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Collage of binary star systems

Many stars travel through space with a binary companion, and large-scale surveys allow us to study enormous numbers of these stellar pairs. What do these surveys tell us about the characteristics of binary stars in the Milky Way?

hubble image of interacting galaxies with tidal tails

Tidal forces are perhaps best known for generating tidal tails and streams in interacting galaxies, but a galaxy’s tidal pull can have subtle effects on binary star systems as well. [NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team and ESA]

Stars Awash in the Galactic Tide

The orbital parameters of a binary star system — namely, the distance between the stars and how eccentric (non-circular) their orbits are — can encode information about the formation and evolution of the binary system as well as the evolution of the stars themselves. The orbits of binary stars are susceptible to outside influence, too; gravitational nudges from passing stars, nearby gas clouds, and the overall tidal pull of the galaxy can change a binary system’s orbits over time.

When we examine the orbits of binary systems in the Milky Way with observations from the sky-mapping Gaia spacecraft, we find unexpected trends in the orbital parameters of binary systems near the Sun. Namely, among binary systems separated by large distances (>1,000 au), there are more systems with highly eccentric orbits than expected. What’s the origin of this trend?

Nature vs. Nurture

plot of initial and final eccentricity distribution functions from the model

The final distribution of eccentricities (black lines) and best-fitting power laws (green lines) acquired from various initial distributions (red lines). These results show that a superthermal eccentricity distribution, as is seen in binary systems near the Sun, can only arise from a distribution that is initially superthermal. [Adapted from Hamilton 2022]

As Chris Hamilton (Institute for Advanced Study) explains in a recent research article, understanding the current orbits of binary stars in the Milky Way requires separating the effects of nature (the eccentricities that the binary systems are born with) and nurture (the outside gravitational effects of passing stars and the background galactic pull).

Hamilton approached this problem by modeling the effects of the Milky Way’s overall gravitational pull on populations of synthetic binary stars in the outer regions of the galaxy. In order to test the effects of nature as well as nurture, Hamilton modeled populations with different initial eccentricity distributions: uniform (all eccentricities are equally common), thermal (the binaries have reached statistical equilibrium through gravitational interactions; represented by the gray lines in the figure to the right), subthermal (fewer eccentric binaries than the thermal case), and superthermal (more eccentric binaries than the thermal case, as we see near the Sun).

Maybe They’re Born with It, Maybe It’s the Tidal Influence of the Milky Way

The model results show that the tidal pull of the Milky Way tends not to change the eccentricity distribution of a population of binary stars. Put another way, this means that the high number of wide, eccentric binary systems in the solar neighborhood can’t have been caused by the Milky Way’s gravitational influence — another factor, such as the combined effects of individual gravitational nudges from passing stars and gas clouds, must have caused this trend, or binary systems in the solar neighborhood must be born with a similar distribution of eccentricities.

As is so often the case, there’s plenty more work to be done to understand this issue fully. In particular, modeling the effects of gravitational tugs from passing stars and applying new techniques to study the time evolution of binary systems will be critical to reaching a conclusion.

Citation

“On the Phase-mixed Eccentricity and Inclination Distributions of Wide Binaries in the Galaxy,” Chris Hamilton 2022 ApJL 929 L29. doi:10.3847/2041-8213/ac6600

photograph of earth's aurora taken from the international space station

Researchers have applied a powerful simulation tool to a fundamental question about the solar wind: how do sparse plasmas exchange energy?

A Tenuous Topic

Earth and the solar wind

This artist’s impression of the solar wind shows a constant torrent of particles filling the heliosphere and streaming past Earth. [NASA Goddard’s Conceptual Image Lab/Greg Shirah]

In a dense plasma, particles exchange energy through collisions and eventually reach thermal equilibrium. Space plasmas, however, aren’t dense enough to trade much energy in this way — at Earth’s location, the solar wind is less dense than the best vacuum we can create on Earth.

Particles bounce around equally in all directions in a substance in thermal equilibrium, but in a tenuous, magnetized plasma like the solar wind, the particles can move at very different speeds parallel and perpendicular to the magnetic field. Since temperature is a measure of the average kinetic energy of a collection of particles, this means the temperature of a plasma can be different in the direction parallel to the magnetic field lines than it is perpendicular to them!

Early plasma theory predicted that the solar wind plasma would be tens of times hotter parallel to the magnetic field than perpendicular to it. In reality, at Earth’s location, the temperature parallel to the magnetic field is, on average, just 20% hotter than the temperature perpendicular to the field lines. With few to no collisions to help the plasma come to thermal equilibrium, how is this possible?

An Unstable Solution

photograph of clouds exhibiting the kelvin-helmholtz instability

The Kelvin–Helmholtz instability, which plays an important role in the atmospheres of stars and planets alike, is revealed by rare, wave-like patterns in clouds. [UCAR]

In a new publication, a team led by Rodrigo López (University of Santiago, Chile) used simulations to explore the impact of a plasma instability on the temperature of solar wind plasma. Plasma instabilities kick in when certain physical conditions are present, and they can have a big impact on a plasma’s large-scale characteristics, like temperature and density. While instabilities might seem abstract, they actually play a role in many common situations; weather systems, volcanic clouds, and lava lamps are all examples of instabilities at work. In each of these cases, what we observe is the result of the system spiraling away from equilibrium when it is disturbed.

Similar to these examples, instabilities can arise in a plasma in which the temperature parallel to the magnetic field is much higher than the temperature perpendicular to the magnetic field. In today’s article, López and collaborators investigated what happens to this setup when the fire hose instability comes into play. Previous work has explored the impact of the fire hose instability in plasmas where the electrons or the protons have a much higher temperature parallel than perpendicular, but relatively little work has explored what happens when both types of particles have this feature.

Approaching Equilibrium

plot of simulation results

The change in the perpendicular to parallel temperature ratio over time for the simulated electrons (top) and protons (bottom). In Case 1, the electrons start out with a temperature ratio of 1. In Cases 2 and 3, the electron temperature ratios are 0.4 and 0.3, respectively. [Adapted from López et al. 2022]

Using plasma physics equations and two-dimensional particle-in-cell simulations, López and coauthors found that when solar wind protons and electrons have much higher temperatures in the parallel direction, the fire hose instability takes hold of the protons much faster than it would if only the protons had this feature, while the electrons behave similarly regardless of what the protons are up to. In addition, the protons’ parallel and perpendicular temperatures draw closer when the electrons undergo the fire hose instability as well, suggesting that the electrons’ behavior is an important factor in explaining the observed temperatures in the solar wind.

Overall, the authors’ findings confirm that the fire hose instability plays an important role in moderating the temperature of the solar wind plasma, and future work should consider the influence that electrons have on the behavior of protons in the solar wind and other sparse plasmas.

Citation

“Mixing the Solar Wind Proton and Electron Scales. Theory and 2D-PIC Simulations of Firehose Instability,” R. A. López et al 2022 ApJ 930 158. doi:10.3847/1538-4357/ac66e4

Globular star cluster featuring multiple bright stars

Crab Nebula; an extended structure containing filaments of different colors (representing different elements)

Hubble Space Telescope image of the Crab Nebula, the remnant of a supernova that took place in the year 1054 AD. [NASA, ESA, J. Hester and A. Loll]

Just by knowing the mass of a star, can we predict if it will end its life in fire (a supernova) or ice (a white dwarf that eventually fades into a cool black dwarf)? A team led by astronomers at the University of British Columbia tries to answer that question by observing white dwarfs in order to find exactly where that dividing line is between a death of fire and ice. 

…But Which Road Leads to a White Dwarf?

When a star runs out of fuel, it can either eject its outer layers in an explosion so violent that it outputs more energy than the Sun will in its 10 billion years of life, or the star may simply expand and settle down into a stable star called a white dwarf about the size of our moon. What determines which route the star takes is its mass: lower masses die a death of ice, higher masses of fire. Though we believe the dividing line is somewhere around 8 solar masses, this number doesn’t always agree with what we observe.  

Color-magnitude diagram with F225W - F336W plotted on the x-axis with values ranging from -2 to 5, and F225W plotted on the y-axis with values ranging from ~26 to ~15. A roughly vertical line is shown toward the middle left and a cluster of points forming a trend that points up toward the upper left is also shown.

The color–magnitude diagram of the Milky Way globular cluster 47 Tucanae. The x-axis shows the color, the left y-axis shows the apparent magnitude at 47 Tucanae’s distance, and the right y-axis shifts the cluster to the distance of the Large Magellanic Cloud. This diagram shows that even at the distance of the Large Magellanic Cloud, these massive WDs are detectable. [Richer et al. 2022]

In Two Words I Can Sum Up Everything I’ve Learned About Stars: They Evolve 

If all stars greater than 8 solar masses end their lives in the fire of a supernova, we would see a lot more supernova explosions (specifically, Type II supernovae) than we actually do. This dearth of Type II supernovae could indicate that the maximum mass of a star that can end its life as a white dwarf is actually closer to 12 solar masses rather than 8. Constraining this mass limit of stars that can become white dwarfs could inform the formation rate of compact objects as well as the metal content of galaxies. The more massive a star is, the more massive its white dwarf remnant is. Therefore, by hunting for massive white dwarfs, we can effectively hunt for massive progenitor stars that weren’t heavy enough to end in a supernova. A team led by Harvey Richer at the University of British Columbia has looked deep into young open star clusters outside our own galaxy to try to identify massive white dwarfs. 

Previous searches for massive white dwarfs in young Milky Way open clusters only found white dwarfs up to 1.1 solar masses, which come from stars no larger than 6.2 solar masses. To probe whether even more massive stars can become white dwarfs, Richer and coauthors searched young clusters in the Large Magellanic Clouds. The team looked at four Magellanic Cloud clusters in which stars of 5.7 to 10.2 solar masses were just about to enter the asymptotic giant phase (a late evolutionary stage in an intermediate–mass star’s life at which point the star has exhausted its main fuel source), which would mean the white dwarfs in these clusters must have come from stars more massive than that. They also chose these specific clusters because of their distance; the Magellanic Clouds are far enough away that there would be new clusters to search, but not so distant that Gaia parallaxes are unreliable and there is confusion with field white dwarfs.  

The Universe Is Lovely, Dark, and Deep, But We Need More Data To Put This Mystery To Sleep

Two plots both showing distance from the center cluster in pixels vs. cumulative fraction. The left graph shows background objects, all stars, white dwarfs, and bright stars in NGC 2164, and the right image shows them for NGC 330. The right image shows the majority increasingly diagonally up to the right, whereas the left plot shows more spread out lines (all pointing up toward the right but getting there more slowly/gradually)

Distributions of the various populations of stars in two of the clusters. The white dwarfs in the leftmost panel are the five potential white dwarf candidates. [Richer et al. 2022]

The team found five potential candidates in the oldest of the four clusters they studied by looking at the ages and populations of the clusters. These stars represent the first extragalactic single white dwarfs ever discovered. This study demonstrated that it is possible to detect white dwarfs in nearby galaxies with only moderate exposure times with Hubble. However, to study them spectroscopically and determine their masses and ages, the team needs more resolution, which will come with future 30+ meter telescopes. Confirmation of these heavy white dwarfs may finally lead us to the point where the roads of stellar evolution diverged.

Citation 

“When Do Stars Go Boom?” Harvey B. Richer et al 2022 ApJL 931 L20. doi:10.3847/2041-8213/ac6585 

image of coronal loops on the Sun

Though solar flares are common — the Sun releases as many as 20 per day — these explosive events remain mysterious. What can models tell us about the uncertain origins of the X-rays we see at the onset of a solar flare?

time evolution of high-energy flux and observations of coronal loops

Time evolution of flux in several energy bands (left) for the coronal loops shown on the right. Contour lines indicate the 25–35 keV flux. Click to enlarge. [Adapted from Shabalin et al. 2022]

A High-Energy Inquiry

In the early stages of a flare, solar plasma begins to glow in high-energy X-rays. These X-rays are associated with coronal loops — plasma suspended on arching magnetic field lines above an active region, where electrons in the hot, dense plasma travel at breakneck speed. Most of the X-rays arise from the base of these loops, where the density of the plasma is highest. However, observations show that X-rays can also arise from the top of the loop, high in the Sun’s tenuous upper atmosphere, or corona, where the density is far lower. What causes this emission?

diagram of the collapsing trap model and plot of the time evolution of the magnetic field

Diagram of how magnetic field lines move in the collapsing trap model (left) and an example of how the magnetic field strength at the top of the loop varies with time as the field lines collapse (right). Click to enlarge. [Adapted from Shabalin et al. 2022]

It’s a (Collapsing) Trap!

Alexander Shabalin (Ioffe Institute) and collaborators approached this question by testing ways to generate X-ray emission in the rarefied environment of the solar corona, focusing on the collapsing trap model.

The “trap” in this model is a magnetic one: when magnetic field lines near the top of a coronal loop rearrange into a new configuration, the field lines lower down retract, snapping the “trap” shut and ensnaring any electrons that are present. The electrons bounce back and forth between the boundaries of this trap, and as the boundaries draw closer, the electrons’ velocities skyrocket. In theory, this process could concentrate and accelerate electrons enough for them to generate the X-ray emission we observe.

A Numerical Solution

Shabalin and coauthors numerically solved a system of equations that describe how the trapped electrons behave as the magnetic field in the coronal loop evolves. Their investigations focused on the effect of electrons traveling with different orientations relative to the field lines, and they explored how much these electrons were accelerated as individual strands of the coronal loop collapsed in sequence.

plot of flux percentage as a function of time

Percentage of the total 29–58 keV flux of the coronal loop that arises from the top of the loop as a function of time. The red dotted line shows the collapsing trap model results and the black solid lines shows a model in which the plasma density and magnetic field along the loop are constant with time. [Adapted from Shabalin et al. 2022]

For a magnetic trap that takes 8 seconds to collapse, the team found that electron energies could increase by 20–200%, depending on model inputs. These higher-energy electrons increased X-ray emission in the 29–58 keV range, boosting the loop top’s contribution to the overall X-ray flux by 20–70%. The authors also found that their results depended greatly on the energies and orientations of the electrons before they became ensnared in the trap; when the captured electrons had lower energies, the collapsing trap increased their energies by a larger factor, and boosting the energies of electrons traveling perpendicular to the field lines was especially important.

The results show that the collapsing trap model is a viable explanation for the bright X-ray sources seen in the solar corona at the onset of a solar flare — and as the twenty-fifth solar cycle ramps up toward solar maximum in 2025, there should be plenty of solar flares to test these predictions!

Citation

“Early-stage Coronal Hard X-Ray Source in Solar Flares in the Collapsing Trap Model,” Alexander N. Shabalin et al 2022 ApJ 931 27. doi:10.3847/1538-4357/ac65fe

composite image of the active galaxy Centaurus A

Stars have been singing the same song since the beginning of the universe: you’re born, you fuse hydrogen into helium, you drift off the main sequence, and finally, you’re recycled into the cosmos. Under the right conditions, though, stars could become immortal. How is this possible, and what does it mean for these stars’ surroundings?

Live Fast, Die Never

illustration of an active galactic nucleus with dusty disk and polar winds.

Artist’s illustration of the surroundings of a supermassive black hole at the heart of an active galaxy. [ESO/M. Kornmesser; CC BY 4.0]

Many galaxies host an active galactic nucleus — a luminous disk of gas and dust circling a central supermassive black hole. Extreme though this environment may be, stars can live within these disks, and astronomers suggest that some of these stars might be immortal.

As these stars churn hydrogen into helium in their cores, they constantly replenish their hydrogen stores from the surrounding disk. As a result, they never run out of fuel, never leave the main sequence, and never die. Now, a team led by Adam Jermyn (Flatiron Institute) has explored how these extreme stars might affect the evolution of the disk that surrounds them.

Schematic of stars being captured in an active galactic nucleus disk

The disk around an active galactic nucleus (AGN) captures most stars within rcap. The stars within rmax grow and become immortal, while those outside the disk or beyond rmax accrete little gas. Click to enlarge. [Jermyn et al. 2022]

Drinking from the Stellar Fountain of Youth

Jermyn and collaborators considered disks with short (0.1 million years) and long (10 million year) lifetimes, estimating that these disks would capture 1,000 and 20,000 stars, respectively, from the inner regions of their galaxies. The short-lived disk only contains enough mass to raise 300 of these stars to immortality, while the long-lived disk can support all 20,000 stars.

In both disks, the immortal stars grow to 300 solar masses and have massive convective cores. The constant churning brings fresh hydrogen to their cores and transports helium outward to their surfaces. From there, fierce stellar winds carry the helium-rich gas out into the disk, boosting the abundance of helium near the black hole. The consequences of this chemical enhancement aren’t yet clear — it might rob immortal stars of their superpowers, since sucking up helium-rich material will make them burn through their hydrogen faster than it can be replenished — but measuring helium abundances in active galactic nuclei may provide a way to test the degree of chemical enrichment.

Immortal No More

diagram of the wind-fed disk model

This illustration shows that the gas in stellar winds feeds the disk in the innermost regions where the escape velocity is high and escapes from the disk in the outer regions, where the escape velocity is low. Click to enlarge. [Jermyn et al. 2022]

Do immortal stars help or hinder the disk’s survival? Both outcomes are possible. These stars’ winds likely replenish the inner regions of the disk, but they may also drive material to escape the outer regions of the disk. In addition, active galactic nuclei don’t remain active forever — as the disk begins to dissipate, the stars shed much of their mass, giving the disk a last boost before the stars cross back to the mortal realm and evolve into black holes.

The authors note that their estimates are still uncertain, but it’s clear that immortal stars can play an important role in the evolution of the innermost regions of a galaxy. Future work might explore the consequences of helium-enriched gas spiraling around a supermassive black hole or assess the impacts of stars that form within the disk itself. Clearly, immortal stars provide plenty of work for modelers and observers alike — immortality might be beyond our reach, but at least we can live vicariously through these stars!

Citation

“Effects of an Immortal Stellar Population in AGN Disks,” Adam S. Jermyn et al 2022 ApJ 929 133. doi:10.3847/1538-4357/ac5d40

astrophotograph of the pelican nebula

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

Stellar Surveys

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

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

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

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

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

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

Digging In to the Data

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

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

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

Broad Applicability

plot of flyx asymmetry versus quasiperiodicity

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

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

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

Citation

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

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

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

Making Sense of Microlensing

illustration of a gravitational microlensing event

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

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

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

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

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

New and Improved

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

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

A Curated Catalog

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

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

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

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

Citation

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

Galaxy surrounded by a large dark matter halo

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

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

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

Peering Back into the Universe’s History 

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

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

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

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

Probing Dark Matter on Small Scales 

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

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

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

Using the Power of a Wide Range of Measurements 

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

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

Citation 

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

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

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

A Black Hole Bonanza

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

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

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

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

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

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

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

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

Inspecting Images

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

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

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

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

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

Modeling a Massive Black Hole

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

modeling results and simulated image

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

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

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

Looking Ahead

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

Side-by-side comparison of the angular sizes (how large objects appear on the sky) of M87* (left) and Sgr A* (right). [EHT Collaboration; CC BY 4.0]

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

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

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

Locations of the telescopes that make up the EHT array

Locations of the telescopes used to create the Sgr A* images (yellow symbols) as well as the facilities added to the EHT array after 2018 (blue symbols). Click to enlarge. [ESO/M. Kornmesser; CC BY 4.0]

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

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

Citation

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

collage of 18 fields containing submillimeter galaxies

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

photograph of alma telescopes

Arrays of radio telescopes like the Atacama Large Millimeter/submillimeter Array are a powerful tool for studying galaxies in the early universe. [Pablo Carrillo – ALMA (ESO/NAOJ/NRAO); CC BY 4.0]

Early Galactic Goings-On

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

Submillimeter Survey

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

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

  • plot of redshift versus 870 micron flux density

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

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

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

Interaction Implications

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

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

Citation

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

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