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astrophotograph of the pelican nebula

Since 2018, the Zwicky Transient Facility has kept tabs on fleeting events and variable objects in the universe, scanning the entire northern sky every two days. What can this expansive survey tell us about the variability of young stellar objects?

Stellar Surveys

map of the nebular complex with locations of the sample stars overlaid

Full sample of young stellar objects overlaid on an image of the North America Nebula (light-colored area on the left) and the Pelican Nebula (light-colored area on the right). The final sample of variable stars is indicated by the darker blue circles. [Hillenbrand et al. 2022]

Stars of all ages show a broad variety of variability, from slow pulsations to quick bursts of accretion. Young stellar objects seem especially prone to interesting variations, perhaps thanks to the influence of their natal nebulae. In a new publication, Lynne Hillenbrand (California Institute of Technology) led a team of astronomers, which included four high school students, on a hunt for variability in young stellar objects.

The team surveyed stars in the North America and Pelican Nebulae, which lie 2,590 light-years away in the direction of the brightest star in the constellation Cygnus. Though they bear different names, the nebulae are actually connected, separated visually by a dark band of dust. The North America and Pelican Nebulae complex contains thousands of candidate young stellar objects, dozens of which have been monitored for signs of variability. Astronomers have previously used Zwicky Transient Facility data to study individual young stellar objects, but never a large sample — until now.

example plots showing different values of the flux asymmetry and quasiperiodicity parameters

Examples of light curves with low, medium, and high (from left to right) values of the flux asymmetry parameter (M, top row) and quasiperiodicity parameter (Q, bottom row). Click to enlarge. [Hillenbrand et al. 2022]

Digging In to the Data

Hillenbrand and collaborators analyzed more than two years of Zwicky Transient Facility observations of 392 young stars, searching for signs of variability and classifying each star’s quasiperiodicity and flux asymmetry. As the names suggest, quasiperiodicity refers to how periodic or random an object’s variation is, while flux asymmetry quantifies how symmetrical an object’s light curve is. Both metrics are valuable ways to describe a variable star’s behavior, distinguishing those that vary smoothly and reliably from those that suddenly and haphazardly burst or flare.

The team found that 323 of the stars in their sample vary in brightness, and 15% do so with a regular period and symmetric light curves. Another 39% of the stars have symmetric light curves but vary either quasiperiodically or randomly. Roughly 14% of the stars are “bursters” and 29% are “dippers,” with abrupt increases and decreases in brightness, respectively.

The team also examined the colors of the objects, since color changes can give us clues as to why an object’s brightness is varying. The color analysis suggested that dippers might be due to changes in the dusty, light-absorbing material surrounding the young stars, while bursters may signal accretion episodes.

Broad Applicability

plot of flyx asymmetry versus quasiperiodicity

Categorizations of all stars in the sample according to their flux asymmetry and quasiperiodicity. Click to enlarge. [Hillenbrand et al. 2022]

This work not only provides information about hundreds of young stars, but also reflects on the metrics we use to study them. The quasiperiodicity index was designed for data from space telescopes, and some studies suggest that it can’t be used with less precise and lower cadence ground-based data. However, Hillenbrand and collaborators make a compelling case that applying thoughtful boundary conditions makes the quasiperiodicity index viable for ground-based applications as well.

This study marks the first time that Zwicky Transient Facility data have been used to investigate a large sample of young stellar objects, but surely not the last — be on the lookout for more exciting results from this survey!

Citation

“A Zwicky Transient Facility Look at Optical Variability of Young Stellar Objects in the North America and Pelican Nebulae Complex,” Lynne A. Hillenbrand et al 2022 AJ 163 263. doi:10.3847/1538-3881/ac62d8

Map of the observing area for the fourth phase of the Optical Gravitational Lensing Experiment overlaid on a picture of the milky way

Researchers have reanalyzed nearly 10,000 light curves from the Optical Gravitational Lensing Experiment. The resulting catalog, which is publicly available, provides new opportunities to study black holes, exoplanets, and much more.

Making Sense of Microlensing

illustration of a gravitational microlensing event

An illustration of a gravitational microlensing event. In this case, the lensing object is a star with an exoplanet in orbit around it. Click here to see an animation of this event. [NASA]

When one astronomical object passes in front of another, the background object’s light is lensed, or focused, by the foreground object’s gravity, and we see a temporary jump in the background object’s brightness. This process of gravitational microlensing can clue us in to the presence of objects that emit little or no light, like black holes, exoplanets, and dark matter candidate objects, as they pass in front of stars or other luminous sources. Gravitational microlensing events are one of the most promising ways to find isolated stellar-mass black holes, which have long been difficult to track down.

The Optical Gravitational Lensing Experiment (OGLE) has observed more than 10,000 microlensing events since the survey began in 1992. But recording the event is just the first step to understanding what caused it. Researchers model microlensing light curves to estimate the properties of the objects involved, but many factors can complicate these calculations; our vantage point — Earth — is in constant motion, stars vary in brightness for a multitude of reasons, and instruments aren’t perfect. How can we account for all those factors and extract useful information from a microlensing light curve?

plot of the magnitude of a target of the OGLE survey as a function of time

An example of a gravitational microlensing event drawn from the sample analyzed in this work. In this event, OGLE BLG 156.7.141434, the brightness of the background source is variable. Click to enlarge. [Golovich et al. 2022]

New and Improved

That’s where today’s article comes in. In a new publication, a team led by Nathan Golovich (Lawrence Livermore National Laboratory) reanalyzed nearly 10,000 microlensing events in the third and fourth OGLE catalogs. The team’s new model accounts for Earth’s motion — which affects our perception of how quickly the background and foreground objects move relative to one another — as well as variability in the brightness of the background object and systematic instrumental effects.

This type of model has been applied to a single microlensing event, but it has never been used on a full survey because of the immense computing power it requires — Golovich and coathors used about a million hours of computer processing time to analyze their sample! The team showed that their model was able to separate the desired signal from competing factors like Earth’s motion and the variability of the object being lensed, greatly reducing sources of bias.

A Curated Catalog

plot of a microlensing event and models fit to that event's light curve

An example light curve and model fit for OGLE BLG 156.7.141434, the same event shown in the previous figure. Click to enlarge. [Golovich et al. 2022]

What does this updated catalog mean for the search for isolated black holes? Golovich and collaborators used the open-source Population Synthesis for Compact object Lensing Events tool (PopSyCLE) to simulate microlensing events and identify locations in parameter space that isolated black holes are likely to inhabit. Based on the results of these simulations, the authors estimate that 50% or more of the 390 OGLE events in that region of parameter space are most likely due to foreground black holes.

The catalog — the largest of its kind, to date — is free to anyone who wishes to use it; if you’re interested in black holes, exoplanets, or any other kind of dark object, there’s no better time to be on the hunt!

Citation

“A Reanalysis of Public Galactic Bulge Gravitational Microlensing Events from OGLE-III and -IV,” Nathan Golovich et al 2022 ApJS 260 2. doi:10.3847/1538-4365/ac5969

Galaxy surrounded by a large dark matter halo

Astronomers have long sought to probe deep into our universe’s early history. What was the nature of matter back then? How did small galactic seeds grow into the gas-siphoning monsters we see today, and what was the nature of the mysterious substance that weighs down their halos yet eludes our earthly detectors? A team of astronomers may have uncovered a new tool that will allow us to probe this mysterious matter on smaller scales than ever before. 

Image showing galaxies of all shapes and sizes scattered on the sky

The Hubble eXtreme Deep Field, which shows galaxies from when the universe was just 500 million years old. [NASA, ESA, G. Illingworth, D. Magee, and P. Oesch, R. Bouwens, and the HUDF09 Team]

Peering Back into the Universe’s History 

One of the key tasks of modern astronomy has been to understand the early universe and how it evolved to get to the state it’s in today. The Hubble Space Telescope took us back to when the universe was just 500 million years old, and the Planck mission allowed us to peer back at the universe when it was just 380,000 years old, using the cosmic microwave background radiation (CMB) (light from the very early universe that’s been stretched to the microwave regime as the universe has expanded). One of the keys to understanding the early universe is understanding how both ordinary matter and dark matter behaved at this time.  

A clue to how dark matter acts on small scales might be found in the dark matter halos surrounding galaxies in the early universe. These dark matter halos were much less massive than those that surround galaxies today, so probing these halos in the early universe would provide us with a new window to look at dark matter on smaller scales and could help us understand the nature of this mysterious substance that pervades our cosmos. 

1+z (redshift) vs. k (wavenumber) showing the range that CMB lensing, CMB, LRG, Cosmic shear, MW Sat. Lyman-alpha, this work, and 21 cm probe (also showing the corresponding halo masses at redshift z = 0 on the y-axis)

Different areas probed by different experiments, showing redshift against wavenumber, which characterizes the spatial scale explored with the measurements. [Sabti et al. 2022]

Probing Dark Matter on Small Scales 

A group of scientists led by Nashwan Sabti from King’s College London has used a decade of observations from the Hubble Space Telescope to study dark matter at very small scales, looking at distant galaxies and their halos using a method complementary to the range of local probes and the CMB. The team first determined the ultraviolet galaxy luminosity function (UV LF), which captures the abundance of galaxies as a function of their UV luminosity. Because the UV LF is dependent on the mass distribution of dark matter halos, this technique allowed the authors to indirectly probe how dark matter is distributed on different scales during this early period in the universe’s history, revealing clues as to how the early structure of our universe formed and evolved. 

Power spectrum plotted as a function of wavenumber, showing a curve that starts ~10^3 Mpc^3 on the y-axis (10^-4 wavenumber), peaks around a power spectrum of 10^5 and wavenumber 10^-2, and then falls off

Power spectrum as a function of wavenumber k for seven different works. Wavenumber is a measure of the spatial scale, and the matter power spectrum indicates what the matter density perturbations look like on any given scale. The results of this study (black crosses) are plotted along with previous measurements, showing that this work probes smaller scales (larger k) than any other experiment has before. [Sabti et al. 2022]

Using the Power of a Wide Range of Measurements 

The authors’ UV LF measurements cover a wide range, from when the universe was 48 million years old all the way up to 156 million years old, and probe scales beyond what the CMB allows us to explore. The authors model the resulting matter power spectrum — a measure of how matter clusters on different spatial scales — with different parameters to test a range of theoretical models describing dark matter. The team found that their modeled power spectra were consistent with the theoretical predictions of the lambda cold dark matter model of cosmology (the standard model of the universe) up to a certain point. The power spectra disfavor other models, such as the warm dark matter model, which doesn’t predict structure consistent with what the team found on small scales.  

These new results show that measuring the UV LF is a unique, powerful technique for probing the nature of dark matter. The newly launched JWST and the Nancy Grace Roman Space Telescope, which is set to launch in mid-2027, will observe galaxies farther back in the universe’s history and probe dark matter halos on smaller scales, making this an exciting time for dark matter astronomers! 

Citation 

“New Roads to the Small-scale Universe: Measurements of the Clustering of Matter with the High-redshift UV Galaxy Luminosity Function,” Nashwan Sabti et al 2022 ApJL 928 L20. doi:10.3847/2041-8213/ac5e9c 

image of the milky way's supermassive black hole from the event horizon telescope

In 2019, the Event Horizon Telescope collaboration published an image of the supermassive black hole at the center of the galaxy Messier 87 — the first image of its kind ever created. Now, the long-awaited first image of the Milky Way’s central supermassive black hole has been released. The results of this international endeavor are summarized in a new Focus Issue of The Astrophysical Journal Letters.

A Black Hole Bonanza

composite x-ray and infrared image of the center of the milky way

This composite (blue) and infrared (red and yellow) view shows the crowded environment of the galactic center. The close-up X-ray view of the innermost half light-year zooms in on Sgr A*. [X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI]

Over the past decade, gravitational-wave observatories, telescope arrays, and detailed computer models have advanced our understanding of black holes throughout the universe. Now, our knowledge of the black hole at the center of our galaxy — Sgr A*, pronounced “sadge-ay-star” — is getting a huge boost thanks to years of work by the Event Horizon Telescope (EHT) collaboration.

The EHT is a global network of radio dishes stretching from Antarctica to Spain that can be combined to make a planet-size telescope. In April 2017, eight telescopes in six locations around the world collected the faint radio emission from Sgr A*, Messier 87’s central black hole (M87*), and a handful of other objects. The images of M87* released in 2019 helped us to determine this black hole’s mass, spin, and orientation, and they provided a valuable test of how massive objects bend spacetime.

hubble space telescope image of the galaxy M87 and it's particle jet

Messier 87, located 53 million light-years away, emits a powerful jet of electrons and other particles in this visible-light image from the Hubble Space Telescope. [NASA and The Hubble Heritage Team (STScI/AURA)]

Although M87* and Sgr A* appear around the same size from Earth — Sgr A* is roughly the scale of a donut placed on the surface of the Moon, as viewed from Earth — the two supermassive black holes are vastly different. M87* is known to be active, shooting out a jet of particles that stretches 5,000 light-years from the galaxy’s center, while our own supermassive black hole is relatively subdued. M87* is 2,000 times farther away than Sgr A*, but it’s also 1,500 times more massive and far more luminous. M87* likely feeds on massive reservoirs of gas while Sgr A* sips from the stellar winds of a few dozen stars in its neighborhood.

Sgr A*’s size and location create some unique observational challenges. Because Sgr A* is much smaller than M87*, our black hole’s appearance can change on much faster timescales, leading to hourly brightness variations that made processing the EHT data challenging. On top of that variability, there’s the difficulty of viewing angle: to observe Sgr A*, the researchers had to peer through 27,000 light-years of gas and dust between Earth and the galactic center. How did the EHT collaboration handle these challenges, and what do the resulting images tell us?

Inspecting Images

images of the milky way's supermassive black hole constructed from event horizon telescope data

Image of Sgr A* constructed from observations made on April 7, 2017. The bottom row shows average images derived using different data-processing techniques and the prevalence of each morphology (bar graphs in lower-left corners). These images do not show the time variability of Sgr A*. Click to enlarge. [EHT Collaboration et al. 2022]

Unlike M87*, which looked similar each day it was observed, Sgr A*’s brightness varied during a single day’s observations. Building on the programs used to create the image of M87*, the team devised new algorithms to process their data, accounting for Sgr A*’s rapid variation and removing the effects of scattering by interstellar gas and dust.

The articles published today focus on the best set of observations, which were taken on April 7, 2017. An image created from these data reveals a bright, asymmetrical ring of emission from the superheated gas that orbits the black hole. At the center of the ring is a dark region called the shadow, which contains the black hole’s event horizon — the surface beyond which nothing, not even light, can escape the black hole’s grasp.

These stunning images provide the first visual confirmation that the massive object at the center of the Milky Way is indeed a black hole. The diameter of the ring — 51.8 microarcseconds, encircling a shadow with a diameter of 48.7 microarcseconds — precisely matches the predictions of the general theory of relativity for a black hole four million times as massive as the Sun, providing yet another validation of this leading theory of gravity.

Modeling a Massive Black Hole

After constructing the images, the team used models to explore conditions close to Sgr A*, determine the black hole’s properties, and test theories of gravity. Overall, the simulations match the observations beautifully, with models that incorporate strong magnetic fields, involve a moderate rate of spin, and place Sgr A* at a slight angle relative to our line of sight matching the observations best.

modeling results and simulated image

Left: Snapshot of a simulation of the emission from Sgr A* that matched all but one of the team’s observational criteria. Center: The same simulation, but time-averaged to match the EHT observing cadence. Right: Simulated EHT image based on the modeling results. Click to enlarge. [EHT Collaboration et al. 2022]

However, none of the models passed all of the team’s stringent tests. In particular, the models were unable to match Sgr A*’s level of variability, which may mean that they don’t fully capture the behavior of the hot ionized gas in the black hole’s vicinity.

The team also used the decrease in brightness at the center of the ring to constrain the nature of Sgr A*. The brightness decrease is consistent with the presence of a black hole with an event horizon and specifically rules out several exotic black-hole alternatives like naked singularities, boson stars, and certain types of wormholes.

Looking Ahead

side-by-side comparison of M87* and Sgr A* images

Side-by-side comparison of the angular sizes (how large objects appear on the sky) of M87* (left) and Sgr A* (right). [EHT Collaboration]

Combining the EHT results for Sgr A* and M87* with insights gleaned from gravitational-wave measurements of colliding stellar-mass black holes, we have validated the predictions of Einstein’s general theory of relativity over an impressive range of masses and distances.

Though the EHT’s results give definitive answers to many questions about the Milky Way’s central black hole, many questions about supermassive black holes remain. How do supermassive black holes like M87* create their particle jets? Do supermassive black holes hinder star formation in the galaxies they inhabit, or do they spur it onward? And how and when did supermassive black holes form in the first place?

Future analysis of the EHT observations will delve into the structure of the magnetic field near Sgr A*, as well as investigate structural changes that may have accompanied a flare that was seen at X-ray wavelengths on the final day of EHT observations.

Locations of the telescopes that make up the EHT array

Locations of the telescopes used to create the Sgr A* images (yellow symbols) as well as the facilities added to the EHT array after 2018 (blue symbols). Click to enlarge. [ESO/M. Kornmesser]

While the existing observations of Sgr A* already promise a wealth of future discoveries, there are even more exciting things to come. More detectors and recording bandwidth have been added to the EHT since 2017, greatly enhancing the capabilities of this planet-spanning telescope and potentially enabling the first video of hot gas circling a black hole. It’s a thrilling time to be studying black holes!

For more information, check out the full ApJL Focus Issue here: Focus Issue on First Sgr A* Results from the Event Horizon Telescope

Citation

“First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way,” EHT Collaboration et al 2022 ApJL 930 L12. doi:10.3847/2041-8213/ac6674

collage of 18 fields containing submillimeter galaxies

Early in the history of the universe, huge, dusty galaxies churned out new stars. A new survey performed with a highly sensitive array of radio telescopes is poised to teach us more about this active period in our universe’s history.

photograph of alma telescopes

Arrays of radio telescopes like the Atacama Large Millimeter/submillimeter Array are a powerful tool for studying galaxies in the early universe. [Pablo Carrillo – ALMA (ESO/NAOJ/NRAO)]

Early Galactic Goings-On

Galaxies that shine at submillimeter wavelengths — often referred to simply as submillimeter galaxies — offer a window into the evolution of massive galaxies in the distant past. These galaxies generate copious amounts of optical and ultraviolet radiation from their furious star formation, but they’re so dusty that most of the emission that reaches us is longer in wavelength, in the submillimeter range. Submillimeter galaxies are a challenge for observers and modelers alike, as the two groups struggle to agree on how numerous these galaxies are and how their brightness varies over the course of the universe’s history. Can a new, high-resolution survey of the brightest of these galaxies shine a light on the matter?

Submillimeter Survey

In a recent publication, a team led by Chian-Chou Chen (陳建州) from the Academia Sinica Institute of Astronomy and Astrophysics, Taiwan, presented the first results from a new survey of bright submillimeter galaxies from when the universe was 1.2–3.3 billion years old. The sample was drawn from the Cosmological Evolution Survey, which was carried out with the Submillimetre Common-User Bolometer Array 2 instrument, giving the survey the moniker SCUBA-2 COSMOS.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), the team explored several characteristics of 18 of the brightest galaxies surveyed in SCUBA-2 COSMOS:

  • plot of redshift versus 870 micron flux density

    Redshift distribution as a function of flux density at 870 microns for this survey (black circles) and previous works (blue squares and peach diamonds). Click to enlarge. [Chen et al. 2022]

    Redshift distribution: The median redshift for this sample is z = 3.3, which is higher than that of less luminous galaxies, suggesting that brighter submillimeter galaxies are found at higher redshifts (i.e., earlier in the universe).
  • Galaxy pairs: Five of the 18 galaxies investigated in this study are actually two galaxies. Of these, two appear to be physically associated. This implies that 40% of the paired galaxies in the sample are interacting.
  • Magnification: By modeling the gravitational field surrounding the galaxies, the team finds that only one is likely to be strongly lensed by a foreground galaxy.
  • Mass and number density: The authors estimated the average mass (350 billion solar masses, or about a third of the mass of the Milky Way) and number density (roughly 1 x 10-6 cMpc-3) of the galaxies to compare them to galaxy populations later in the universe.
plots and spectra of the five galaxy pairs in the sample

Images at a wavelength of 870 microns for the five galaxy pairs in the sample (top row). Spectra of the primary (middle row) and secondary (bottom row) galaxies in each pair. Click to enlarge. [Chen et al. 2022]

Interaction Implications

What do these findings imply about galaxies during this time period? The high proportion of submillimeter galaxies that are paired up suggests that interactions between galaxies play a large role in star formation at this stage in the universe’s development. And the true proportion of interacting galaxies may be higher — future surveys tailored to detecting faint emission lines from companion galaxies might reveal more interactions.

Given the masses and numbers of the galaxies studied in this work, it’s possible that bright submillimeter galaxies are an important missing link in the lineage of ancient galaxies: they may be the forebears of quiescent (i.e., not star-forming) galaxies that appear later in the universe — a population whose galactic ancestors have been difficult to determine.

Citation

“An ALMA Spectroscopic Survey of the Brightest Submillimeter Galaxies in the SCUBA-2-COSMOS Field (AS2COSPEC): Survey Description and First Results,” Chian-Chou Chen et al 2022 ApJ 929 159. doi:10.3847/1538-4357/ac61df

collage of six views of the sun from solar orbiter

You’ve heard of sunspots, but how about Sun dots? A recent solar mission has snapped photos of tiny, bright dots on the Sun, and astronomers are contemplating their cause.

Something New on the Sun

infographic of important dates in the Solar Orbiter mission

An illustration of the dates of major orbital milestones in the Solar Orbiter mission. Click to enlarge. [ESA-S.Poletti]

In the short time since its launch in February 2020, Solar Orbiter has already returned some incredible photos and data of the Sun. The joint European Space Agency–NASA mission is expected to have its biggest scientific breakthroughs when it tilts its orbit to be able to see the Sun’s poles — a region that we’ve never photographed — but it has already revealed never-before-seen phenomena like the miniature explosions on the Sun’s surface called solar campfires.

Today’s article dives into some of the exquisite data from Solar Orbiter to investigate an even smaller-scale phenomenon: the fleeting appearance of dozens of tiny, bright dots.

Seeing Spots

A team led by Sanjiv Tiwari (Lockheed Martin Solar and Astrophysics Laboratory and Bay Area Environmental Research Institute) began their investigation of these dots with Solar Orbiter observations from 20 May 2020, when the Sun’s dynamic activity cycle was at a simmer. Using images from the Extreme Ultraviolet Imager, Tiwari and collaborators analyzed a region where loops and strands of magnetic flux were poking through the Sun’s surface. The images revealed tiny spots faintly visible against the bright background of hot plasma.

side-by-side images of the dots studied in this work

Solar Orbiter Extreme Ultraviolet Imager (left) and post-processing image (right) of the dots, which are indicated by the yellow boxes. [Adapted from Tiwari et al. 2022]

After applying image processing techniques to enhance the appearance of the dots, the authors were able to characterize 170 of them as they emerged and faded over the course of an hour. On average, the dots are round, 30% brighter than their surroundings, about the size of Germany, and last just 50 seconds before fading. About half of them retained their roundness throughout their short lives, while others split into two, merged with other dots, or elongated and gave rise to small explosive events.

Making a Magnetic Connection

side-by-side images and magnetic field maps of the dots studied in this work

A Solar Dynamics Observatory extreme-ultraviolet image with an unsharp mask applied (left) and the line-of-sight magnetic field strength for the same region (right). The dot being studied is indicated by the yellow square. [Adapted from Tiwari et al. 2022]

The team also analyzed images and magnetic field measurements from the Solar Dynamics Observatory, a spacecraft that surveys the Sun from Earth orbit. (The team noted that any features that pop up in the Solar Orbiter images do so 3.22 minutes before they appear in the Solar Dynamics Observatory images due to the time it takes light to travel between the two spacecraft.) The magnetic field measurements revealed that the dots tended to be concentrated in areas with strong magnetic fields or regions where inward- and outward-pointing magnetic field lines are found close together.

So, what causes these dots? Using magnetohydrodynamic simulations, the team confirmed that most dots are associated with interacting magnetic field lines and potentially occur when field lines snaking out from beneath the solar surface connect with those already above the surface. However, a smaller number of dots were free of tangled magnetic field lines, and they may have instead been formed by shock waves moving through the region.

With Solar Orbiter making its way closer to the Sun over the next few years, we should be able to get a better view of these dots and better understand their origins. Who knows what kind of dots we’ll be able to spot then!

Citation

“SolO/EUI Observations of Ubiquitous Fine-scale Bright Dots in an Emerging Flux Region: Comparison with a Bifrost MHD Simulation,” Sanjiv K. Tiwari et al 2022 ApJ 929 103. doi:10.3847/1538-4357/ac5d46

projection of the milky way

How can we measure something we can’t see? When it comes to dark matter, astronomers are always finding new ways to track it down.

On a Hunt for Dark Matter

Galaxy with a big halo [made of dark matter]surrounding it

Illustration of a galaxy and its dark matter halo (shown in blue). [ESO/L. Calçada]

The motion of each star in our galaxy reflects the combined gravitational influence of all the stars, gas, dust, and dark matter in the Milky Way. In theory, we should be able to separate out the effect of the dark matter, giving us a sense of how it’s distributed throughout the galaxy. In practice, though, this is a tricky thing to measure!

Previous work has attempted to suss out the Milky Way’s gravitational field — and, by extension, its dark matter distribution — by measuring tiny shifts in the timing of the signals from extremely dense, rapidly spinning stellar remnants called pulsars. However, pulsars are relatively rare, leading astronomers to search for ways to discern the Milky Way’s dark matter distribution by keeping a close eye on some of the most common stars in the galaxy.

Measuring Midpoints

Sukanya Chakrabarti (Institute for Advanced Study and Rochester Institute of Technology) and collaborators explored the possibility of using binary stars as a probe of the Milky Way’s gravitational field. This technique hinges on making careful measurements of eclipsing binaries — those in which the stars repeatedly pass in front of each other as seen from our perspective. The team proposed that it’s possible to tease out the tiny nudge of the Milky Way’s overall gravitational field by measuring changes in the timing of the eclipse midpoint, when one star is perfectly centered on the other.

plot of time shift in seconds as a function of the binary period in days

The shift in the timing of the eclipse midpoint over the course of a decade due to individual physical mechanisms. The binary system is taken to have nearly circular orbits with an eccentricity of 0.01. The spins are assumed to be unsynchronized, which means that the effect of tidal decay is an upper limit. [Adapted from Chakrabarti et al. 2022]

However, the effect of the Milky Way’s pull is small — shifting the eclipse midpoint by just 0.1 second over the course of a decade — and other factors might also impact the eclipse timing: exoplanets tug on their parent stars, relativistic effects slowly alter elongated orbits, and stars in tight binary systems draw closer together over time. To determine if there are systems in which these other effects are small compared to the effect of the Milky Way’s pull, Chakrabarti and collaborators analyzed a sample of ~800 eclipsing binary systems. They estimated that more than 400 of these systems have orbits that are circular enough and periods that are long enough — lessening relativistic and tidal effects — for the Milky Way signal to be discernible, and they found that exoplanets don’t influence the timing of the eclipse midpoint much at all. The eclipse timing of ~200 of these systems can be measured to within 0.5 second, with some within 0.1 second.

Capable Spacecraft

plot of a simulated eclipse

Example of a simulated eclipse for KIC 4144236, demonstrating that Hubble can measure the eclipse timing to within 0.1 second. Click to enlarge. [Chakrabarti et al. 2022]

While this work by Chakrabarti and coauthors demonstrates the feasibility of this technique, putting it into practice will require patience. The authors demonstrated that the Hubble Space Telescope is able to determine eclipse timings to within 0.1 second for some systems, and JWST and the Nancy Grace Roman Space Telescope will make even more exacting measurements. Luckily, the shift due to the Milky Way’s overall gravitational pull grows over time; the longer we look, the better our measurements and our understanding of the Milky Way’s dark matter distribution will be.

Citation

“Eclipse Timing the Milky Way’s Gravitational Potential,” Sukanya Chakrabarti et al 2022 ApJL 928 L17. doi:10.3847/2041-8213/ac5c43

Compact star with a strong magnetic field giving off a burst of light

What’s the mechanism behind millisecond-duration bursts of radio energy coming from outer space? A team of astronomers performed a systematic search of optical transients to see if they could match one radio burst with another object, which would help constrain where these bursts come from.

Fantastic, Radiant, Baffling 

Fast radio bursts (FRBs) are energetic pulses of radio waves that burst onto the scene in 2007. Their origin is one of the biggest recent mysteries in astronomy. As new telescopes such as the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Telescope in Canada have come online, more FRBs have been found, but even with all of these new sources, we don’t yet know with certainty what causes them. Some FRBs repeat, some have been localized, and a few are accompanied by persistent radio emission. One of the most promising theories is that FRBs are caused by bursts from magnetars — super dense neutron stars that have extremely high magnetic fields — but no one is quite sure. 

2D map of sources on the sky

The distribution of FRBs (orange circles) and astronomical transients (blue circles) in the sky with the position of FRB 180916B and AT2020hur denoted with a red circle. [Li et al. 2022]

Searching for Signals of a Coincident Companion

Though some models predict multiwavelength counterparts to FRBs, only a few have been found. This may be because these counterparts are very faint, have very short durations, or there’s too much of a delay between the FRB and the counterpart for them both to be detected in one observation. Two FRBs have been found to be accompanied by persistent radio emission — FRB 190520B and FRB 121102 (which is located near radio emission consistent with a superluminous supernova) — while FRB 200428 is located coincident with an X-ray burst. These detections of multiwavelength counterparts led to the theory that there may be some connection between FRBs and other transient sources. A team led by Long Li (Nanjing University) decided to see if there are any optical transients that coincide with FRB 180916B, the only known FRB to repeat at regular intervals. What they found may help shed light on the origin of these bursts. 

To see if any astronomical transients are coincident with FRB 180916B, the team searched through transients contained in the Open Supernova Catalog (OSC) and the Transient Name Server (TNS), both of which contain supernovae, unidentified transients, and some gamma-ray bursts. They discovered that one unidentified source, AT2020hur, seemed to line up with the location of FRB 180916B. The authors calculate that the probability that the sources are connected is 99.96%, meaning the alignment most likely didn’t happen by chance.  

MJD plotted against flux density (mJy), showing how the light curves of the two match up

The radio light curves for the FRB and counterpart. Circles represent detections and triangles represent upper limits. [Li et al. 2022]

Mysterious Magnetars or Fantastic Flares? 

So what does mean for the origin of FRB 180916B? The authors postulate that the FRB could be caused by a flaring magnetar, while the optical counterpart comes from the afterglow of one or more giant flares emitted by that magnetar. However, the authors find this scenario unlikely because the energy of the flares would have to be much larger than what is typical for giant flares. In addition, there’s a lot of fine-tuning and coincidences required for this model to work. Another possibility is that the optical counterpart could come from two or more optical flares that originated from the source of the FRB, which would make sense because the transient is detected during one of the emission windows of FRB 180916B.  

Though the possibility of FRBs having optical counterparts is exciting and could help us solve the mystery of these bursts, more observations of FRBs and their optical counterparts are needed to better understand what processes may be at work in these systems. 

Citation 

“AT2020hur: A Possible Optical Counterpart of FRB 180916B,” Long Li et al 2022 ApJ 929 139. doi:10.3847/1538-4357/ac5d5a 

illustration of rocky material bombarding the young Earth

Earth’s crust contains chemical elements that we’d expect to find in its core, not near its surface. What can detailed simulations of planet formation tell us about the likely origins of these elements?

A Crash Course in Earth History

plot of elemental abundances in Earth's crust

A plot of the abundances of individual chemical elements in Earth’s crust. The siderophile elements, outlined in yellow, are rare in Earth’s crust, though not as rare as expected. [Gordon B. Haxel, Sara Boore, and Susan Mayfield from USGS]

Early in the solar system’s history, rocky planetesimals collided to form larger bodies and eventually planets. As early Earth accreted material through collisions, siderophile (“iron-loving”) elements like gold and platinum dissolved into the young planet’s iron-rich core. However, present-day Earth has an unexpectedly large amount of these elements in its crust, indicating that they were added to the planet late in its formation.

The number, size, origin, and composition of the objects that delivered this final sprinkling of siderophile elements is still uncertain, though. Now, astronomers have used simulations to make sense of the elements found in Earth’s crust and reconstruct our home planet’s formation history.

Location, masses, and origins of planetesimals in the Grand Tack simulation (left) and the calm accretion simulation (right). The top row shows the beginning of the simulation and the bottom row shows the end. In the calm accretion scenario, the planetesimals tend to contain material sourced from their location (indicated by the symbol color). In the Grand Tack model, the planetesimals tend to become “bluer” because of material moved inward by Jupiter. Click to enlarge. [Adapted from Carter & Stewart 2022]

Plentiful Planetesimals

Philip Carter (University of Bristol, UK, and University of California, Davis) and Sarah Stewart (University of California, Davis) set out to understand if Earth’s unusual crustal composition could be due to collisions with planetesimals late in the planet’s formation history. To do so, the team used numerical models to track the composition of tens of thousands of simulated planetesimals as they migrated and collided over a period of 21 million years. The authors explored two scenarios for the dynamics of the inner solar system: the Grand Tack model, in which a simulated Jupiter barrels into the inner solar system before retreating to its current location, and the calm accretion model, in which there is no disturbance from a giant planet.

In the calm accretion model, planetesimals tended to collect material from very close to their birthplace. Since the composition of the planet-forming disk changes as a function of distance from the Sun, this means that planetesimals forming at different distances from the Sun had different compositions.

In the Grand Tack model, on the other hand, Jupiter’s migration mixes the material in the inner solar system, leading to the formation of planetesimals containing a blend of material from throughout the inner solar system. In this scenario, planetesimals at a range of distances from the Sun had similar compositions.

Ample Earth-Like Material

The Grand Tack model concentrates mass in a region located 0.8–1.3 au from the Sun. The Jupiter-induced mixing in this region results in a substantial fraction of planetesimals with similar composition to the planetary embryos that are present within 0.2 au. [Adapted from Carter & Stewart 2022]

If Jupiter’s migration shook up the inner solar system, it may have created plenty of planetesimals similar in composition to Earth. If those planetesimals collided with Earth late in its formation, they could distribute the siderophile elements in their cores over Earth’s surface.

This scenario could also explain why the Moon has the same chemical signature as Earth; if the Mars-sized protoplanet hypothesized to have collided with Earth to form the Moon contained elements in similar ratios to Earth, that would naturally explain the Moon’s composition.

Plenty of questions remain, but the new simulations make a compelling case that collisions between early Earth and material similar in composition could explain many aspects of present-day Earth. For more details and future prospects, be sure to read the full article cited below!

Citation

“Did Earth Eat Its Leftovers? Impact Ejecta as a Component of the Late Veneer,” Philip J. Carter and Sarah T. Stewart 2022 Planet. Sci. J. 3 83. doi:10.3847/PSJ/ac6095

photograph of the lupus 3 star-forming region

For nearly 50 years, models have predicted that the Milky Way should be forming new stars far faster than it currently is. Can a reassessment of our models solve this long-standing mystery?

Stymied Star-Formation or Mistaken Models?

map of the milky way's star forming clouds

The Milky Way hosts thousands of clouds of molecular gas — the sites of star formation. This map shows the locations and surface densities of molecular hydrogen clouds with a map of the Milky Way’s spiral arms on top. [Miville-Deschênes et al. 2017]

All across the galaxy, cold clouds of molecular gas are churning and collapsing, forming dense cores where stars are born. Each year in the Milky Way, 1.65–1.90 solar masses of gas are converted into stars, but theoretical work claims that this number should be 150–180 times larger.

Theorists have suggested that magnetic fields, turbulence, and massive stars injecting energy into their natal clouds suppress the Milky Way’s star-formation rate, but these solutions require unrealistically strong magnetic fields and constant, widespread turbulence. And this star-formation conundrum extends beyond the Milky Way — studies have found that models predict a speedier star-forming rate for our galactic neighbors as well. What might be amiss with our models?

A Milky Way Mystery

A team led by Neal Evans (University of Texas at Austin) approached this long-standing problem by considering the two main quantities that determine a galaxy’s star-formation rate: the masses of molecular gas clouds and how efficiently they form stars.

plot of collapse efficiency as a function of the virial parameter

Plot of the efficiency with which molecular clouds collapse into star-forming cores per unit freefall time as a function of the virial parameter, αvir, which describes whether the cloud is gravitationally bound. (A high αvir value indicates that the cloud is not gravitationally bound and thus forms stars less efficiently.) The blue squares represent the new simulations from this work. The shaded green area indicates a constraint from observations. [Evans et al. 2022]

The mass of a molecular cloud determines, in part, whether or not the cloud is gravitationally bound and how long it will take to collapse. Since we can’t put molecular clouds on a scale, and they’re mainly composed of hard-to-detect molecular hydrogen, we measure emission from other molecules that are present in the clouds to determine the clouds’ total masses. The authors used maps of carbon monoxide emission in the Milky Way to estimate the masses of the star-forming clouds, using a conversion factor that depends on the abundance of metals (elements heavier than helium) and varies with distance from the galactic center.

The authors also considered how to improve our estimates of the star-formation efficiency — the fraction of gas in a star-forming cloud that eventually forms stars. Simple models of star formation assume that all gravitationally bound clouds will be entirely converted into stars, while those that are not bound won’t form any stars at all. However, the authors posit that star-formation efficiency likely varies from cloud to cloud rather than being all or nothing. To capture this subtlety, they developed a framework in which turbulence, high-energy radiation from newly formed stars, and energy injected by supernovae moderate the star-formation efficiency.

Closing In on a Solution

plot of star forming rate as a function of distance from the galactic center

Star-formation rate surface density as a function of distance from the galactic center. Observational estimates are shown in magenta. The numbers in parentheses give the total star-forming rate of each model in solar masses per year. [Evans et al. 2022]

Using a range of input values derived from observations, the authors calculate the star-formation rate in the Milky Way to be between 0.50 and 5.93 solar masses per year — neatly encompassing the observed range of 1.65–1.90 solar masses per year. This work shows that the mystery of the Milky Way’s slow star-formation rate can largely be solved by accounting for the impact of metallicity on the calculation of the mass of molecular clouds and better constraining the star-formation efficiency. The authors anticipate that continued modeling and future surveys of star-forming clouds that search for emission lines from other molecules will further improve our understanding of the star-forming capabilities of the Milky Way and galaxies beyond.

Citation

“Slow Star Formation in the Milky Way: Theory Meets Observations,” Neal J. Evans II et al 2022 ApJL 929 L18. doi:10.3847/2041-8213/ac6427

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