Features RSS

Hubble Space Telescope image of SN1987A

Astronomers observe and model exploding stars to understand the details of stellar evolution, map the chemical enrichment of our galaxy, and even estimate the expansion of the universe. Today we take a look at five recent approaches to supernova science, from assessing the behavior of a single star to making plans for future surveys that will detect thousands of supernovae every year.

Radio Investigation of an Exploding Star

observations and models of the radio evolution of SN 2016gkg

Radio spectra observed with the Jansky Very Large Array (circles) and the Giant Metrewave Radio Telescope (squares) compared to best-fitting models (solid lines). [Nayana A. J. et al. 2022]

Nayana A. J. (Indian Institute of Astrophysics) and collaborators monitored the supernova SN 2016gkg for nearly four years, starting just eight days after the initial explosion. Modeling of SN 2016gkg’s optical light curve suggest that it’s a Type IIb supernova, meaning that it resulted from the collapse of a massive (>8–10 solar masses) star that once possessed a hydrogen-rich envelope of material, most of which was lost before the star collapsed.

While it’s possible to identify the star that went supernova in images taken before the explosion, researchers rely on models of supernova light curves to extract details about the material surrounding the star, which can hold clues about the star’s behavior before it went supernova. The team modeled SN 2016gkg’s radio light curves and spectra, simulating the effects of a shock wave plowing into material previously released by the star. The results suggested that the shock wave from SN 2016gkg expanded outward at 10% the speed of light into stellar wind material produced in the last hundred years of the star’s life.

Accounting for Cooling

C.-I. Björnsson (Stockholm University, Sweden) investigated the radio-wavelength behavior of another supernova, SN 2020oi. SN 2020oi is classified as a Type Ic supernova, which means the explosion resulted from the collapse of a massive star that had lost its hydrogen envelope. Type Ic supernovae are distinguished from other types of supernovae by their spectra, which have weak or no spectral features from silicon or helium.

Björnsson modeled the supernova’s radio emission, which is largely produced by electrons traveling in helical paths around magnetic field lines. Where Björnsson’s modeling differs from previous modeling efforts is the inclusion of radiative cooling by these spiraling electrons in the spectral fitting process, rather than assessing the cooling rate after the fitting is done. The author demonstrated that accounting for cooling during the fitting process alters the inferred properties of the event; for example, while previous analyses have invoked a varying pre-explosion mass-loss rate to explain some features of SN 2020oi’s light curve, Björnsson found that incorporating cooling removed the need for a changing mass-loss rate.

derived upper limits on the instrinsic strain for NS 1987A

Upper limits on the intrinsic strain (a dimensionless quantity related to the amplitude of a gravitational wave; shown here in blue points) compared to an indirect limit on the strain set by the conservation of energy. [Adapted from Owen et al. 2022]

A Search for Gravitational Waves

SN1987A is one of the best-studied supernovae, but astronomers are still debating what it left behind. Multiple lines of evidence suggest that the explosion resulted in a neutron star — a city-sized stellar core so compressed that protons and electrons are squished together into neutrons — but the remnant has never been detected directly. Recently, a team led by Benjamin Owen (Texas Tech University) searched for gravitational waves from the purported neutron star. Unlike two objects colliding, which produces a sudden gravitational wave chirp, a young neutron star would emit gravitational waves constantly, powered by the slow spin-down of the rapidly spinning star.

Guided by knowledge of the neutron star’s likely distance, age, and size, the team searched through observations from the Laser Interferometer Gravitational-Wave Observatory (LIGO) but found no signal. However, the search allowed them to place upper limits on the neutron star’s gravitational wave emission and place constraints on the object’s properties, such as its ellipticity and magnetic field strength. As more precise search methods and gravitational wave data become available, Owen and collaborators anticipate more exacting searches will be possible in the future.

Preparing for the Future

Brian Hsu (Center for Astrophysics ∣ Harvard & Smithsonian) and collaborators looked ahead to future surveys, like the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST), which will produce a vast archive of photometric data. While LSST presents an incredible opportunity to study transient events, including supernovae, it also presents a challenge — how do we use photometric data to identify transient events that normally require spectroscopic data to classify? As an example, the authors considered superluminous core-collapse supernovae, which account for just 0.1% of all core-collapse supernovae but are overrepresented in surveys due to their extreme brightness (10–100 times brighter than a typical supernova).

posterior distributions of magnetar parameters

Posterior distributions of magnetar parameters derived from light curve modeling. Click to enlarge. [Hsu et al. 2022]

Hsu and collaborators used photometric data from the Pan-STARRS1 Medium Deep Survey to train a machine-learning algorithm to identify superluminous supernovae using only photometry. The team fit the light curve for each event with a model representing the spin-down of a magnetar — a neutron star with an extremely strong magnetic field — which is thought to power these events. This allowed them to determine the likely properties of the supernovae in their sample and compare these properties to those of supernovae that were selected spectroscopically. The team found that the two supernova samples had similar properties overall, though the photometrically selected sample captured events driven by magnetars that spun more slowly. While the team noted several challenges related to their technique, they anticipate that machine-learning tools will provide a way to extract valuable insights from the upcoming wealth of photometric data.

Studying Standard (and Over-Luminous) Candles

Not all supernovae are the result of massive stars collapsing; Type Ia supernovae occur when a stellar companion contributes mass to a white dwarf — an Earth-sized stellar core left behind when a low- or intermediate-mass (<8 solar masses) star exhausts its supply of hydrogen and loses its outer layers. All Type Ia supernovae are hypothesized to occur when a white dwarf reaches 1.44 solar masses, resulting in explosions of (theoretically) identical brightness. This makes Type Ia supernovae extremely valuable as standard candles, helping researchers gauge the distances to galaxies and even study the expansion of the universe.

Histogram of absolute magnitude during peak brightness of typical Type Ia supernovae and over-luminous Type Ia supernovae

Histogram of absolute magnitude during peak brightness of typical Type Ia supernovae (black) and over-luminous “91T/99aa-like” Type Ia supernovae (red). [Adapted from Yang et al. 2022]

However, in reality, not all Type Ia supernovae are equally luminous, and understanding how these events differ is critical to calibrating our observations and ensuring that Type Ia supernovae remain useful as standard candles. Jiawen Yang (Texas A&M University) and collaborators used observations of two varieties of Type Ia supernovae — 37 over-luminous Type Ia supernovae and 87 typical ones — to assess the differences between them. The authors found that the over-luminous supernovae are systematically 0.2 magnitude brighter than typical supernovae, even after applying several corrections to their light curves.

Because the two types of supernovae fade similarly in the months after exploding, Yang and collaborators point out that identifying over-luminous supernovae as they approach peak brightness is necessary. Otherwise, these over-luminous supernovae might be mistaken for typical supernovae, biasing calculations of cosmological distances. Accounting for differences in the light curves of Type Ia supernovae will become especially important for interpreting the results of future all-sky surveys, since these surveys will discover an outsize number of over-luminous Type Ia supernovae.

Citation

“Radio Evolution of a Type IIb Supernova SN 2016gkg,” Nayana A. J. et al 2022 ApJ 934 186. doi:10.3847/1538-4357/ac7c1e

“Radio Spectra of SN 2020oi: Effects of Radiative Cooling on the Deduced Source Properties,” C.-I. Björnsson 2022 ApJ 936 98. doi:10.3847/1538-4357/ac87aa

“First Constraining Upper Limits on Gravitational-wave Emission from NS 1987A in SNR 1987A,” Benjamin J. Owen et al 2022 ApJL 935 L7. doi:10.3847/2041-8213/ac84dc

“Photometrically Classified Superluminous Supernovae from the Pan-STARRS1 Medium Deep Survey: A Case Study for Science with Machine-learning-based Classification,” Brian Hsu et al 2022 ApJ 937 13. doi:10.3847/1538-4357/ac87ff

“Using 1991T/1999aa-like Type Ia Supernovae as Standardizable Candles,” Jiawen Yang et al 2022 ApJ 938 83. doi:10.3847/1538-4357/ac8c97

artist's impression of a protoplanetary disk

Recent observations of the protoplanetary disks hosted by a pair of young stars suggest the presence of hot, turbulent water vapor. Though many possibilities exist, researchers propose that a compact disk around a young planet could be the source of this rare spectral signature.

Planet Formation Locations

Illustration of the protoplanetary disks around stars in a binary system

Illustration of the protoplanetary disks around stars in a binary system. [R. Hurt (NASA/JPL-Caltech/IPAC); CC BY 4.0]

Protoplanetary disks are the sites of planet formation, and studies of these disks hold the key to understanding the origins of the planets in our solar system and beyond. However, protoplanetary disks are complex, and teasing out the promised planetary origins relies on understanding the interconnected facets of disk chemistry, structure, and kinematics.

To make matters more complicated, throw another star into the mix: by studying the disks around young binary stars, which are expected to form at the same time from the same material, researchers can probe other aspects of disk development. For example, it’s not yet known whether disk evolution is deterministic (meaning that two disks with the same initial properties will evolve in the same way) or random (meaning that the evolution of identical disks will diverge). In a recent research article, astronomers set out to study dual disk development in a binary system — and found something unusual along the way.

Spectrum of VV CrA A

Spectrum of VV CrA A (black) overlaid with models of water vapor emission at different temperatures (green, orange, and blue). Click to enlarge. [Adapted from Salyk et al. 2022]

Infrared Investigation

Colette Salyk (Vassar College) and collaborators analyzed high-resolution infrared spectra of the two disks in VV Corona Australis (VV CrA), a two-million-year-old binary system containing stars roughly half the Sun’s mass. The disk around one of the stars, VV CrA A, showed an unusually large number of emission lines due to the presence of water vapor — and only one other protoplanetary disk is known to show water emission lines at such long wavelengths.

The second disk in the binary system, VV CrA B, has some of the same spectral features, but emission from water vapor was detected only weakly. This doesn’t necessarily mean that the disk lacks water; instead, water vapor might be present at a lower temperature or density.

Disks Find Themselves in Hot Water

diagrams of emitting geometry and corresponding modeled spectra

Diagrams of the emitting geometry for VV CrA A’s disk and the modeled emission for each scenario. Click to enlarge. [Salyk et al. 2022]

Salyk and collaborators modeled VV CrA A’s spectrum and found that the emission likely arises from water vapor that is hot (1500K), dense, turbulent, and spans an area of just 0.003 au2. Intriguingly, further modeling showed that the emission could arise from a water-rich ring circling the central star or a compact ring surrounding a planet in the process of formation — a circumplanetary disk. Detections of circumplanetary disks are rare and often tentative, so being able to identify them via their water emission would be exciting. However, the only stellar systems in which this water feature has been identified are young, which could mean that the presence of hot water vapor is instead tied to a process at work in young disks, like accretion or disk winds.

As is often the case in the study of protoplanetary disks, delving into one question prompts many others. The authors suggest several avenues for future work, including observing VV CrA at near-infrared or submillimeter wavelengths and expanding their modeling of emission from circumplanetary disks. Hopefully, further analysis will illuminate the cause of this unusual water feature!

Citation

“An Unusual Reservoir of Water Emission in the VV CrA A Protoplanetary Disk,” Colette Salyk et al 2022 AJ 164 136. doi:10.3847/1538-3881/ac8878

polarized-light image of the supermassive black hole in Messier 87

When the supermassive black holes at the centers of galaxies siphon gas from their surroundings, the superheated gas radiates at wavelengths ranging from X-ray to radio. A recent research article explores whether the composition of the accreted gas affects the radiation we observe.

What Do Black Holes Eat?

visualization of the accretion disk around a supermassive black hole

A visualization of the accretion disk around a supermassive black hole with the main features of the gravitationally warped image labeled. Click to enlarge. [NASA’s Goddard Space Flight Center/Jeremy Schnittman]

In 2019 and 2022, the Event Horizon Telescope captured images of M87* and Sgr A*, the supermassive black holes at the centers of Messier 87 and the Milky Way, respectively. The glowing rings of radio emission surrounding these black holes are produced by electrons spiraling around magnetic field lines in an extremely hot gas — so hot that the atoms are split apart, forming a sea of bare nuclei and electrons.

The gas gobbled up by M87* and Sgr A* is likely a mix of hydrogen and helium with just a dash of heavier elements, but we don’t know its exact composition. In a new publication, George Wong (Institute for Advanced Study) and Charles Gammie (University of Illinois) pose a relevant question: does the composition of this gas affect the electromagnetic radiation emitted by a supermassive black hole’s accretion disk?

plots of how modeled paramters change with changing gas composition.

Effect of varying the amount of helium in the gas accreted my M87* (blue solid line) and Sgr A* (red dashed line) on model outputs. Θe is the electron temperature and τS, τQ, and τV are varying types of optical depth. Click to enlarge. [Wong & Gammie 2022]

Hydrogen vs. Helium

To approach this question, Wong and Gammie began with a simple model that allowed them to estimate some of the effects of tuning the gas composition from pure hydrogen to pure helium. In each case, the team used the Event Horizon Telescope observations of M87* and Sgr A* as a benchmark, adjusting the model parameters until the simulated flux equaled the observed flux.

These simulations suggest that as the amount of helium climbs, the electrons must be at a higher temperature, the plasma must be less dense, and the magnetic field must be weaker to produce the observed flux. In other words, changing the gas composition requires altering other physical properties of the system to get the same amount of radiation. These physical alterations may in turn affect other observational properties, such as the polarization, or orientation, of the emitted light waves.

Analyzing Accretion Options

In order to investigate changes in the polarization and other observational properties, Wong and Gammie used the results of these simple models to inform more complex relativistic fluid dynamics models and generate synthetic images. The authors considered two extreme cases — one in which the gas surrounding the supermassive black hole is pure hydrogen, and one in which it’s pure helium. The team also explored two proposed models for how gas is accreted — one in which the material forms an accretion disk that steadily feeds material to the black hole, and one in which the material is accreted in random bursts.

model results for polarized images of the Milky Way's supermassive black hole

Modeled polarized-light images for Sgr A* at three frequencies. For these models, which incorporate a smooth accretion flow, the composition of the accreted gas affects the observed polarization fraction. Click to enlarge. [Adapted from Wong & Gammie 2022]

The authors found that the gas composition affects the polarization that we observe, with the helium-only model having a more orderly polarization pattern. In addition, changing both the gas composition and the accretion method (steady or random) results in complex outcomes including changing where in the disk the emission is generated. These results show that the presence of helium can affect the electromagnetic radiation emitted by an accreting black hole, suggesting that future models should consider the composition of the accreted gas an important variable.

Citation

“Effects of Hydrogen versus Helium on Electromagnetic Black Hole Observables,” George N. Wong and Charles F. Gammie 2022 ApJ 937 60. doi:10.3847/1538-4357/ac854d

extreme-ultraviolet image of the Sun

The first data from the Extreme-ultraviolet Snapshot Imaging Spectrograph show that the instrument’s unique design can help capture the behavior of a dynamic region of the solar atmosphere, hopefully helping researchers understand how the Sun’s upper atmosphere reaches its extreme temperatures.

Transition Region Reconnaissance

plot of the temperature of the Sun's atmosphere as a function of height

Temperature of the solar atmosphere as a function of height. [Andrew Fraknoi, David Morrison, and Sidney C. Wolff via OpenStax; CC BY 4.0]

Between the realm of sunspots and the place where space weather begins lies a narrow region of the solar atmosphere that is challenging to study. The aptly named transition region connects the Sun’s lower atmosphere, which consists of the relatively dense and cool photosphere and chromosphere, to the hot, tenuous upper atmosphere, or corona. In the span of just a few tens of kilometers, the temperature jumps to millions of degrees, creating an environment that requires high spatial, spectral, and temporal resolution to understand fully.

Traditional spectrographs struggle to separate spatial and temporal changes in the transition region, since the time needed to map a narrow slit across the region exceeds the region’s rapid variability. Newer slitless imaging spectrographs can capture spectra over a wide area simultaneously, but disentangling the spatial and spectral components of the data can be difficult. In a new publication, a team led by Jacob Parker (NASA’s Goddard Space Flight Center) describes the first flight of a new spectrograph specifically designed to study the dynamic environment of the solar transition region.

cartoon diagram of the ESIS instrument

This schematic shows the locations of the octagonal field stop, onto which the mirror (not pictured) focuses sunlight, and the four detectors. The locations of three prominent emission lines on the four detectors are marked with the colored octagons. [Parker et al. 2022]

A New Design

The Extreme-ultraviolet Snapshot Imaging Spectrograph (ESIS) has a novel design that incorporates four diffraction gratings that survey the solar atmosphere at 58.4–63.0 nanometers — a wavelength range suited to capture plasma motions in the transition region. Each grating is paired with a detector, and the varying angles of the four gratings disperse the incoming light at a different angle along each detector.

The ESIS data from each detector can be arranged into a data cube, which records the intensity of the Sun’s radiation as a function of location (two dimensions of the cube) and wavelength (the third dimension of the cube). A “slice” of this cube, then, is an image of the field of view at a single wavelength. The processing steps for this type of data include correcting for effects like vignetting and charged-particle impacts before aligning and combining data from all four detectors into a single data cube.

Event Estimation

comparison of an ESIS image of the Sun to one from the Solar Dynamics Observatory

Processed image from a single ESIS detector at a wavelength of 63 nm (left) compared to a 30.4-nm image taken by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly at roughly the same time (right). [Parker et al. 2022]

ESIS’s abilities were tested on a sounding rocket flight, during which the instrument soared to a height of 250 kilometers and collected data for roughly 5 minutes. The Sun was quiet on the day of the launch, but ESIS observed numerous small-scale events. Parker and coauthors described their preliminary analysis of five of these events, which showed plasma moving at roughly 100 km/s. These observations potentially captured complex, three-dimensional magnetic reconnection, in which the solar magnetic field relaxes into a new configuration and releases energy that can power solar eruptions, though more work is needed to interpret the observations fully.

Future work for the team will involve testing new ways to extract spectral information from the raw observations. Though the work thus far is preliminary, Parker and collaborators have shown that ESIS is able to track small events as they evolve on tens-of-seconds timescales — key to understanding the detailed physics of the solar transition region.

Citation

“First Flight of the EUV Snapshot Imaging Spectrograph (ESIS),” Jacob D. Parker et al 2022 ApJ 938 116. doi:10.3847/1538-4357/ac8eaa

Hubble Space Telescope image of galaxy cluster Abell 2744

After years of anticipation, JWST is operating beautifully, and the results are starting to roll in. Today marks the publication of a new focus issue of the Astrophysical Journal Letters, in which researchers describe the first results from the Grism Lens-Amplified Survey from Space (GLASS) Early-Release Science program. The observing program, led by Tommaso Treu from the University of California, Los Angeles, aims to illuminate the epoch of reionization: the period when the first stars ignited, ionizing the universe’s opaque neutral gas and allowing light to shine through.

The details of reionization, including when it started, when it ended, and what the primary sources of ionization were, are difficult to determine, requiring observations of galaxies less than a billion years after the Big Bang. In addition to exploring this crucial phase in our universe’s history, GLASS-JWST will also study gas in and around galaxies, helping researchers understand how star formation and galaxy structures have evolved over time.

To achieve these goals, GLASS-JWST uses JWST’s Near Infrared Imager and Slitless Spectrograph (NIRISS), Near Infrared Camera (NIRCam), and Near Infrared Spectrograph (NIRSpec) to observe distant galaxies behind the galaxy cluster Abell 2744, which is located about 4 billion light-years away. Why search for galaxies behind a galaxy cluster? The concentrated mass of the cluster bends and magnifies the light from more distant sources — a phenomenon called gravitational lensing — allowing us to study galaxies that would otherwise be out of reach. The Abell 2744 field has been studied in depth as a Hubble Frontier Field, and the GLASS-JWST observations deepen our understanding of this region. The first five articles of the GLASS-JWST focus issue were published today, with 12 more articles to follow.

A High-Redshift Search

composite JWST and Hubble image of the galaxy cluster Abell 2744

This representative-color image combines new observations with JWST and previous observations with the Hubble Space Telescope. The two high-redshift galaxies confirmed in this work are marked with magenta squares. [Roberts-Borsani et al. 2022]

In the first of the five articles published today, a team led by Guido Roberts-Borsani (University of California, Los Angeles) carried out the first search for continuum emission from galaxies behind Abell 2744 with redshift z ≥ 7, corresponding to less than about 800 million years after the Big Bang.

Many searches for high-redshift galaxies rely on identifying prominent emission lines, but these spectral features are weakened from absorption by the neutral hydrogen gas present early in the epoch of reionization. As a result, high-redshift galaxies identified through spectral features in this time period are biased toward those with exceptionally bright emission lines. In order to identify galaxies more typical of this epoch, Roberts-Borsani and collaborators searched for continuum emission from highly lensed galaxies using 15 hours of NIRISS observations. Ultimately, the team confirmed the detection of two galaxies from that epoch with redshifts z = 8.04 and z = 7.90. The team also made tentative identifications of ultraviolet emission lines redshifted into JWST’s observing range, the presence of which may indicate low-metallicity star-forming regions or non-thermal photon sources.

image of Abell 2744

Reduced image of the field (split into module b, left, and module a, right) in the F444W filter, the reddest of the NIRCam wide-band filters. [Adapted from Merlin et al. 2022]

Image Reduction and Catalog Production

Emiliano Merlin (Italy’s National Institute for Astrophysics — Astronomical Observatory of Rome) and collaborators described their efforts to process and make available the NIRCam observations of Abell 2744. The observations, which are centered on the galaxy cluster, consist of images taken in seven filters from 0.9 to 4.4 microns.

In preparation for their planned photometric analysis, the team generated synthetic images from mock galaxy catalogs and used these images to test their image reduction pipeline, which is a customized version of the official JWST pipeline provided by the Space Telescope Science Institute. Ultimately, the team extracted 6,368 sources from the images and measured colors and fluxes for each source, all of which have been made publicly available along with the processed images.

Distant Galaxy Candidates

plot of the photometric measurements for one of the high-redshift galaxy candidates

Photometry and best-fitting spectral energy distribution for the galaxy candidate at z = 12.2. The images across the top of the plot show the source in the seven wide-band NIRCam filters. [Adapted from Castellano et al. 2022]

Marco Castellano (Italy’s National Institute for Astrophysics — Astronomical Observatory of Rome) and coauthors pushed the search for early galaxies to extremely high redshifts. The team used two methods to identify potential galaxies with redshift z ~ 9–15 — just a few hundred million years after the Big Bang. The first method relies on a synthetic galaxy catalog — using observations of low- and moderate-redshift galaxies to predict the characteristics of high-redshift galaxies — to estimate the colors of galaxies in the desired redshift range. The second method uses photometric redshift, which relies on the overall reddening of a galaxy’s emission with increasing distance.

In total, the team identified six potential galaxies with redshifts in the z ~ 9–15 range. Two sources with redshifts of z = 10.6 and z = 12.2 were found to be especially intrinsically bright, with high rates of star formation. The remaining candidates, which were placed at slightly lower redshifts, were identified less robustly, and the authors note that spectroscopic followup and further deep imaging is needed to make a final identification.

From Reionization to Cosmic Noon

The penultimate article of those released today explores a galaxy somewhat less remote, situated in the star-formation heyday of z ≅ 1–3. A team led by Xin Wang (University of the Chinese Academy of Sciences; Chinese Academy of Sciences; California Institute of Technology) used JWST’s high-resolution spectroscopic capabilities to investigate a redshift z = 3.06 galaxy in detail, mapping the properties of its gas as a function of location within the galaxy.

This set of observations — the first instance of using JWST’s slitless spectroscopy to obtain spatially resolved observations of a high-redshift galaxy — allowed the team to measure the galaxy’s star-formation rate, metallicity, and extinction. The team found that the galaxy’s metallicity gradient is inverted, meaning that the galaxy’s outskirts contain more metals than the galactic center. This suggests that a past interaction between the target galaxy and a nearby galaxy that’s 100 times more massive caused metal-poor gas from the galaxy’s outer regions to be funneled in toward the center.

maps of stellar surface density and several emission lines within the galaxy GLASS-Zgrad1

Maps of the galaxy GLASS-Zgrad1 constructed from JWST NIRISS observations. [Adapted from Wang et al. 2022]

A Visual-Wavelength View of Galaxy Sizes

Finally, Lilan Yang (University of Tokyo) and collaborators investigated the size–luminosity relation for galaxies with redshift z > 7. While the Hubble Space Telescope has allowed researchers to measure the sizes of distant galaxies at rest-frame ultraviolet wavelengths, the size–luminosity relationship at rest-frame optical wavelengths has been more difficult to determine.

plots of the size–luminosity relationship for galaxies in the early universe.

Size–luminosity relationships for galaxies observed in five NIRCam wavelength bands. The solid and dot-dashed black lines in the rightmost panel (F150W) show the results from previous studies with Hubble data. Click to enlarge. [Yang et al. 2022]

Using NIRCam, Yang and coauthors measured the sizes of 19 bright galaxies with redshifts 7 < z < 15. The team found that the slope of the size–luminosity relationship is somewhat steeper in the shortest wavelength band, caused by the faintest galaxies in their sample appearing comparatively small in that wavelength range. This may support previous findings based on Hubble observations that the size–luminosity relationship is best represented by a broken power law, with the faintest galaxies behaving differently from the brightest galaxies. However, the authors caution that this work is preliminary, and further analysis on a more complete sample of galaxies should provide greater clarity.

While the first five articles in the focus issue stick close to the stated goals of the GLASS-JWST program, the remaining articles will branch out to explore a wide range of topics — proto-globular clusters in Abell 2744, galaxy shapes during the epoch of reionization, intriguing foreground stars in the Abell 2744 field, and much more. Whet your appetite for early-release science with today’s research articles, and stay tuned for more results from GLASS-JWST!

Citation

“Early Results from GLASS-JWST. I: Confirmation of Lensed z ≥ 7 Lyman-break Galaxies behind the Abell 2744 Cluster with NIRISS,” Guido Roberts-Borsani et al 2022 ApJL 938 L13. doi:10.3847/2041-8213/ac8e6e

“Early Results from GLASS-JWST. II. NIRCam Extragalactic Imaging and Photometric Catalog,” Emiliano Merlin et al 2022 ApJL 938 L14. doi:10.3847/2041-8213/ac8f93

“Early Results from GLASS-JWST. III. Galaxy Candidates at z ∼9–15,” Marco Castellano et al 2022 ApJL 938 L15. doi:10.3847/2041-8213/ac94d0

“Early Results from GLASS-JWST. IV. Spatially Resolved Metallicity in a Low-mass z ∼ 3 Galaxy with NIRISS,” Xin Wang et al 2022 ApJL 938 L16. doi:10.3847/2041-8213/ac959e

“Early Results from GLASS-JWST. V: The First Rest-frame Optical Size–Luminosity Relation of Galaxies at z > 7,” L. Yang et al 2022 ApJL 938 L17. doi:10.3847/2041-8213/ac8803

A photograph of an astronaut on the lunar surface standing over an instrument with large solar panels

In the half-century since astronauts walked on the Moon, many of the practices and techniques used by earth-bound planetary scientists have evolved. Recently, a team of planetary scientists reprocessed Apollo-era seismic data to meet modern forms, ensuring that these precious measurements remain usable for the next generation.

Far-Flung Seismographs

The Apollo astronauts left more than footprints when they departed the Moon over fifty years ago. A few of these remnants were abandoned intentionally to continue the scientific mission in the years following each lunar departure: these were the passive seismometers, or devices designed to measure tiny displacements of the ground caused by seismic waves traveling through the Moon’s interior.

Despite the remoteness of their locale and its penchant for destroying complex technology, these instruments performed spectacularly. While they sat patiently listening, each of the five packages recorded numerous lunar quakes and meteor impacts, which collectively demonstrated that the lunar subsurface was a highly fractured, messy arrangement of rock.

A plot showing the vertical displacement of the ground as a function of time following a shallow moonquake. The displacement oscillates around zeros, and its amplitude decays with time.

The seismic profile of a representative shallow moonquake. The y-axis denotes vertical ground displacement in nanometers. This event was recorded by the package deployed at the Apollo 12 landing site. [Adapted from Nunn et al. 2022]

Initial Transmission

The success of these seismometers was hard fought, however, and depended just as much on the ground logistics as it did on the space hardware. The relatively simple instruments could not store data, and instead screamed their findings in real time across the void towards Earth. These shrieks, manifested as radio transmissions, were picked up by NASA’s Deep Space Network and recorded on magnetic reels, which later were copied to cassette tapes and eventually digitized.

This process offered numerous opportunities for errors. For one, the pace of the seismometer’s measurements was not steady, making timestamping each discrete sample difficult. For another, certain sections were missing or duplicated, depending on the number of Deep Space Network dishes able to receive broadcasts. Finally, small sections of data were corrupted each time a magnetic tape was copied over the intervening decades.

As a result, the previously digitized version of the seismometer data was full of holes, repeats, and uneven timestamps. Though it could be used, it could not be imported into modern seismic data processing software and required intimate knowledge of mission specifics not familiar to most practicing planetary scientists.

A two panel plot showing a seismogram before and after cleaning. The "before" version contains several spikes greater than 20x the amplitude of the signal. These spikes are missing in the "after" version.

Top: A portion of the seismic data prior to any cleaning. Bottom: The same portion after application of the authors’ cleaning and timestamp alignment procedures. [Nunn et al. 2022]

Cleaning and Archiving

In this new study, Ceri Nunn (NASA JPL) and collaborators undertook the Herculean task of extracting the data from the magnetic tapes, massaging them into a modern format, and archiving it digitally for future use. The team states that they “hope that the new archive will make it easier for a new generation of seismologists to use these data to learn more about the structure of the Moon,” and thanks to their efforts, that new generation can lean on the efforts of their predecessors.

The newly archived data can be found here.

Citation

“A New Archive of Apollo’s Lunar Seismic Data,” Ceri Nunn et al 2022 Planet. Sci. J. 3 219. doi:10.3847/PSJ/ac87af

Black hole with a ring around it that has a tail, which shows the black hole pulling material off a star that's out of the frame.

Tucked away in the deep corners of the universe may lie intermediate-mass black holes: the missing link between supermassive black holes, which sit at the centers of most galaxies, and stellar-mass black holes, which result from supernovae. Could white dwarfs help us find these elusive enigmas? 

Tidal Tale Signs of an Intermediate Black Hole

Though intermediate-mass black holes have been challenging to find, they may reveal themselves when they rip apart a white dwarf and cause a burst of nucleosynthesis, the process that transforms light elements into heavier elements. Modeling the interaction of intermediate-mass black holes with white dwarfs can give us clues to what the electromagnetic signature of these events looks like, which we can then search for with telescopes.  

However, the 3D simulations we’d ideally use for this work are computationally expensive. Modeling these interactions requires looking at timescales from microseconds to hours to capture detailed nuclear physics and massive accretion flows. We also need to model length scales from tens of meters to thousands of kilometers to study hotspots of nuclear ignition as well as track the white dwarf throughout its orbit. Though 3D simulations might capture the physics most accurately, working in 2D reduces the computational cost and eliminates factors that may not be vital to understanding the system as a whole. A team led by Peter Anninos (Lawrence Livermore National Laboratory) simulated these tidally disrupted white dwarfs in 2D to test how well these simulations stack up against their 3D counterparts. 

Three panels on top of each other; top panel shows a line going from the top of the y-axis (value at 0.5) sloped down, hitting the x-axis at 0.5, with a fairly uniform gas density at roughly 4e+04; middle panel shows a line again starting at 0.5 on the y-axis but sloping down more gradually and landing at roughly 6.5 on the x-axis (with just small bumps from 6.0 to 6.5) and a fairly uniform density but a higher density from 6.0 to 6.5 (around 1e+06); bottom panel shows the same as the middle but with a big bump with contours from 6.0 to roughly 7.5 on the x-axis

The gas density over time for the 0.15 solar mass helium white dwarf with a fairly strong tidal force. The top panel is at a time of 1.25 seconds, the middle at 1.36 seconds, and the bottom at 1.45 seconds. The color bar represents the density of the gas in g/cm-3. The x and y axes plot distances, which are given in terms of 104 km. [Anninos et al. 2022]

Only Time Will Tell

The team simulated the interaction of a 0.15-solar-mass helium white dwarf and a 0.6-solar-mass carbon–oxygen white dwarf with intermediate-mass black holes at various distances — which correspond to various tidal strengths — and observed the conditions that triggered nucleosynthesis in both helium and carbon–oxygen white dwarf encounters.

After setting the initial conditions at a wide range of spatial scales, the authors ran time forward to see when specific elements were formed and when detonation (the start of nucleosynthesis) occurred. In the various scenarios, helium burned to carbon before detonation occurred, and the rising gas temperature triggered a detonation wave that burned carbon, oxygen, and their byproducts into nickel and iron.

Compression Conclusion

Anninos and collaborators found slight differences between the processes that trigger nucleosynthesis in the different strength interactions. Overall, they concluded that detonation of nucleosynthesis is mainly triggered by adiabatic compression — compression without a change in heat. Their tests between helium and carbon–oxygen white dwarfs showed very little difference in their behavior. 

Top panel: a density from 10^-4 g/cm^3 to 10^8 g/cm^3 with an x-axis from 0 to 2.5 (in units of 1e2) km, and showing a black line that has some spikes around y=10^-2 and around x=0.7, slopes up quickly; Bottom panel: same axes but with all of the colors of lines coming together around x=0.5 to x=1.3

Line profiles of the density of the helium (black), carbon (magenta), oxygen (green), calcium (blue), and nickel (red). The top panel shows densities just before the detonation and the bottom shows after. [Anninos et al. 2022]

The authors’ work reveals that 2D simulations are comparable to those in 3D, specifically for the modeled density and temperature profiles. Understanding the onset of nucleosynthesis in the tidal disruption events of white dwarfs allows us to predict the signature of an intermediate-mass black hole, which may finally lead to the detection of these mysterious objects.  

Citation 

“Resolution Study of Thermonuclear Initiation in White Dwarf Tidal Disruption Events,” Peter Anninos et al 2022 ApJ 934 157. doi:10.3847/1538-4357/ac7b87  

computer visualization of a binary supermassive black hole system

Astronomers study gravitationally linked pairs of black holes to understand how stellar-mass black holes form. What can we learn from black holes that are off kilter from the binaries they belong to?

You Spin Your Way, and I’ll Spin Mine

a diagram illustrating positive and negative effective spin

Top: The black hole spins are aligned with the system’s orbital angular momentum (positive effective spin). Bottom: The black hole spins are misaligned with the system’s orbital angular momentum (negative effective spin). [AAS Nova/Kerry Hensley]

Collecting the gravitational waves from merging stellar-mass black holes allows us get at one of the fundamental questions in high-energy astrophysics: how did the black holes that exist in the universe today come to be? A black hole’s mass and spin hint at whether it formed directly from the collapse of a massive star or through a process called hierarchical merging — the formation of large black holes through successive mergers of smaller ones.

One property we can use to probe the origins of black holes in merging binary systems is the effective inspiral spin parameter, denoted χeff, which is a measure of how aligned the spins of the black holes are with the orbit of the binary pair. In some systems, the black holes spin in the same direction as they orbit, giving the system positive χeff. In others, one or both black holes are tilted, and they spin in directions that are at odds with their orbital motion, giving the system negative χeff. A recent publication searches for the most misaligned black hole binaries to understand the origins of the black holes in the universe today.

plots of the effective spin distribution for binary systems containing one or two black holes formed through hierarchical merging

Theoretical probability distribution functions (top) and cumulative distribution functions (bottom) for the effective inspiral spin parameters (χeff) of binary systems containing one (orange) or two (blue) black holes formed through hierarchical merging. The different lines in the top panel show the effect of different initial black hole spins. [Fishbach et al. 2022]

Monitoring Misalignment

Maya Fishbach (Northwestern University) and collaborators began their investigation by predicting the spins of black holes in binaries in which at least one member formed through hierarchical merging. The team’s calculations showed that hierarchical merging results in a substantial number of black holes with spins misaligned from their direction of orbital motion. Specifically, the team predicted that 16% of black hole binaries would have χeff less than −0.3, regardless of whether one or both members of the binary system arose through hierarchical merging.

With this prediction in hand, Fishbach and coauthors turned to the data, assessing the χeff values of 69 merging black hole pairs in the third Gravitational-wave Transient Catalog (GWTC-3), which is the most recent catalog of gravitational wave events detected by LIGO and Virgo. Depending on the model used to analyze the data, the team found that the maximum proportion of binary systems with χeff less than −0.3 is 4.2%.

Evidence for Meager Mergers

plot of expected number of misaligned binary mergers as a function of total number of events observed and

The expected number of gravitational wave events for systems with χeff less than −0.3 that we expect to observe as a function of the hierarchical merger (HM) fraction and the total number of events observed. [Fishbach et al. 2022]

Given the small fraction of strongly misaligned black hole systems, the team found that hierarchical mergers likely populate no more than 26% of black hole binaries. Intriguingly, only 69% of massive black holes (60 solar masses) are expected to be products of hierarchical merging, meaning that some of these high-mass black holes most likely result from enormous collapsing stars — a finding that has implications for our understanding of stellar interiors and nuclear physics.

Fishbach and collaborators note that if black hole binaries with χeff less than −0.3 aren’t detected in the future, that would point to hierarchical merging being even less common. If the LIGO, Virgo, and KAGRA gravitational wave detectors find no such systems during their next observing runs, the team limits the percentage of black holes formed through hierarchical merging to no more than 2.5%.

Citation

“Limits on Hierarchical Black Hole Mergers from the Most Negative χeff Systems,” Maya Fishbach et al 2022 ApJL 935 L26. doi:10.3847/2041-8213/ac86c4

The logo of the Astropy Project, which is a stylized snake curled into the shape of a spiral galaxy

If producing new results in astronomy research is like constructing a house, different pieces of software used in the process are like the builder’s tools. One of the most fundamental of these tools, a screwdriver of the astronomy world, just got a major update.

The Python Era

Over the past 30 years, the common workflows used to analyze and present scientific findings in astronomy have evolved. While Fortran was the most common programming language discussed in peer-reviewed astronomy publications in the 1990s, IDL overtook this long-leading language to become the dominant method in the early 2000s. Most recently, this top position was usurped again just prior to 2016 when Python became the most-mentioned language. Since its reign began, Python’s popularity has only grown, and now it is regularly used in over 1,000 publications per year.

A plot showing the number of times a given programming language mentioned in refereed astronomy articles per year between 1990 and present. Five languages are shown, and Python has been the most popular since 2016.

A plot illustrating the trends in usage of various programming languages in refereed astronomical literature over time. [The Astropy Collaboration et al. 2022]

One driver for this ascendence in popularity is the ease of creating, modifying, and tweaking major software libraries. In 2013, a loose network of collaborators unveiled one of these libraries that aimed to handle many routine tasks in astronomical research, like unit manipulation, coordinate transforms, and more. Called Astropy, it was quickly adopted by the community, and as of this writing its release publication is the fifth most cited refereed astronomy article from the last decade. 

In the intervening years, the Astropy Collaboration and the software it maintains have both grown in scope. Astropy now underpins an “ecosystem” of other more specialized astronomy packages, and many new features have been added to the original core. In August of this year, a team of over 100 Collaboration members published the third article in the Astropy series which details their latest efforts and announces the newest major release of Astropy, version 5.0.

New Features

According to the authors, the core Astropy package is designed to stay as broadly applicable to as many subfields of astronomy as possible. As such, most domain-specific updates live in one of more than 50 “affiliated” packages, some of which are maintained by the central Astropy Collaboration, others by independent groups. 

However, there were still several updates deemed useful for the entire community that made it into v5.0, including a new subpackage to handle uncertainties and invoke various statistical distributions, and a new subpackage for manipulating time series data directly within the Astropy environment. Other major additions are listed in Section 2 of the Collaboration’s article.

Collaboration Updates

Besides documenting the new feature additions, the authors also included an update on the governance and health of the Collaboration. Astropy is not maintained by one central institution, but rather by interested members of the community who interact via its GitHub repositories. Until recently, the over 1,500 contributors have worked together without a formal charter, but in fall 2021 the collaboration adopted a more explicit governance structure.

A two-panel plot showing the number of commits to the Astropy core package per month over time on top, and the number of unique Collaboration members submitting those commits on the bottom. Both curves are relatively flat between 2011 and present.

A plot illustrating the number of active collaborators and the number of their additions to the Astropy code base. [The Astropy Collaboration et al. 2022]

Additionally, the Collaboration pursued and acquired several large grants for the first time in its history, both from private foundations and from NASA, which it intends to use to seed longer-term stability.

This useful experiment in open-source, collaborative research code continues to evolve, and further additions are rolled out regularly.

Citation

“The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package,” The Astropy Collaboration et al 2022 ApJ 935 167 doi:10.3847/1538-4357/ac7c74

Full moon against a black sky

Though it is our closest neighbor, the Moon remains partially shrouded in mystery. Under the surface of the Moon lie cryptomaria, remnants of ancient lava flows that were buried during periods of unknown lunar activity. Can radar observations help us map these hidden maria and uncover the secrets of the Moon’s volcanic past?

Title "Moon features you can see from Earth for Northern Hemisphere observers" and different maria and craters labeled

A map of different features on the Moon. [NASA/GSFC/Arizona State University/The Planetary Society]

It’s a Crater…It’s a Highland…It’s…Hidden Lava Flows?

The Moon is full of interesting geological features, from mountains that form the rims of craters to maria left over from ancient lava flows. However, not all of these features lie on the surface: cryptomaria are maria that formed early in the Moon’s history and have been buried by crater ejecta, hiding them from view. Cryptomaria can be mapped using a number of techniques: photographs reveal their locations on the surface (as they’re usually surrounded by dark-halo craters) and spectroscopic analysis shows their characteristic basaltic composition. A team led by Ali Bramson (Purdue University) has taken a new approach to searching for these buried seas: using radar to map their boundaries.  

Top image: 60 degrees S to 60 degrees N latitude by 135 degrees W to 135 degrees E longitude map of the Moon with a small red square circle from ~70 degrees W to 38 degrees W latitude and 60 degrees to 30 degrees S. Bottom: that zoom-in with five areas in boxes with dotted lines labeled #1-#5. Regions of white (cryptomare, Whitten & Head 2015a), light gray (light plaines, Meyer et al. 2020), and yellow (mare, Nelson et al. 2014) are shown.

A map of the estimated areas of cryptomaria from previous studies. [Bramson et al. 2022]

Looking for Cryptomaria in All the Right Places

Researchers use radar to probe below the surface of the Moon by sending radio waves toward the lunar surface and measuring the waves that bounce back. Observing using this approach can help uncover cryptomaria that are buried too deep to have been noticed in visible images and spectroscopic analyses, and it can also reveal ones that have not been exposed by impact craters.  

The team chose the Schiller–Schickard region in order to build on previous volcanic studies, which have used spectroscopy, visible images, and gravity signatures to study the multiple dark-halo craters in the area. Bramson and collaborators study used ground-based radar from the Arecibo and Green Bank telescopes, as well as space-based data from the Lunar Reconnaissance Orbiter Miniature Radio Frequency Instrument (LRO Mini-RF) to observe the region in a new way. 

Putting the “Rad” in “Radar” 

The team used radar at two different frequencies—P-band (430 MHz) and S-band (2380 MHz)—to study the area. Since the two frequencies probe different depths below the surface and structures of different sizes, any differences in the reflected radar signals would indicate similarities or differences between subsurface structure. Bramson and her team looked at the reflected radar signal in five locations and found new cryptomaria outside of previously mapped areas, indicating that the Schiller–Schickard region contains a large network of ancient lava flows. They also used the radar observations to estimate how deep the cryptomaria are buried and found that, in this region, the ancient lava flows are anywhere from ~10 to ~130 meters below the surface. This also shows that the cryptomaria in the region are more extensive than previously thought. If this is true, it could mean that we’re underestimating the amount of cryptomaria beneath the surface of the Moon by more than a factor of two. 

Title: "Interpretation of Schiller Schickard Volcanism. A region (60 degrees to 30 degrees South latitude and 70 degrees to 40 degrees West latitude) showing different colors for Mare (boundaries set by Nelson et al. 2014), very shallow cryptomare, shallow cryptomare, deep cryptomare, and deepest cryptomare (boundaries by Whitten and Head 2015a). Splotches of all colors are shown (along with just gray areas of the Moon)

A map of the depth of cryptomaria detected in the Schiller–Schickard region. [Bramson et al. 2022]

This study only focused on one region where cryptomaria are known to be prevalent, but it shows how useful radar can be in searching for these geologic features. The authors hope that future work will look at other areas where cryptomaria have been hypothesized in order to see if those areas have been underestimated as well.

Citation 

“Burial Depths of Extensive Shallow Cryptomaria in the Lunar Schiller–Schickard Region,” A. M. Bramson et al 2022 Planet. Sci. J. 3 216. doi: 10.3847/PSJ/ac8670

1 … 41 42 43 44 45 … 122