürgi, a Swiss clockmaker and mathematician. On his way there, he passed the Old City Hall, which had an astronomical clock that measured time since the previous sunset. The clock read 21 ⅓ hours. Once in Bürgi’s workshop, Kepler repeated his camera obscura observation, traced the Sun again, and noted that the Sun was setting when he finished. See Figure 2 for a depiction of his observations.

two drawings of the Sun and sunspots

Figure 2: Kepler’s 28 May 1607 sunspot drawings. Left: Observation at Kepler’s house. Right: Observation at Bürgi’s workshop. [Adapted from Hayakawa et al. 2024]

Diagram of a camera obscura

Figure 3: Diagram of a camera obscura. As the light travels through the pinhole, the projected image is inverted. [Fizyka z 1910 via Wikimedia Commons; Public Domain]

From Kepler’s account the authors were able to piece together that the observation times were around 5:30 PM at his house and 7:40 PM at Bürgi’s workshop. Next was the problem of orientation. Kepler oriented his drawings to match the Sun as it appeared in the sky, except they were upside down as a result of the camera obscura pinhole projection (see Figure 3). Finally, to overlay heliographic coordinates on the drawings, the authors aligned the vertical axes of the drawings toward the local zenith and ground in Prague at the time of the observations, as is shown in Figure 4.

Kepler’s sunspot drawings overlaid with heliographic coordinates

Figure 4: Kepler’s sunspot drawings overlaid with heliographic coordinates after reorientation. The Sun’s equator is the line that passes through the midpoint of each circle. Left: Observation at Kepler’s house. Right: Observation at Bürgi’s workshop. [Hayakawa et al. 2024]

In both drawings, the large sunspot group appears near or on the Sun’s equator. The small difference in sunspot location between the drawings isn’t surprising due to a number of possible errors — either in the rudimentary observation technique (although Kepler was meticulous) or the large uncertainties from interpreting the times of observations. In either case, the drawings are qualitatively similar and point to one important conclusion — the sunspot group is near the equator, so Kepler was observing the end of a solar cycle in 1607.

Regular Solar Cycle Durations

Because many sunspots were visible at high latitudes in the years following 1610, the authors were able to place the beginning of Solar Cycle −13 (and end of Solar Cycle −14) somewhere between 1607 and 1610. Solar Cycle −13 ended around 1620, meaning it had a regular duration of 11–14 years. As an end date for Solar Cycle −14, these results also agree with carbon-14 solar cycle models in which Solar Cycle −14 lasts around 14 years.

In recent years, the Sun’s activity appeared to be decreasing and discussion arose around the possibility that we could be approaching another Maunder-like event. Though not a cause for concern, the possibility highlights the importance of studying and predicting changes in the Sun’s activity. Using Kepler’s pre-telescope drawings to show that previously contentious solar cycle durations were typical before the Maunder Minimum adds another piece to this unsolved puzzle.

Original astrobite edited by Caroline von Raesfeld.

About the author, Annelia Anderson:

I’m an astrophysics PhD student at the University of Alabama, using simulations to study the circumgalactic medium. Beyond research, I’m interested in historical astronomy and hope to someday write astronomy children’s books. Beyond astronomy, I enjoy making music, cooking, and my cat.

illustration of the planets in our solar system and their orbits

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Eccentricity Distribution Beyond the Snow Line and Implications for Planetary Habitability
Authors: S. Kane and R. Wittenmyer
First Author’s Institution: University of California, Riverside
Status: Published in ApJL

A fundamental question in exoplanet science is to discern how common or rare our solar system is compared to the other planetary systems across the galaxy. While we have found many thousands of exoplanetary systems, so far, none “look” like our own, with small, rocky planets interior to gas giant planets. One reason we haven’t found any lookalikes is an observational bias that effectively prevents us from being sensitive to these systems. For example, small planets comparable to Earth in size are very difficult to detect through planetary transits (which give the planet’s radius) and impossible currently to detect with radial-velocity measurements (which give the planet’s mass). Therefore, we don’t expect to be finding systems like our own solar system, at least not yet (we’re getting closer every day!). So how can we begin to answer this question of solar system–like architecture occurrence rate in the meantime?

Another special feature of our solar system is Jupiter’s low eccentricity. As a quick refresher, eccentricity is the measure of how elliptical a planet’s orbit is. The more elliptical, the more eccentric. The more circular the orbit, the less eccentric. In our solar system, all planets have orbital eccentricities of less than 0.2 (most are near 0.05), which means all orbits are very nearly perfect circles. But we have found exoplanets with very high eccentricities that have interesting implications for planet formation and the dynamical evolution of exoplanetary systems, as explored in this astrobite. Jupiter’s low eccentricity of 0.04 has big implications for how Jupiter likely scattered material into the inner solar system during its formation, potentially seeding the inner solar system with the very material needed for our own planet Earth to form and become habitable to life as we know it — in particular, ices, which in planetary science can be water ice, things like CO2 ice aka “dry ice,” or even things like methane that condense into solids at the very cold temperatures of the outer solar system. Today’s daily article summary looks at a study that attempts to investigate how efficient this scattering of ices to the inner system is for Jupiter-like planets in hypothetical planetary systems.

First, the authors look to quantify how common or rare it is for known Jupiter-like planets to have low eccentricities like our own Jupiter. They took all planets with well-measured masses and cut out all those with masses below 0.3 Jupiter mass, or about the mass of Saturn. They then split this sample of 846 planets into those interior and exterior to the “snow line.” In a planetary system, the snow line is the minimum distance from the host star where it is cold enough that water exists only as ice. For each star, the snow line exists at a different semi-major axis, based entirely on the temperature of the host star. Giant planets beyond the snow line can scatter these ices towards the inner system, but giant planets within the snow line cannot bring this material from beyond the snow line to within the snow line. The authors find that the mean eccentricity for these giant planets within the snow line is 0.18, but the median is only 0.01. The big discrepancy between the mean and median suggests that this sample is heavily skewed by outliers. Additionally, there is a bias toward very short-period planets because orbits that were eccentric can become circular over time through tidal forces (see here). However, the mean and median eccentricity of giant planets beyond the snow line, like our Jupiter, are 0.29 and 0.23; these numbers being similar suggests that we are neither biased nor skewed within this sample.

The authors compared the eccentricities of the planets interior and exterior to the snow line using a Kolmogorov–Smirnov (K–S) test. In short, this statistical test is used to determine if two populations are drawn from the same distribution or from completely different distributions (see this earlier astrobite). For K–S test values near 1.0, it is most likely that the two groups are drawn from the same parent distribution, but for values near 0.0, it is most likely that we are truly seeing two different distributions — in this case, different distributions of eccentricities, which would imply different formation mechanisms. The results of this K–S test on the population of giant planets within and beyond the snow line yields a value very near 0.0, so they are truly different populations. However, when the authors cut out all planets with orbital periods less than 10 days (all within the snow line) and repeat the test, the K–S value is now 0.977, implying that they were from the same distribution. This is in line with earlier works that show that very close-in planets circularize their orbits, so when we exclude these very close-in giants, we recover that generally giant planets at all orbital separations are one population. With it established that giant planets (except for very close-in planets) form out of the same distribution of eccentricities regardless of orbital separation, the authors now ask a new question: does the eccentricity of a giant planet help or hurt the scattering of ices to the inner system?

To answer this, the authors run a suite of dynamical simulations. They initialize each simulation the same way: with a 1-solar-mass star and a 1-Jupiter-mass planet at a separation of 5.2 au (Jupiter’s true separation). Then they initialize the simulation with many thousands of “ice particles” at various locations within the system. Lastly, in one simulation, they give the Jupiter planet a small eccentricity of 0.05 and in another they set the eccentricity at 0.23. They then let the simulations run for 10,000 years and at the end, they count up all the ice particles that were scattered by the planet’s gravity into the inner system, measuring both how many cross within the snow line and how many cross within 1 au, Earth’s orbital separation. The results can be seen in Figure 1. They find that the high-eccentricity planet is much more efficient than the low-eccentricity Jupiter at scattering these ice particles into the inner system, at a rate of double for getting ices within the snow line and at eight times higher for getting ices within 1 au.

plots of scattering efficiency as a function of orbital semi-major axis

Figure 1: The scatter rate of ices that start at a variety of orbital separations due to a low-eccentricity Jupiter (top) vs. a high-eccentricity Jupiter (bottom). At every orbital period, the high-eccentricity Jupiter is better at scattering ices both within the snow line and within 1 au. [Kane & Wittenmyer 2024]

This has big implications for the formation of small, Earth-like planets and specifically for forming these planets with lots of water. It suggests that systems with higher-eccentricity Jupiter-like planets may be more efficient at forming small, watery planets. However, the authors note a few caveats to this finding. First, they acknowledge the bias in detecting more eccentric Jupiter-like planets. This in itself may have biased their choice of using 0.23 for the high-eccentricity case simulation. Second, they state that a higher scattering rate does not directly convey a higher delivery rate of these ices to small planets, it merely makes the collision events that do deliver these ices to be more probable. Lastly, they note that there are many other theories for how Earth received its water that are not necessarily tied to the delivery of ices from the outer solar system.

In all, this study sheds light on the population of Jupiter-like planets across the galaxy and shows that higher-eccentricity Jupiters are better at scattering ices to the inner system. This could point the way for further follow-up surveys to target systems with eccentric Jupiter-like planets in our search for small watery planets.

Original astrobite edited by Skylar Grayson.

About the author, Jack Lubin:

Jack received his PhD in astrophysics from UC Irvine and is now a postdoc at UCLA. His research focuses on exoplanet detection and characterization, primarily using the radial velocity method. He enjoys communicating science and encourages everyone to be an observer of the world around them.

artist's impression of the first stars in the universe going supernova

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: A Hide-and-Seek Game: Looking for Population III Stars During the Epoch of Reionization Through the HeIIλ1640 Line
Authors: Alessandra Venditti et al.
First Author’s Institution: Sapienza University of Rome
Status: Published in ApJL

The Dawn of the Universe

A long time ago, in a galaxy far, far away… the very first stars lit up the universe, ending the cosmic dark ages and ushering in the cosmic dawn. In our current best model of the universe, this happened around 13.4 billion years ago, around 100 million years after the Big Bang. Finding evidence of these very first stars, also called Population III (Pop III) stars, is one of the ultimate treasure hunts in astronomy, and one that JWST was specifically designed for.

These Pop III stars were formed from highly pristine gas, i.e., from clouds that are almost exclusively hydrogen and helium (the lightest elements in the periodic table). It’s these first stars that then began to form and release heavier elements, and after many billions of years of star formation, the universe today is a lot more chemically evolved. However, some pockets of pristine gas reservoirs are predicted to hang around even after cosmic dawn, meaning that Pop III stars could still exist as late as 12.5 billion years ago (which corresponds to a redshift of z = 6).

The authors of today’s article carry out a very interesting experiment to predict just how many Pop III stars we might be able to find at these later times (at redshifts of z = 6–10, which correspond to ~12.5–13 billion years ago). The first important question they address is how we even go about finding these stars.

 Oh Look, a Clue!

Formation within a pristine environment is a key characteristic of Pop III stars, and it actually helps us find them. Thanks to their hydrogen-rich composition, Pop III stars are theorised to be much more massive than modern stars and capable of powering very energetic (hard) radiation fields. With this huge amount of energy, they are able to double ionise the helium in the surrounding gas, which then causes emission of the helium-II recombination line at 1640 Angstroms. This emission line is therefore a great clue for finding Pop III stars!

We can even predict how many Pop III systems should exist at each point in time and how strong this emission line should be. To do this, the authors use the dustyGadget cosmological simulations. Essentially, they simulate the evolution of a universe of a certain size (volume) and include as much physics as possible (for example, star formation recipes). These simulations are currently the largest-volume simulations that also include models for Pop III stars. In the top panels of Figure 1, you can see how many Pop III systems exist in the simulations as a function of stellar mass and at different redshifts (different points in cosmic history), indicated by the solid grey line.

plots showing the number density of expected population III systems and the fraction of systems predicted to be missed by JWST

Figure 1: Top panels: Number density of Pop III systems expected at a given redshift in haloes within a given range of stellar mass. The total number density is shown as a solid grey line, while the numbers observable by JWST NIRSpec Integral Field Unit (IFU) are shown by the golden lines and those observable through NIRSpec Multi-Object Spectroscopy (MOS) by the brown lines. The solid/dashed line style refers to the best-/worst-case observations. Bottom panels: The fraction of Pop III stars missed in JWST/NIRSpec observations. The authors find that a significant number of Pop III systems can be overlooked by JWST. [Venditti et al. 2024]

The downside is that this emission is faint and difficult to observe, so next the authors need to consider the capabilities of our current best instrument (JWST).

Is JWST up to the Task?

JWST hosts a variety of instruments, and for this work we are mainly interested in the NIRSpec Integral Field Unit and the NIRSpec Multi-Object Spectroscopy modes. We can make a pretty good estimate of just how capable these instruments are at detecting helium-II 1640 by calculating their sensitivity limits, i.e., how strong does the emission need to be for us to detect it? The authors calculate the sensitivity limits of these instruments for a variety of observing times and set ups. They then compare the sensitivity limit to the predicted emission line strength of the simulated Pop III systems to work out how many of those systems they would be able to observe (coloured lines in top panels) and what fraction are missed (bottom panels).

These results indicate that only the brightest Pop III systems (within the most massive haloes) can be observed. Very low-luminosity systems might be missed even with ~50-hour exposures. However, there’s still hope! Even with these limitations, the authors predict that more than 400 Pop III systems could be discovered within current JWST surveys — although spectroscopic follow-ups would be necessary to identify them.

Overall, today’s article makes some very exciting predictions about finding Population III stars using JWST, which would help astronomers understand the very first light in the universe.

Original astrobite edited by Nathalie Korhonen Cuestas.

About the author, Lucie Rowland:

I’m a first-year PhD student at Leiden Observatory in the Netherlands, studying massive, star-forming galaxies in the early universe with ALMA and JWST. It’s a really exciting time to be interested in astronomy, so I hope to make groundbreaking new research more accessible!

Artist's impression of an exoplanet orbiting its host star

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Climate Regimes Across the Habitable Zone: A Comparison of Synchronous Rocky M and K Dwarf Planets
Authors: Ana H. Lobo and Aomawa L. Shields
First Author’s Institution: University of California, Irvine
Status: Published in ApJ

When it comes to planet hunting, the hot star on the block is the humble M dwarf: the coolest and dimmest type of star in the universe. They’re also the most abundant type of star, which combined with their small size makes them pretty good targets for rocky Earth-like planet hunting.

However, the jury’s still out on whether M dwarfs are likely to host habitable rocky Earth-like planets, as habitability — as we know it — requires a vast set of conditions on top of those needed to form a planet that simply resembles Earth in size and composition. What we do know is that one requirement for Earth-like life is the maintained and continued presence of water.

We have the luxury of living within our Sun’s “Goldilocks zone,” i.e., a “just right” distance from the Sun where it isn’t too hot or too cold for water to either boil off or freeze up permanently. An exoplanet’s atmosphere must also survive the onslaught of stellar storms brought on by its host star, which, in the case of an M-dwarf host, can be extreme from the very beginning.

This, unfortunately, considerably pares down the chances of finding a habitable planet orbiting an M dwarf, although there are scenarios in which it could be possible.

Today’s authors propose an alternate target: K dwarfs.

Choose Your Host Star!

K dwarfs are essentially M dwarfs’ slightly more massive and more stable siblings, albeit overshadowed. The gentler stellar activity that K dwarfs offer gives them an edge over M dwarfs, as fewer violent storms enhance volatile delivery and biosignature detection, and overall provide more of a fighting chance for life. However, we’re uncertain how common small rocky planets are around K dwarfs, and there hasn’t been much study on the kinds of climates K-dwarf planets could have. The authors thus set out to simulate two potentially habitable scenarios on both K-dwarf and M-dwarf planets with the goal of understanding how dayside climate and water availability vary between host stellar types.

For both scenarios, they assume tidally locked planets with Earth-like radii and surface gravities, and simplified atmospheres and orbits. They model the M-dwarf host star after AD Leonis and the K-dwarf host star after Epsilon Eridani. For both stellar classes, today’s authors play with water worlds and sand worlds (planets with surfaces made entirely of sand) by varying their orbital periods and instellation fluxes — the amount of stellar flux received by a planetary surface. An overview of how these planets’ temperatures vary across their surfaces can be seen in Figure 1.

plots of the surface temperature of simulated planets orbiting M dwarfs or K dwarfs

Figure 1: A diagram of the surfaces of different K-dwarf and M-dwarf hosted planets simulated by the authors projected onto a 2D map, and how their temperature varies across the surface. The center of each projection corresponds to a latitude of 0 degrees and a longitude of 0 degrees. The top two rows correspond to planets hosted by an M dwarf, where the top row contains water worlds, and the bottom row contains sand worlds. The bottom two rows contain planets hosted by a K dwarf, where the top contains water worlds and the bottom contains sand worlds. Instellation flux values increase from left to right. [Lobo & Shields 2024]

It’s an Eyeball Summer

The K-dwarf water world planets today’s authors simulate are home to an “eyeball” regime, where the dayside contains a habitable region with moderate temperatures on surface areas where the star is nearly directly overhead, somewhat resembling an eyeball, and the nightside is dominated by freezing cold temperatures. These make up the third row in Figure 1.

The authors find that with increasing instellation, these planets inch closer to the moist greenhouse limit, the point at which surface temperatures become too high to support liquid water, leading to the evaporation of a planet’s atmosphere. We might expect this to happen with increased instellation flux or decreased orbital distance. One simulated K-dwarf planet surpasses this limit once instellation flux is ramped up over 50%. But surprisingly, the nightside is still frozen! Meanwhile, a similar M-dwarf planet with 50% increased instellation flux enters a runaway greenhouse state, where evaporating oceans become trapped in the atmosphere, preventing heat escape. This leads to an uninhabitable rise in temperature, much like a pressure cooker.

The K-dwarf planets’ resistance to increased instellation seems to support the notion of them being more stable than their M-dwarf-hosted cousins, but there’s more to the tale than this.

Life in The Terminator Zone

The second scenario is characterized by a “terminator band.” This is a world of extremes where the dayside is home to hellish temperatures and the nightside is uninhabitably cold, yet a temperate band persists at the boundary where day and night meet. As seen in Figure 1, temperate terminator zones can exist even on “eyeball” planets. It turns out that, depending on whether a planet has reached the moist greenhouse limit, the terminator might be easier to analyze on actual observed planets, as it tends to be cloud-free at lower instellations.

Unfortunately, the terminator zone never reaches habitable temperatures on water worlds around K dwarfs or M dwarfs. Terminator habitability does occur for some M-dwarf and K-dwarf sand worlds, and one K-dwarf planet even becomes a borderline case, where the eyeball center is uninhabitably hot but most of the dayside is habitable.

Simulated M-dwarf and K-dwarf sand planets have been found to fall into terminator habitability zones in the inner habitable zone, and one might be tempted to conclude that cooler dayside temperatures make K-dwarf planets less likely to contain habitable terminators when placed in the outer habitable zone. However, the authors state this may vary on planetary surface composition and orbital distance, as darker materials such as basalt could absorb more stellar energy, keeping the terminator warmer for longer at greater orbital distances.

Which is Better for Habitability?

While planets hosted by M dwarf and K dwarfs experience similar climate trends, it takes different instellations to incur different states, which in themselves can look different depending on the host stellar type. While it took much more instellation flux for a K-dwarf planet to reach the moist greenhouse limit, its nightside was still frozen. This might seem like straightforward evidence for planets around K dwarfs being more stable in general, but the authors state that dayside water retention might be more difficult than expected, as much of the water would be packed into the nightside.

While K-dwarf systems might be more stable for habitability, water worlds may be easier to find in the mid to outer Goldilocks zone. Rocky worlds might be better found in the inner zone, although we don’t know how common they are around K dwarfs yet.

Where Would You Live?

With future expected advances in telescope power, such as the upcoming Large UV/Optical/IR Surveyor (LUVOIR) mission, it should eventually become easier to observe K-dwarf-hosted planets and learn more about their compositions and atmospheres. Then we can find out more about their ability to retain water.

With this in mind, where would you want to live? A planet hosted by an M dwarf or a K dwarf?

Original astrobite edited by Kat Lee.

About the author, Diana Solano-Oropeza:

I’m a first-year astronomy PhD student at Cornell University, where I study exoplanets, stars, and habitability using Gaia data. I earned my BS in physics at Drexel University before entering the Bridge to the PhD in STEM program at Columbia University. There, I researched TESS-detected exoplanets for two years. My hobbies include practicing Muay Thai, fictionwriting, and playing video games. You can check out my website at https://dianasolano-oropeza.com/.

illustration of a black hole in a galaxy

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Merger-Driven Growth of Intermediate-mass Black Holes: Constraints from Hubble Space Telescope Imaging of Hyper-luminous X-Ray Sources
Authors: R. Scott Barrows et al.
First Author’s Institution: University of Colorado Boulder
Status: Published in ApJ

Who doesn’t love a good origin story? How did your favorite superheroes and supervillains become one in the first place? Sometimes, it may boil down to a single instant, like that fateful night when Bruce Wayne and his parents took a turn down a dark alley. Occasionally, they evolve slowly, through several internal battles, to emerge as the greatest supervillain in the Galactic Empire. Regardless of how they became some of the most powerful characters in the fictional (or real) universe, there is no question that origin stories help us understand them better.

Our very universe has (super) entities whose origin story is shrouded in mystery. These are the supermassive black holes, which are powerful objects that occupy the centers of nearly every galaxy in the universe. These black holes are believed to have been around for billions of years (some were even formed when the universe was very young) and are some of the most massive and luminous objects in the universe. However, we still don’t understand how the first black holes formed. The enthusiastic astronomy community has been hard at work trying to come up with an origin story. Some astronomers believe that the first black holes formed when the first stars in the universe died, while others believe they formed when dense gas in the early universe directly collapsed into black holes without forming stars first.

So, how can we determine the true origin story of the black hole? We look for clues in the universe that we see through our telescopes. One way is by looking at the masses of certain black holes called intermediate-mass black holes. These are believed to be relics of early black holes that have survived relatively unscathed to our present universe. If most of these black holes have low masses (102 – 104 solar masses), they were likely to have formed from the collapse of early stars. If they instead have masses around 104– 105 solar masses, then they were formed from the collapse of dense gas. This difference in mass is a likely consequence of the conditions involved in the collapse scenario.

It is challenging to detect intermediate-mass black holes in the first place, let alone measure their masses. The authors of today’s article set out to achieve this mighty task! They first identify a sample of hyper-luminous X-ray sources. As the name suggests, these are highly luminous X-ray sources, and they are detected at off-center locations in a galaxy. This characteristic makes it likely that they are linked to intermediate-mass black holes. Intermediate-mass black holes are more likely to wander and can be found in different parts of a galaxy, as opposed to supermassive black holes, which are always found at the centers of galaxies. The authors then use the Hubble Space Telescope to look at these hyper-luminous X-ray sources and their surroundings to understand them better. Let’s see the clues the authors gather to formulate their theory of the formation mechanism of black holes.

Clue No. 1: Violent Disruptions Found at the Scene of the Crime!

The authors find that the strongest X-ray sources are associated with systems showing signs of mergers between galaxies. The hyper-luminous X-ray sources do not reside in a definite galaxy (Figure 1) but rather in a compact source that more closely resembles the core or center of a galaxy. The authors also measure the masses of the objects that host the X-ray sources using spectral energy distributions and the flux measured by Hubble. They find that nearly all the masses of the objects are larger than typical globular clusters and, thus, they are more likely to be the leftovers of dwarf galaxies that have merged or are merging with the more massive galaxy.

Sloan Digital Sky Survey and Hubble Space Telescope images of hyper-luminous X-ray source host galaxies

Figure 1: The Sloan Digital Sky Survey images (left) and Hubble images of the hyper-luminous X-ray source candidates (highlighted by the magenta pointers). Each candidate lies outside/on the edge of a larger galaxy in the center of each Hubble image. [Adapted from Barrows et al. 2024]

Clue No. 2: What Are the Masses of Those Black Holes?

The authors then use several scaling relations and different Eddington ratios (which give a sense of how quickly the black hole is accreting based on its luminosity) to determine the mass of the black hole. Looking at the lowest-mass black holes (as they are more similar to the early seed black holes) (Figure 2), they conclude that 28% of their samples agree with the direct collapse scenario (which forms more-massive black holes) and 21% with the formation in dense stellar clusters (which forms less-massive black holes).

plot of cumulative black hole mass fraction

Figure 2: The total fraction of black hole masses (determined from scaling relations) in hyper-luminous X-ray sources, added over the sum of each contribution from black holes at different mass ranges. The data has been corrected to be mass-complete, which determines how many sources are present in a field based on the number of sources detected. [Barrows et al. 2024]

The Plot Thickens!

Looking at the larger masses of the intermediate-mass black holes, it is tempting to conclude that we have narrowed the origin story of black holes to the direct collapse scenario. However, since most of the hyper-luminous X-ray sources are found in galaxies closely associated with merger events, the authors argue that the gas falling into the black hole during the galaxy merger could have triggered the increase in the size of the black hole seeds from their original mass. They determine that the low-mass X-ray sources all have larger X-ray luminosities than expected from their host stellar masses. This could hint that accretion likely increases the size of the black hole, which leads to enhanced luminosity. The original black hole seeds were thus likely much smaller, suggesting they were formed from stars.

Well, that was full of twists and turns! This article has added some evidence favoring the stellar collapse formation mechanism. With more data and further analysis, we will one day end up with convincing numbers to help us determine how the first black holes in the universe formed. And hey! If I could sit through all of those 7 hours and 1 minute to find out how Anakin Skywalker became Darth Vader, I think I would be fine waiting a couple more years (hopefully!) to uncover the origin story of these mighty black holes!

Original astrobite edited by Storm Colloms.

About the author, Archana Aravindan:

I am a PhD candidate at the University of California, Riverside, where I study black hole activity in small galaxies. When I am not looking through some incredible telescopes, you can usually find me reading, thinking about policy, or learning a cool language!

illustration of a tidal disruption event

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Late-Time Radio Flares in Tidal Disruption Events
Authors: Tatsuya Matsumoto and Tsvi Piran
First Author’s Institution: Kyoto University
Status: Published in ApJ

Hungry (and Loud) Black Holes

Tidal disruption events arise when a star wanders too close to a supermassive black hole that then exerts a tidal force across the star, shredding it. These events are relatively rare: we have only discovered a few hundred. When the star is disrupted, about two-thirds of the material remains bound. The remaining material is ejected from the supermassive black hole into the “circumnuclear medium,” or the region immediately surrounding the supermassive black hole. We typically discover tidal disruption events from the optical emission resulting from the initial disruption, which lasts several weeks. However, tidal disruption events are known to be multi-wavelength events visible across the electromagnetic spectrum. Before optical tidal disruption events were discovered, almost all of the tidal disruption events were found in the X-ray, where the formation of an accretion disk around the supermassive black hole may be powering some high-energy activity. On the other end of the spectrum, the radio properties of tidal disruption events have proven to be unique. Today’s article aims to explain the radio light curves of tidal disruption events.

The radio emission from tidal disruption events is caused by the material that survives the disruption of the star and is ejected away from the supermassive black hole. This stellar material runs into the ambient density surrounding the supermassive black hole, causing shocks inside the material. These shocks give rise to synchrotron radiation, an emission caused by free electrons in a plasma spiraling around magnetic field lines. Directly related to the density and energy of the material, the synchrotron radiation is emitted across the radio spectrum, typically at frequencies lower than 10 GHz, making it an excellent choice for instruments like the Very Large Array.

Second Peak, Second Life?

Although we know about a third of the material from the star is ejected away from the supermassive black hole after the disruption, we do not understand how the black hole launches this material. For example, supermassive black holes in active galactic nuclei can launch powerful relativistic jets as they accrete massive amounts of material. Or, in a less energetic scenario, a jet does not have to be launched, and the outflows could be in all directions and essentially non-relativistic. In yet another situation, the delayed formation of an accretion disk may induce a relativistic jet to be launched much later than the initial disruption. To complicate matters further, it is almost certain that tidal disruption events do not originate from an underlying homogeneous population and that a spectrum of disruption scenarios results in many different ejecta geometries.

Today’s article uses the non-relativistic approach to model the tidal disruption event scenario. The authors model a shock quasi-spherically propagating first through a circumnuclear medium with a radially decreasing density and then through an interstellar medium with constant density. Using a standard set of code and modeling packages for synchrotron emission, they produce light curves for what this model should look like. In this model, there are two peaks caused by differing effects. The radio emission is “self-absorbed” in the first peak and transitions to optically thin, eventually peaking. By measuring the peak frequency and luminosity, we can estimate the radius of the outflow and local circumnuclear medium density. Then, depending on the spectral index of the circumnuclear medium’s radial density profile, the light curve will fall and eventually reach a minimum at the Bondi radius of the supermassive black hole. At this point, the radial density profile becomes flat (i.e., constant density interstellar medium), and the radio light curve will rise again as the shock wave sweeps up material. The brightness will continue to increase until the swept-up mass is comparable to the mass from the original ejected outflow. After the second peak, the radio brightness decreases indefinitely.

How does this model compare to some real scenarios? The authors of today’s article select two well-known events from the literature and gather radio observations to compare with their modeled light curves. When comparing AT2019dsg and AT2020vwl, the double-peaked feature is evident in both light curves, as seen in Figure 1. The authors note that while the rapid t3 initial rise is well explained for both sources, other radio-loud tidal disruption events, such as AT2018hyz, rise even faster like t5 and thus are better candidates for relativistic models. The authors state that further observations at even later times will enable improvements to this model and constrain their parameters.

plot of radio light curves of two tidal disruption events

Figure 1: The late-time C-band (6 GHz) radio light curves of AT2019dsg and AT2020vwl. These sources have some of the best data quality and quantity in the literature. The double-peaked feature of our authors’ model is evident in the light curves of both events. [Matsumoto & Piran 2024]

Original astrobite edited by Archana Aravindan.

About the author, Will Golay:

I am a graduate student in the Department of Astronomy at Harvard University and the Center for Astrophysics | Harvard & Smithsonian, advised by Edo Berger. I study radio emission from transient astrophysical objects like tidal disruption events.

simulation of the large-scale structure of the universe

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: A Detection of Cosmological 21-cm Emission from CHIME in Cross-Correlation with eBOSS Measurements of the Lyα Forest
Authors: CHIME Collaboration
Status: Published in ApJ

A Map of the Universe

Throughout the history of astronomy, we’ve been able to map the universe in a couple of different ways. Large surveys of galaxies, such as the Sloan Digital Sky Survey (SDSS) or the Dark Energy Spectroscopic Instrument survey, are extremely useful — they give astronomers a huge sample of galaxies to characterize, and the positions of those galaxies tell us about how the universe as a whole is set up. At extremely high redshifts, the cosmic microwave background shows us the structure of the universe right as it began. More recently, a totally separate technique has been developing: (spectral) line-intensity mapping. In this technique, you essentially take a very blurry picture of a large region of the universe at a wavelength that corresponds to a specific spectral line. Instead of targeting specific galaxies, you get all of the emission from that spectral line in that region of the universe. This way, you can get light from things that are much fainter than a traditional galaxy survey can see.

The fact that you’re targeting a specific spectral line is also important: because of the expansion of the universe, emission from different distances is redshifted to different observed wavelengths, and so the end product of a line-intensity mapping experiment is a 3D map of the universe instead of just a 2D picture. Because of the expansion of the universe, this also means you map the universe through time. Figure 1 shows a (very idealized) picture of what a single-wavelength slice of this could look like.

illustration of the evolution of the large-scale structure of the universe and the line-intensity mapping technique

Figure 1: A simulated line-intensity map from a single slice. [NASA / LAMBDA Archive Team]

Neutral About Hydrogen

The line-intensity mapping technique was originally developed for studies of the 21-cm hydrogen line, which is a spin-flip transition of neutral hydrogen mainly emitted by diffuse gas. This is a very powerful technique: it’s extremely difficult to observe neutral hydrogen gas in any other way, and a lot of the universe is made up of this gas, especially at high redshift!

Today’s article is also looking for this 21-cm emission, specifically by using the Canadian Hydrogen Intensity-Mapping Experiment (CHIME; shown in Figure 2). This experiment has been discussed in several astrobites over the past few years, both for its 21-cm work (x) and for its work with fast radio bursts (x, x, x).

photograph of the CHIME instrument

Figure 2: The CHIME instrument. [Wikipedia user Z22; CC BY-SA 4.0]

Fighting Foregrounds with Friends

Detecting 21-cm emission is significantly complicated by the fact that there are lots of things in the way! Between Earth and the distant galaxies astronomers are actually after, there are many other sources that emit around a wavelength of 21 cm, including Earth’s ionosphere and synchrotron emission from the Milky Way. These are “foregrounds,” and they’re far brighter than the 21-cm emission astronomers are looking for! Although you can work around foregrounds using 21-cm data alone, combining your 21-cm measurement with other measurements of the structure you’re trying to see (taken using other wavelengths of light) makes for a much more robust approach. Because the two measurements are taken using different wavelengths, it’s very unlikely that they’ll show the same foreground structure. When you use a statistical technique such as cross-correlation to combine the two, the foregrounds (more or less) disappear! This has already been done for the CHIME 21-cm data at low redshifts, but not at redshifts above z = 1.5 (9 billion years ago).

In this research article, the authors combine their 21-cm data with Lyα forest measurements from the extended Baryonic Oscillation Spectroscopic Survey (eBOSS), which is a part of SDSS. Importantly, the Lyα forest traces absorbing hydrogen gas, and the 21-cm data trace emitting hydrogen gas. Density determines whether gas absorbs or emits: denser gas tends to absorb radiation, and more diffuse gas tends to emit radiation. This means that on small scales, the two signals are actually expected to be anti-correlated (because the measurements are coming from different kinds of gas), and the cross-correlation signal will be negative.

A Negative Detection!

Indeed, after reducing the CHIME 21-cm data, the authors do detect anti-correlation! Figure 3 shows the cross-correlation signal. This is measured by introducing an artificial offset in the x-axis (in this case, the frequency axis) between the 21-cm measurement and the Lyα forest measurement, and then measuring how much these two signals correlate or anti-correlate at that offset. The amount of correlation is then shown as a function of this frequency offset. You can see a large negative spike right at zero offset, where the two signals should anti-correlate the most. The dotted black line shows a model the authors came up with for this cross-correlation, and the two agree quite well (the bottom panel shows the residuals between the signal and the model).

plot of the cross-correlation signal

Figure 3: Top: The cross-correlation signal shown as a function of offset between the two datasets in the frequency axis. The blue line is the actual CHIME x eBOSS data, and the dotted black line is a model of what the signal should look like, fit to the data in amplitude but nothing else. Bottom: The residuals between the data and the model, expressed in terms of their significance. [Adapted from CHIME Collaboration 2024]

This is very exciting — it’s the first detection of 21-cm radiation at a redshift greater than z = 1.5, and its cross-correlation with the Lyα forest looks pretty much as expected! There is a lot of information to be gained from this measurement. In particular, the amplitude in the y-axis of the cross-correlation signal is set by the spatial relationship between dense and diffuse hydrogen gas in the universe. However, the authors leave determining that exact relationship for a future work, because it requires some extremely detailed cosmological and hydrodynamic simulations. Also, even in this cross-correlation measurement, the authors found a lot of foreground emission! Improvements to the CHIME instrument and its data reduction will help get rid of this contamination in the measurement, but it’s still a very difficult problem. For now, it’s incredible that this faint but all-important signal has been detected so far away.

Original astrobite edited by Nathalie Korhonen Cuestas.

About the author, Delaney Dunne:

I’m a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency line-intensity mapping experiment based in California’s Owens Valley.

1 2 3 4 5 6 … 44