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

The first stars in the universe, known as Population III stars, formed just a few hundred million years after the Big Bang. While researchers expect that most Population III stars led bright, brief lives, those with masses less than 0.8 solar mass would still be shining faintly today. Can magnetic fields in the early universe explain why we’ve yet to find these stars?

Producing Population III Stars

infrared image of the Orion star-forming region with a map of magnetic field lines on top

An infrared image of the Orion star-forming region with magnetic field lines measured by the Stratospheric Observatory for Infrared Astronomy traced on top. [NASA/SOFIA/D. Chuss, et al., and European Southern Observatory/M.McCaughrean, et al.]

There are many possible explanations for why low-mass Population III stars are so elusive, but the simplest explanation might be that they never existed at all. It’s challenging to prove that something doesn’t exist in the universe, but simulations give us a way to explore whether low-mass Population III stars could have formed in the conditions that existed when the first stars were born.

Previous research suggests that the early universe was suffused with a subtle magnetic field that was about a trillion times weaker than the fields measured in typical star-forming regions in the universe today. Magnetic fields play an important role in shaping — and sometimes suppressing — star formation in the local universe, leading researchers to wonder if the weak magnetic fields in the early universe could have halted the formation of low-mass stars entirely.

plot of simulation results

Simulation results for the unmagnetized case (top row) and the case in which the magnetic field strength was 10-20 Gauss (middle and bottom row). The top and middle rows show how the density of the gas varied between the two simulations at 0, 10, and 1,000 years of simulation time. The bottom row shows how the magnetic field strength was amplified and evolved over time in the magnetized case. Click for high-resolution version. [Hirano & Machida 2022]

Magnetic Magnification

Shingo Hirano (University of Tokyo and Kyushu University, Japan) and Masahiro Machida (Kyushu University, Japan) modeled the collapse of a cloud of gas under the conditions present in the early universe to understand how magnetic fields might have influenced the formation of the first stars.

The team performed magnetohydrodynamic simulations of a gas cloud with weak initial magnetic field strengths of 0, 10-20, 10-15, and 10-10 Gauss. In the unmagnetized case, the cloud fragmented into a massive (~200 solar masses) central protostar and a handful of smaller protostars, all of which persisted until the end of the simulation at 1,000 years.

In the magnetized cases, on the other hand, the rapid rotation of the massive young star forming at the center of the cloud wound the magnetic field tighter and tighter, boosting the magnetic field strength up to 1,000 Gauss. The strong magnetic field prevented the cloud from fragmenting further, and the few small protostars that started to form dissipated entirely, leaving only the massive protostar at the end of the simulation. This result suggests that even a weak magnetic field can be amplified enough to stop small protostars from forming, meaning that the first stars may have all formed alone.

plots of the number of protostellar fragments formed and the total mass of protostars

The number of protostellar fragments formed (top) and the overall mass of protostars formed (bottom) in all simulations. [Adapted from Hirano & Machida 2022]

A Turbulent Twist

Hirano and Machida caution that while their simulations suggest that the magnetic fields in the early universe can prevent the formation of low-mass stars, other factors may influence the formation of the first stars; if the star-forming gas is turbulent, for example, it might be more inclined to fragment into multiple stars. Similarly, the slow process of diffusion could prevent the magnetic field from growing strong enough to play an important role.

In future work, the authors plan to introduce turbulence into their simulations, test the effects of different rotation rates of star-forming clouds, and extend the simulation out to 100,000 years. In the meantime, the search for Population III stars continues!

Citation

“Exponentially Amplified Magnetic Field Eliminates Disk Fragmentation around Population III Protostars,” Shingo Hirano and Masahiro N. Machida 2022 ApJL 935 L16. doi:10.3847/2041-8213/ac85e0

Hubble Space Telescope image of Alpha Centauri A and B

Across the Milky Way, pairs of nearly identical stars orbit each other, separated by vast distances. What can recent survey data tell us about how these systems form?

Binary Star Breakthroughs

By studying binary stars, astronomers hope to discern the details of star formation as well as how repeated gravitational encounters can shape stellar systems after they’ve formed. Common though binary stars may be, they’re not without their mysteries, and recent data have revealed intriguing details about the binary stars in our galaxy.

projection of the milky way

A view of the Milky Way containing 1.7 billion stars observed by the Gaia spacecraft. [ESA/Gaia/DPAC, CC BY-SA 3.0 IGO]

One finding is that widely separated binary systems in which the stars have nearly the same mass — wide twin binaries — are more common than expected. Twin binaries are expected to form from a single disk of gas and dust, but these disks tend to be far smaller than the present-day separations of these systems.

If these distant binary companions formed close together in a single disk before being driven to their current locations by gravitational encounters, these systems should have extremely elongated, or eccentric, orbits — and thanks to the Gaia spacecraft, we can test that prediction for thousands of stars.

plot of observed and modeled distributions of the angle between the two vectors

Observed distributions of the measured v–r angle for wide twin (blue) and non-twin (orange) binaries. The black lines show the simulated distributions for binary systems with eccentricities, e, of 0 (solid line) and 0.90 (dashed line). [Hwang et al. 2022]

Exploring Eccentricities

Widely separated binary systems take more than a thousand years to complete a single orbit, making it challenging to measure the eccentricity of an individual system. Instead, Hsiang-Chih Hwang (Institute for Advanced Study) used a statistical technique to study nearly a million binary systems at once. Using stellar position and velocity data from Gaia, the team measured the angle between two vectors: one that describes the difference in the binary members’ motion across the sky (v) and one that connects the two stars (r).

By comparing the angle between those vectors to theoretical predictions for stellar populations with different eccentricities, the team determined that twin binaries with orbital separations of 400–1,000 au tend to have extremely eccentric orbits. Specifically, there appear to be a high number of systems with eccentricities between 0.95 and 1.0.

Formation Possibilities

comparison of observed and modeled v minus r angle distributions for wide twin binaries

Left: Angle distributions for twin binaries with orbital separations of 400–1,000 au (blue). A power-law model is shown in black and the result for a simulated population of wide twin binaries in which 18.9% of stars have eccentricities between 0.948 and 0.992 is in red. Right: The modeled eccentricity distribution that corresponds to the dashed red line in the left panel. Click to enlarge. [Hwang et al. 2022]

This finding suggests that wide twin binaries likely form close together before being driven apart, but how these binary systems attain their eccentric orbits is still unclear. Hwang and collaborators explore several possibilities:

  1. An instantaneous “kick” could wrench a close circular orbit into a highly eccentric one, but it’s not clear what process could provide the kick.
  2. Wide, eccentric twin binary systems might instead have three stars, with the third star being a close, unresolved companion of one of the two widely separated stars. However, previous research suggests that unresolved stellar companions are not especially common among twin binaries.
  3. Interactions between a young binary system and the disk surrounding it could increase the system’s eccentricity. This process would affect all close binaries — not just twin binaries — but the results might be more apparent in the twin binary population because twins are more common among close binary systems.

The formation of wide, eccentric twin binaries has implications for single stars as well; Hwang and coauthors outline the possibility that the same process that drives close binary systems into highly eccentric orbits likely separates some systems entirely, creating pairs of “walkaway” stars that meander in opposite directions through the galaxy.

Citation

“Wide Twin Binaries are Extremely Eccentric: Evidence of Twin Binary Formation in Circumbinary Disks,” Hsiang-Chih Hwang et al 2022 ApJL 933 L32. doi:10.3847/2041-8213/ac7c70

ultra-diffus galaxy NGC1052-DF2

Ultra-diffuse galaxies are the size of normal galaxies but far fainter, and many host an unusual abundance of globular clusters. A recent study takes a closer look at how one such galaxy’s globular clusters came to be where they are — and what this might tell us about the galaxy’s dark matter halo.

Copious Clusters

Hubble image of UDG1 and locations of objects in that image

Left: Inverted Hubble Space Telescope image of UDG1. Right: Locations of objects identified in the Hubble observations, separated into magnitude bins. The brightest objects (dark red circles) are found closest to UDG1’s center. Click to enlarge. [Bar et al. 2022]

Observations over the past several years have given rise to numerous theories about the evolution of ultra-diffuse galaxies, and the arrangement of these galaxies’ globular clusters — spherical clusters containing hundreds of thousands of stars — can provide a useful test of these theories. Previous investigations of the ultra-diffuse galaxy NGC5846-UDG1, or UDG1, have shown that it has an exceptional collection of globular clusters for a galaxy of its size: researchers have found 54 candidate clusters, 11 of which have been spectroscopically confirmed.

UDG1’s population of globular clusters is also remarkable because its brightest clusters are concentrated near the center of the galaxy. The arrangement is unlikely to be random — what’s responsible for UDG1’s globular cluster distribution?

plot of projected radial distance versus cluster mass

Projected radial distance of UDG1’s globular clusters as a function of mass, binned three ways, compared to the predictions of simple dynamical friction theory. [Bar et al. 2022]

Influence of a Frictional Force

A team led by Nitsan Bar (Weizmann Institute of Science, Israel) hypothesized that the brightest and most massive globular clusters would naturally migrate to UDG1’s center because of gravitational dynamical friction. Dynamical friction isn’t the same as the friction that allows us to warm chilly hands by rubbing them together; instead, dynamical friction arises when objects interact gravitationally and lose a bit of their momentum in the process. In the case of UDG1, dynamical friction should cause the globular clusters to sink toward the galaxy’s center, and since the most massive clusters should experience the most friction, they should be found closest to the center.

To test this hypothesis, Bar and collaborators first used simple mathematical expressions to calculate where globular clusters with various masses would be located within UDG1 if dynamical friction is at work. Even without capturing the nuances of the system, these simple calculations matched observations fairly well, suggesting that dynamical friction plays an important role in UDG1.

A Test of Dark Matter Distributions

As a further test, the team performed detailed numerical simulations, scattering globular clusters evenly throughout a UDG1-like galaxy and allowing them to drift for 10 billion years under the influence of dynamical friction, cluster mergers, and mass loss. These simulations showed that dynamical friction could have caused globular clusters to migrate to their current positions, likely from an initial arrangement slightly more dispersed than the current arrangement.

plot of mass density as a function of radius for three halo models

Density profiles of three mass models tested. The Stars model is derived from the observed stellar luminosity of UDG1, while the other two models incorporate a massive, extended dark matter halo. [Adapted from Bar et al. 2022]

Bar and coauthors also explored the effects of changing the way mass is distributed in UDG1’s halo, which could give clues to the diffuse galaxy’s dark matter distribution. The team found that UDG1 could be situated in a massive dark matter halo, which would distinguish it from other ultra-diffuse galaxies that are almost entirely lacking in dark matter.

More work remains to be done, and the question of UDG1’s dark matter is not yet settled. The authors suggest new avenues for both theoretical and observational investigations: improved simulations of globular cluster formation can refine model results, and future data from Vera Rubin Observatory and the Nancy Grace Roman Space Telescope should illuminate the faintest globular clusters in ultra-diffuse galaxies.

Citation

“Dynamical Friction in Globular Cluster-rich Ultra-diffuse Galaxies: The Case of NGC5846-UDG1,” Nitsan Bar et al 2022 ApJL 932 L10. doi:10.3847/2041-8213/ac70df

Galaxy with jets coming from the center

Deep inside the dust-shrouded core of radio galaxy Centaurus A, particles are being accelerated to relativistic speeds. What’s causing this acceleration, and what’s the nature of the matter around this energetic core? By using multiple telescopes to observe nearly the entire X-ray spectrum, astronomers may be getting closer to unlocking the answers. 

Unpolarized light randomly oriented then going through a filter and getting polarized in one direction (vertically)

A diagram showing the concept of polarization; unpolarized light has electric fields going in all directions but polarized light has its electric field going in only one direction/vibrating in only one plane. Magnetic fields in space can change the orientation of the electric field, resulting in polarized light. [PhysicsOpenLab; CC BY 4.0]

Getting to the Core of the Mystery 

Polarization, or the way the electromagnetic waves are oriented, is a powerful tool in astrophysics; the same concept that allows sunglasses to reduce glare can also be used to probe the emission mechanism of magnetized neutron stars and study the orientation of magnetic fields. Measurements of the polarization properties of jets around the high-energy cores of supermassive black holes can help illuminate what type of physics is taking place, in particular how the high-energy emission is produced and how it behaves. These polarization measurements provide a valuable counterpart to other ways we probe the physics around black hole jets — like by examining how X-ray intensity varies with frequency.  

Using simultaneous observations from multiple X-ray telescopes, a team led by Steven Ehlert at NASA’s Marshall Space Flight Center explores the polarization of the material and the X-ray spectrum around Centaurus A in order to better understand the material around the galaxy’s core. Centaurus A is of particular interest because it contains an active galactic nucleus — a black hole spewing radio jets into space — and it also emits X-rays. Though many studies have observed the X-ray emission from its core, we still haven’t pinpointed the source of this energetic light. 

Image of the core of Centaurus A with surrounding material

An image of the core and surrounding regions of Centaurus A taken by IXPE. [Ehlert et al. 2022]

A Slew of X-Ray Telescope Observations 

The team used the Imaging X-ray Polarimetry Explorer (IXPE) to observe polarized X-ray emission from Centaurus A. IXPE is a brand new mission dedicated to studying polarized X-ray emission from sources such as neutron stars and supermassive black holes. Launched in December 2021, the first science images from the mission were released this past February. The instrument measured low degrees of polarization in the core of Centaurus A, which suggests that the X-ray emission is coming from a scattering process rather than arising directly from the accelerated particles of the jet. The low degree of polarization, specifically near the core region, indicates that electrons are accelerated in an area around the core where the magnetic field lines are twisted and disordered.

Observations from IXPE, Swift, NuSTAR, and INTEGRAL fit with a power law

The spectrum of the source taken with the various telescopes, fit with a simple power law. [Ehlert et al. 2022]

By combining the IXPE measurements with simultaneous X-ray observations using the NuSTAR, Swift, and INTEGRAL telescopes, the team was able to observe Centaurus A throughout the full X-ray spectrum and see how the X-ray emission behaves from 0.3 keV all the way up to 400 keV. They modeled the spectrum of the source and were able to fit a simple power law to it. The lack of complex spectral features indicates that the X-rays around Centaurus A are passing through an optically thin medium (material in space that the X-rays can pass through, where no scattering or absorption of the light takes place) that’s distant from where the X-rays originate.

A Unique Radio Galaxy? 

This work, which is consistent with previous studies at other wavelengths, shows that the X-rays coming from Centaurus A’s core are produced by particles that are accelerated within about a light-year of the central black hole. Studying other galaxies that host luminous, accreting supermassive black holes will allow scientists to understand if the low degree of X-ray polarization is common, or if Centaurus A is unique among the population. 

Citation 

“Limits on X-Ray Polarization at the Core of Centaurus A as Observed with the Imaging X-Ray Polarimetry Explorer,” Steven R. Ehlert et al 2022 ApJ 935 116. doi:10.3847/1538-4357/ac8056 

Photograph of the aurora taken from the International Space Station

Geomagnetic storms — disturbances in Earth’s protective magnetic shield caused by oncoming solar particles — can have real-world consequences. A recent research article explores how machine learning can be used to create an early warning system for these events.

Geomagnetic Storms on the Horizon

photograph of the solar corona

The ghostly solar corona and a billowing coronal mass ejection (right side of the image) emerge from behind a coronagraph that blocks the Sun from view in this image from the Solar and Heliospheric Observatory. [SOHO (ESA & NASA)]

A spacecraft at a distant vantage point glimpses a tangled mass of plasma and magnetic fields emerging from the Sun — a coronal mass ejection — headed our way. It’ll be hours or days before the coronal mass ejection collides with Earth, potentially disrupting radio communications, damaging spacecraft electronics, and threatening power grids. How can we predict if a coronal mass ejection will cause these disastrous consequences?

In a recent publication, a team led by Andreea-Clara Pricopi (Technical University of Cluj-Napoca, Romania) tested the ability of machine learning to predict whether a coronal mass ejection will disrupt Earth’s magnetic shield. This technique may provide a way to anticipate geomagnetic storms days in advance.

An Expansive Sample

Machine learning is a relatively new technique in which computers are trained on a set of inputs with known outcomes. The trained computer can then predict the outcomes of a fresh set of inputs.

Pricopi and collaborators took as inputs the speed, angle, and acceleration of coronal mass ejections identified in white-light images, as well as a measure of the overall solar flare activity. The corresponding output is a measure of how disrupted Earth’s magnetic field became, known as the disturbance storm time index. The team trained the model on these inputs and outputs for a subset of 24,403 coronal mass ejections observed between 1996 and 2014, 172 (0.7%) of which caused geomagnetic storms.

illustration of solar particles impinging on the magnetosphere

Artist’s impression of solar particles interacting with Earth’s protective magnetic shield, or magnetosphere, causing a geomagnetic storm. [NASA]

Because so few of the coronal mass ejections in the sample caused geomagnetic storms, Pricopi and collaborators had to be careful about assessing the model’s performance — after all, a model that simply labeled all 24,403 events as not causing a storm would be 99.3% accurate, but it would be useless as a predictor of geomagnetic storms! The team also wanted to be sure that their model correctly predicted all or most storms, even at the risk of false alarms, since the consequences of failing to prepare for a damaging geomagnetic storm are worse than preparing for a storm that never comes.

Prioritizing Powerful Events

Pricopi and coauthors trained their models on 80% of the data set, reserving the remaining 20% for testing the models’ performance. In order to push the models to prioritize finding geomagnetic storms, the team tested several strategies, including penalizing models that misclassified these events and creating synthetic storms based on real data to bulk up the sample size.

visualization of the model output

A visualization of the best model’s performance on the 20% of the data set reserved for testing. The color of the symbols indicates whether the model result was a true negative (TN, blue), false positive (FP, yellow), true positive (TP, green), or false negative (FN, red). Click to enlarge. [Pricopi et al. 2022]

The best model correctly predicted about 80% of storms. The storms overlooked by the model tended to have poor quality data, and false alarms were most common for certain types of coronal mass ejections, giving clues as to how the model might be improved in the future.

These results show that machine learning can be used to predict geomagnetic storms days in advance using a limited number of inputs. However, the authors acknowledge that models that incorporate data from later in a coronal mass ejection’s evolution are more accurate. This suggests that the technique described in this work could be used to flag potentially damaging events, passing them to more precise models to get more information and improve our ability to prepare for an oncoming storm.

Citation

“Predicting the Geoeffectiveness of CMEs Using Machine Learning,” Andreea-Clara Pricopi et al 2022 ApJ 934 176. doi:10.3847/1538-4357/ac7962

Hubble Space Telescope image of the globular cluster NGC 6397

Astronomers have discovered a second millisecond pulsar — a rapidly spinning, ultra-dense remnant of a massive star — in one of the nearest globular clusters to Earth. The new observations might help explain the surprising rarity of millisecond pulsars discovered in dense globular clusters.

Close Encounters of the Stellar Kind

animation still of a millisecond pulsar

Still image from an animation of a millisecond pulsar accreting material from its companion. [NASA]

In the cores of globular clusters, where gravitational encounters between stars are common, compact remnants of massive stars form binary systems with a wide range of properties. This sets the stage for the formation of millisecond pulsars: tiny, dense, rapidly spinning stellar remnants composed entirely of neutrons. All pulsars spin incredibly fast, but millisecond pulsars are the fastest of them all; if you stood on the equator of the speediest known millisecond pulsar, you’d whirl around at 24% of the speed of light. Astronomers believe that most millisecond pulsars started out as more slowly rotating solo acts, but after gaining a stellar companion, pulsars accrete matter and get spun up to “millisecond” status.

Nearby globular cluster NGC 6397 — a glittering, spherical collection of 400,000 stars — is home to a curious binary system that has been detected at X-ray, optical, and ultraviolet wavelengths. Its X-ray emission flashes with the period of the binary orbit, and optical observations show a red star at the same location. Previous research has suggested that this system contains a millisecond pulsar, but the characteristic radio pulses have been elusive.

plot of the radio pulses of the newly confirmed pulsar

Left: Phase-folded radio observations from the Parkes Telescope, showing the characteristic radio signal of the newly confirmed pulsar. Right: Timing residuals as a function of orbital phase. The pulsar is not visible when the phase is between 0 and 0.5 and the pulsar is farthest from the observer. Click to enlarge. [Zhang et al. 2022]

In Pursuit of a Pulsar

In a new article, a team led by Lei Zhang (Chinese Academy of Sciences and Swinburne University of Technology, Australia) reports the results of their observations of the system made between 2019 and 2022 using the Parkes (Murriyang) radio telescope in Australia and the MeerKAT array in South Africa. Zhang and collaborators discerned faint but detectable radio pulses every 5.8 milliseconds, and the pulses were modulated with a period of 1.97 days — the same period as the orbital period of the X-ray-emitting binary system at the same location.

This confirms that the system contains a millisecond pulsar, dubbed NGC 6397B, and further analysis of the timing of the pulses suggests that the pulsar is also the source of the X-ray emission detected previously.

Implications of an Intermittent System

plot of orbital period versus companion mass for known millisecond pulsars

Orbital period and companion mass for millisecond pulsars (MSPs) discovered in globular clusters (filled circles) and in the field (empty circles). The black and gray symbols indicate whether the companion is a white dwarf (WD), main-sequence star (MS), or an ultralight or planet-mass object (UL). Click to enlarge. [Zhang et al. 2022]

Even after the team tracked down the elusive pulsar, it still managed to give them the slip; the radio pulses became undetectable for 14 months before reemerging in early 2022. The system’s on-again off-again radio emission could point to one of two possibilities: hot, ionized gas flowing out from the companion star could be blocking the radio emission from reaching us when the binary system swings into certain orientations, or the act of accreting material from the companion star — the process that generates the X-rays — could temporarily halt the pulsar’s radio emission.

Previous research has suggested that pulsars in binary systems should be common in globular clusters with exceptionally dense cores, like NGC 6397, but most known pulsars in so-called core-collapse clusters are singletons. The particulars of the newly discovered system may provide an explanation as to why pulsar-hosting binary systems have been elusive in these environments: pulsars in binary systems might have faint or intermittent radio emission, making them hard to track down.

Citation

“Radio Detection of an Elusive Millisecond Pulsar in the Globular Cluster NGC 6397,” Lei Zhang et al 2022 ApJL 934 L21. doi:10.3847/2041-8213/ac81c3

European Southern Observatory image of the spiral galaxy NGC 6744

Milky Way–like galaxies are abundant in the universe. A recent publication scours astronomical surveys to understand how these familiar spirals grow over billions of years.

How Does Your Galaxy Grow?

visible-light image of hundreds of galaxies

A field of galaxies from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. Click to enlarge. [NASA, ESA, P. Oesch and I. Momcheva (Yale University), and the 3D-HST and HUDF09/XDF Teams]

In the first billion years after the Big Bang, galaxies began to form. But how the earliest galaxies evolved into those we see in the local universe today is still up for debate. One facet of this question is the growth of galaxies’ stellar mass — the amount of mass locked up in stars as opposed to gas clouds or dark matter. A galaxy’s stellar mass grows over time as gas clouds collapse to form stars, but it’s not yet clear how this growth proceeds; are galactic centers early hubs of star formation, do the outskirts skirt out ahead, or do all regions gain stellar mass at a similar rate?

In a recent study, a team led by Maryam Hasheminia (Shiraz University and Institute for Advanced Studies in Basic Sciences) turned to survey data to understand how the Milky Way and galaxies like it grew into the galaxies they are today.

Size evolution of Milky Way–like galaxies selected using the first method (red), the second method (blue), and from cosmological simulations (green). The quantities r20, r50, and r80 are the radii within which 20%, 50%, and 80% of the galaxies’ stellar mass is contained, respectively. [Adapted from Hasheminia et al. 2022]

Looking for Lookalikes

When we observe galaxies as they existed billions of years ago, how do we know which galaxies will grow to look like the Milky Way? Hasheminia and collaborators used two methods to pick Milky Way–like galaxies out of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and the 3D-Hubble Space Telescope survey.

In the first method, the team assumed that Milky Way–type galaxies follow the observed relationship between galactic stellar mass and star formation rate for galaxies that are actively forming stars, and the authors selected galaxies out of the survey data that followed that relation. In the second method, the team matched the stellar masses and ages of galaxies in the survey data to the stellar growth history of the Milky Way as derived from chemical evolution models.

Self-Similar Growth

Evolution of the half-mass radius (r50) of Milky Way–like galaxies with increasing stellar mass. [Adapted from Hasheminia et al. 2022]

Hasheminia and collaborators then studied how the stellar masses of the selected galaxies evolved over time and compared their findings against results from cosmological simulations. Both methods showed that the half-mass radius — the radius within which half a galaxy’s stars (by mass) are contained — of Milky Way–like galaxies has changed little over the past 10 billion years.

The results indicate that all parts of these galaxies gain stellar mass at a similar rate. This contrasts with the hypothesis of inside-out growth, in which star formation ramps up in the center of a galaxy first before spreading to the outer regions, as well as the results of cosmological simulations; simulations matched the team’s findings early in the history of the universe, but they diverged around 6 billion years ago.

The team suggests that their findings are consistent with previous Milky Way evolution scenarios, in which a thick disk of stars formed when the universe was about 6 billion years old, before star formation stalled and a bar of stars formed in the center of the galaxy. To fully unravel the growth history of the Milky Way and galaxies like it, we’ll need high-resolution observations of galaxies in the distant past — and with JWST, such observations are in our future!

Citation

“No Evolution in the Half-mass Radius of Milky Way–type Galaxies over the Last 10 Gyr,” Maryam Hasheminia et al 2022 ApJL 932 L23. doi:10.3847/2041-8213/ac76c8

1.25 mm continuum image of the protoplanetary disk AS 209

Astronomers may have found a gaseous disk around a planet orbiting the young star AS 209. This is one of just four circumplanetary disk candidates discovered so far, and these observations enable the first mass estimate of a circumplanetary disk’s gas.

Views of AS 209 in emission from (clockwise from top left) CO, HCN, C2H, HC3N, and H2CO. Click to enlarge. [Adapted from Öberg et al. 2021]

Disks within Disks

The disks of gas and dust that collect around young stars are the sites of planet formation. Thanks to high-resolution radio observations from instruments like the Atacama Large Millimeter/submillimeter Array (ALMA), we can study these disks in incredible detail and attempt to catch planet formation in the act.

Recently, researchers used ALMA to observe five nearby circumstellar disks at high resolution as part of the Molecules with ALMA at Planet-forming Scales (MAPS) observing program. Using MAPS data, a team led by Jaehan Bae (University of Florida) finds evidence that the 1.6-million-year-old star AS 209 hosts a disk within a disk — a circumplanetary disk around a hidden planet orbiting far from the central star.

A Planet, Perhaps?

Bae and collaborators analyzed new ALMA observations of radio emissions from three forms of carbon monoxide gas and archival observations of 1.25-millimeter dust emission. Several pieces of evidence suggest that the team has detected carbon monoxide gas belonging to a circumplanetary disk within the larger disk surrounding AS 209:

  1. The location of the circumplanetary disk (CPD) candidate in the 13CO observations. [Adapted from Bae et al. 2022]

    The carbon monoxide (12CO) observations reveal a 78-au-wide gap in the gas of AS 209’s disk at a distance of 200 au from the central star, in the same location as a gap seen in scattered-light images of the disk. This suggests that a young planet is carving a gap in the disk.
  2. The 12CO data show localized changes in the velocity of the gas near this gap, which can result from an embedded planet disturbing the gas of the disk.
  3. The 13CO observations show a point source 206 au from the central star — right in the middle of the 12CO gap and close to the velocity perturbations. Given ALMA’s resolving power, the point-like nature of the object indicates that it is no larger than 14 au in diameter.

Left: Comparison of the location of the gap (a) and the circumplanetary disk candidate (b). Middle: 12CO emission with the location of the gap marked with the gray dashed lines. Right: A zoomed-in version of the previous panel with the 13CO emission contours placed on top. Click to enlarge. [Bae et al. 2022]

Point-Source Possibilities

Based on these observations, Bae and collaborators estimated that the circumplanetary disk contains roughly 30 Earth masses of gas and just 2.2 Moon masses of dust, suggesting a low dust-to-gas ratio of 0.0009. At a temperature of 35K, the gas is 13K warmer than expected given the distance from the central star. This likely means that there is an additional heat source in the vicinity of the circumplanetary disk, such as accretion by the planet or turbulence within the disk.

AS 209 hosts just the fourth circumplanetary disk candidate ever found, and this work marks the first time that researchers have detected the gas within a circumplanetary disk and estimated its mass. Future observations with ALMA and JWST should help answer lingering questions about the structure of the circumplanetary material, the mass and age of the young planet, and how the planet formed at such a large distance from its parent star.

Citation

“Molecules with ALMA at Planet-forming Scales (MAPS): A Circumplanetary Disk Candidate in Molecular-line Emission in the AS 209 Disk,” Jaehan Bae et al 2022 ApJL 934 L20. doi:10.3847/2041-8213/ac7fa3

hubble space telescope image of the rosette nebula and the young stars at its center

Researchers have modeled the turbulent gas of the interstellar medium in a new way, with important implications for how we interpret observations of distant galaxies.

Probing the Early Universe

optical image of the Milky Way star-forming region S106

The nebulae surrounding young stars, like the S106 star-forming region pictured here, frequently show intricate and irregular structure. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)]

How can we tell what makes galaxies billions of light-years away shine? Astronomers use photoionization models to analyze the photons we collect from galaxies in the early universe and discern what sources of energy, like young stars, shocks, or active galactic nuclei, make them glow.

In a recent study, a team led by Yifei Jin (金刈非; Australian National University and ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions) used photoionization models — with a turbulent twist — to simulate the intricate emission nebulae that surround young, massive stars in galaxies near and far.

plots of the hydrogen beta emission from the modeled nebula as well as the density of the nebular gas

Top: The modeled H-beta emission from the nebula. Bottom: The density of the interstellar gas with the outline of the nebula traced on top. [Jin et al. 2022]

Forming Fractal Gas

Jin and collaborators used state-of-the-art models to simulate realistic emission nebulae from turbulent interstellar gas. The team placed a synthetic O star a million times more luminous than the Sun at the center of a cube 140 light-years on each side. They filled the cube with gas with an average density of 100 particles per cubic centimeter — a typical value for nebulae in the Milky Way — and the same chemical composition as the Sun.

The main advance in this work is the use of a fractal density pattern for the simulated interstellar gas. Unlike previous models, which assumed that the gas had the same density throughout, the team’s model incorporates density variations on large and small scales, resulting in a clumpy interstellar medium similar to what is seen in observations. As high-energy photons from the synthetic O star ionize the gas, they create an emission nebula with a complex and irregular shape.

Volume versus Boundary

To determine the properties of their modeled nebula, Jin and collaborators tracked the strength of the emission lines and categorized each emission line as either a volume species or a boundary species, depending on where in the nebula the line was produced. The volume species — H-alpha, H-beta, and [O III] — are produced mainly in the body of the nebula, while the boundary species — [O I], [S II], and [N II] — are produced along the outer edge.

Comparison of the fluxes of several emission lines and emission-line ratios between the realistic nebula case and the spherical nebula case. A value of zero indicates no change in flux or the flux ratio. Click to enlarge. [Jin et al. 2022]

By comparing against a modeled spherical nebula formed in a region of uniform gas, the team found that the more complex the structure of the nebula, the more prominent the emission from the boundary species. This effect becomes more important the more concentrated an emission line is toward the edge of the nebula; for instance, H-alpha emission is scarcely different between the two models, but [O I] emission — 99.5% of which is produced along the boundary of the nebula — soars by 253% when a realistic nebular shape is adopted.

This study by Jin and collaborators demonstrates that modeling turbulent interstellar gas in a realistic way can have a huge impact on the resultant emission lines — which in turn has implications for how we interpret emission lines from distant galaxies. The team predicts that using fractal geometry for models of interstellar gas will be key to interpreting observations of galaxies early in the universe, when interstellar gas was likely highly turbulent.

Citation

“Theoretically Modeling Photoionized Regions with Fractal Geometry in Three Dimensions,” Yifei Jin et al 2022 ApJL 934 L8. doi:10.3847/2041-8213/ac80f3

image of the Sun releasing two coronal mass ejections

A chance alignment between Earth and a Mars-bound spacecraft has given us a rare glimpse into the movement of high-energy particles from the Sun. The data from this event can help researchers understand the radiation environment near Mars — a key factor in planning crewed missions to our neighboring planet and beyond.

Energetic Particle Parade

illustration of energetic particles being ejected by the sun

Illustration of energetic particles being ejected by the Sun. [NASA’s Goddard Space Flight Center Conceptual Image Lab]

The space between the planets in our solar system is filled with a wispy sea of charged particles that flow out from the Sun’s atmosphere. This particle population is augmented by cosmic rays — speedy protons and atomic nuclei accelerated in extreme environments across the universe — which ebb and flow against the 11-year solar activity cycle. This undulating particle background is punctuated by bursts of high-energy particles from the Sun, which can be unleashed suddenly in violent solar storms.

Spacecraft that venture out from the protection of Earth’s magnetic field must navigate this ocean of particles and weather solar storms. And if we someday wish to send astronauts to other planets, we’ll need to know how high-energy solar particles, which pose a risk to the health of astronauts and electronic systems alike, travel through the solar system.

location of Tianwen-1 relative to Earth, Mars, and other spacecraft

Location of Tianwen-1 (TW-1) relative to Solar Orbiter (SolO), Parker Solar Probe (PSP), and STEREO-A (STA), Earth, and Mars. The black arrow marks the location of the active region that launched the solar storm. [Adapted from Fu et al. 2022]

When Spacecraft Align

In a new publication, a team led by Shuai Fu (Macau University of Science and Technology), Zheyi Ding (China University of Geosciences), and Yongjie Zhang (Chinese Academy of Sciences) studied the high-energy solar particles produced in an event in November 2020, when the Sun emitted a solar flare and a massive explosion of solar plasma called a coronal mass ejection.

This event coincided with a chance alignment of multiple spacecraft along the same solar magnetic field line. This alignment meant that several spacecraft near Earth and the Tianwen-1 spacecraft en route to Mars measured the same burst of energetic particles millions of miles apart, providing a rare opportunity to study how energetic particles from the Sun travel through space along magnetic field lines.

Diffusion and Evolution

By comparing the timing of measurements from Tianwen-1 to those from three spacecraft near Earth, the team discerned that the magnetic field line that connected the spacecraft did not connect back to the origin of the particles. This means that the particles must have traveled, or diffused, across magnetic field lines to reach the spacecraft.

plots of particle flux as a function of energy at eight time steps

Comparison of proton fluence (number of particles collected per unit area) measured by spacecraft at Earth (blue) and by Tianwen-1 at 1.39 au (red). The time increases from (a) to (h). The spectra at Earth and at Tianwen-1 “break” or bend at roughly the same energy, suggesting that there is little evolution as the particles travel outward. Click to enlarge. [Fu et al. 2022]

In addition, the team found that the shape of the particle energy distribution remained the same at moderate and high energies as the particles traveled between Earth and Tianwen-1’s location at 1.39 au. This suggests that the shape of the energy distribution is determined earlier, at the time the particles are accelerated to high energies, rather than as the particles travel through space.

The November 2020 event marked the first solar energetic particle event observed by Tianwen-1, but surely not the last. The spacecraft will continue to monitor high-energy particles from its station in Mars orbit as the solar cycle revs up, collecting valuable data for understanding the radiation environment around Mars and planning future missions.

Citation

“First Report of a Solar Energetic Particle Event Observed by China’s Tianwen-1 Mission in Transit to Mars,” Shuai Fu et al 2022 ApJL 934 L15. doi:10.3847/2041-8213/ac80f5

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