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black holes in a globular cluster

Since the first merger of two black holes detected in 2015, the LIGO/Virgo gravitational-wave detectors have observed a total of 47 confident collisions of black holes and neutron stars through the end of September 2019. What’s the big picture behind these events? The second gravitational-wave catalog is officially out — and the population statistics are in!

A New Catalog

In recent years, the Advanced LIGO detectors in Hanford, WA and Livingston, LA and the Advanced Virgo detector in Europe have kept a watchful vigil for ripples in spacetime that let us know that a pair of compact objects — black holes or neutron stars — has spiraled in and merged.

Plot showing masses of observed black hole and neutron star binary mergers. Plot includes black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange).

The rapidly expanding “stellar graveyard”, a plot that shows the masses of the different components of observed compact binary mergers included in the second gravitational-wave transient catalog (GWTC–2). [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

During LIGO’s first two observational runs (O1 in 2015–16 and O2 in 2016–17) the two LIGO detectors discovered 11 merger events. After a series of upgrades to the detectors, the system came back online in April 2019 for its third run (O3). In just the first 26 weeks of the run (O3a), LIGO/Virgo jointly found another 36 mergers!

In a new publication recently accepted to Physical Review X, the collaboration has released its second catalog of gravitational-wave events (GWTC–2), which includes data from O1, O2, and O3a. And in a companion publication in the Astrophysical Journal Letters, the team has now analyzed the broader set of all 47 mergers in the catalog, using population models to gain deeper insight into the binary properties and how these systems form and evolve.

Learning About Collisions

So what have we learned from the GWTC–2 population?

  1. Black-hole mass is more complicated than we originally thought.
    The merging black holes in O1 and O2 all had primary masses below 45 solar masses, consistent with the theory that black holes of ~50–120 solar masses shouldn’t be able to form. But O3a included several primaries above 45 solar masses, so we can’t model the primary mass distribution as a single power law with a sharp cutoff at 45 solar masses anymore. This may suggest that we’re looking at different populations of black holes that formed in different ways.
  2. Some black holes have spins that are misaligned with the angular momentum of the binary.
    Nine of the recent detections exhibit misaligned spins, which is another clue about their formation. Black hole binaries that form and evolve in isolated pairs are expected to have aligned spins, whereas black hole binaries that form dynamically — due, for instance, to interactions in clusters of stars or in the disk of an active galactic nucleus — should have isotropically distributed spins. The authors show that the spinning GWTC–2 population is consistent with 25–93% of black holes forming dynamically. That’s a large range, but what’s important is that this also indicates there’s more than one formation channel at work!
  3. The black hole merger rate probably increases with redshift.
    Plot showing how merger rate increases with redshift

    The modeled median merger rate density (solid curve) as a function of redshift suggests that the merger rate increases with redshift. Still, the increase is not as steep as the increase in the star formation rate (dashed line). [Abbott et al. 2021]

    Updated estimates suggest that binary black holes merge at a rate of 15–38 Gpc3 yr-1 and binary neutron stars at a rate of 80–810 Gpc-3 yr-1. The merger rate appears to be higher at higher redshift, but this increase doesn’t quite parallel the known increase in star formation rate with redshift. An intriguing mystery!

A Smashing Good Time Ahead

These takeaways clearly represent a dramatic increase in our understanding of how and where black hole binaries form and evolve — but we still have so much left to learn! Luckily, there’s plenty more data ahead: the collaboration is now analyzing the remaining 5 months of data from O3, and the detectors are currently undergoing upgrades in preparation for O4, which is slated to begin in mid-2022.

Citation

“Population Properties of Compact Objects from the Second LIGO–Virgo Gravitational-Wave Transient Catalog,” R. Abbott et al 2021 ApJL 913 L7. doi:10.3847/2041-8213/abe949

A spiral galaxy seen nearly edge-on. The galaxy has a bright center that is surrounded by a disk of dark black and brown clouds. Dim blue light can be seen through the clouds. To the lower left of the galaxy, just below the disk, is a bright white object with diffraction spikes. The background is mostly dark, scattered with a few small dim white objects.

Type Ia supernovae are a cornerstone of extragalactic distance measurements, so it’s important that we understand them very well. Right now, we’re fairly certain that Type Ia supernovae are the result of white dwarfs exploding. But how they explode is still an open question.

To Explode a White Dwarf

A creation mechanism for Type Ia supernovae where a white dwarf accretes mass from a companion (upper panel) till it explodes as a supernova (lower panel). [NASA/CXC/M. Weiss]

White dwarfs are remnants of relatively low-mass stars like our Sun. They are essentially exposed stellar cores, typically dominated by carbon and oxygen with an outer layer of helium. White dwarfs don’t produce energy of their own. Instead, they just cool off, slowly radiating away residual energy from when they were part of a star.

So how do you make something like a white dwarf explode? Just add mass! If a white dwarf accretes enough matter from a nearby companion, it can approach the Chandrasekhar limit of 1.4 solar masses and explode. This process seems fairly straightforward, but it turns out there are several potential ways to explode a white dwarf.

One scenario involves “double detonation”, where the helium shell of a white dwarf detonates and causes the carbon core to detonate in turn. Another scenario considers white dwarfs in a binary, with one white dwarf accreting material and exploding to knock the other one away.

Interestingly, observations suggest that a combination of these two scenarios — double detonation in white dwarf binaries — may be the likely progenitor of many Type Ia supernovae. One important constraint in this model is that the exploding white dwarf’s mass remains just below the Chandrasekhar limit.

With this in mind, a group of researchers led by Ken Shen (University of California, Berkeley) considered sub-Chandrasekhar-mass explosion scenarios with a tricky but realistic assumption: that local thermodynamic equilibrium (LTE) does not hold. 

Model and observed supernova light curves in different bands. The solid lines and the colored circles represent the model, while the unfilled shapes are observed supernovae. The model colors correspond to different white dwarf masses. The white dwarf’s carbon/oxygen ratio was assumed to be 50:50. [Shen et al. 2021]

Explosions Not in Equilibrium

When a system is in LTE, the energies and ionization levels of particles in the system are in some fixed relation with each other while temperature remains consistent across the system. There are astrophysical scenarios where LTE is a safe assumption, like in stars, but LTE certainly doesn’t hold in an event like a supernova.

To model explosions with non-LTE assumptions, Shen and collabors used two different modeling codes. A major difference between the two codes was computation time, and running the same explosion scenarios through both codes allowed Shen and collaborators to determine if the more time-efficient code could stand up to the other. The model outputs included spectra of the ensuing supernovae as well as their light curves across different filters.

Model Matches

A diagram showing the Phillips relation, with peak B-band brightness plotted against the decrease in B-band magnitude 15 days after the peak. Crosses correspond to observations of Type Ia supernovae. The colored shapes correspond to the models, with color representing white dwarf mass, shape representing the carbon/oxygen ratio, and shape outline representing the modeling code used. [Shen et al. 2021]

Shen and collaborators found that the model light curves were excellent matches to observed supernovae out to 15 days after the brightest point in the B-band light curve (“B-band maximum”). This means that the codes also successfully model an observed relation called the Phillips relation — the brighter a supernova’s peak B-band magnitude, the more slowly it will evolve past that peak. This can be seen by plotting peak B-band brightness against B-band brightness 15 days after the peak.

The model spectra are also good matches to observations, sometimes even out to 30 days after the peak. They are especially accurate near the peak, save for spectral features from “intermediate mass elements”, which generally include elements heavier than carbon up to calcium.

All in all, these initial non-LTE models of sub-Chandrasekhar detonations are an excellent match to a broad range of observed Type Ia supernovae near peak brightness! Future models will have to account for more conditions, but non-LTE seems to be the way to go.

Citation

“Non-local Thermodynamic Equilibrium Radiative Transfer Simulations of Sub-Chandrasekhar-mass White Dwarf Detonations,” Ken J. Shen et al 2021 ApJL 909 L18. doi:10.3847/2041-8213/abe69b

Illustration showing a distant galaxy emitting a pulse of light that passes through the halo of an intervening galaxy and arrives at the Milky Way.

In recent years, we’ve recorded hundreds of brief, powerful flashes of radio light originating from outside of our galaxy. In new work, scientists are now leveraging these enigmatic fast radio bursts to learn about the hot gas around galaxies.

An Epic Voyage

FRB

Artist’s conception of a fast radio burst originating in a distant galaxy. [Danielle Futselaar]

Fast radio bursts (FRBs) are intense bursts of radio emission that last only milliseconds. At their source, the light of these powerful eruptions contain as much energy in a single millisecond as the Sun emits across 3 days. But FRBs primarily arise from distant sources that can lie billions of light-years away — so that light has a long journey ahead of it.

To reach us, this emission first passes through the source’s local environment, then through the interstellar medium (ISM) of its host galaxy, and then through that galaxy’s halo. Once free of the galaxy, the light must traverse the intergalactic medium (IGM) — potentially passing through intervening galactic halos — before it eventually enters the circumgalactic environment around the Milky Way. There, it travels through the halo and ISM of the Milky Way and finally arrives at our detectors here on Earth.

This epic voyage is fraught with obstructions: the burst emission encounters clumps of hot, ionized, and turbulent gas that slows its passage and leaves distinct imprints on the signal we eventually see. In a new study led by Stella Ocker (Cornell University), scientists have used these signatures to probe the ionized gas that lies between us and distant FRBs.

FRB dispersion

Schematic illustrating how transient radio signals travel to us. Pulsars (marked by sun symbols) lie in the galaxy, interior to the halo; their signals are affected only by the Milky Way’s interstellar matter. Fast radio bursts (marked by lightning symbol) lie in other galaxies; their signals travel through multiple different regions. [Platts et al. 2020]

Constraints from Bursts and Pulses

Ocker and collaborators combine multiple different diagnostics: 

  • dispersion of bursts, which occurs when different frequencies of light travel at different speeds through intervening gas
  • pulse and angular broadening, or smearing in time and space due to scatter as light travels along multiple different paths through the gas
  • scintillation, or twinkling of a compact source caused by turbulence in the intervening medium

To disentangle the relative contributions of the ionized gas in the different regions of the FRB’s journey, the authors took advantage of data from multiple FRBs along various lines of sight passing through different sections of our galaxy. They combined this information with further constraints from pulsars — pulsing, magnetized neutron stars — that lie within our galaxy, to better understand the density fluctuations along these varying lines of sight.

Downplaying Halo Contributions

Plot showing the relative scattering contributions of different regions

A representation of the scattering contributions estimated by the authors for different regions of ionized gas. The scattering that occurs due to intervening galactic halos (M33 and FG–181112) is consistent with the upper limits found for the Milky Way’s halo (MW Halo), and substantially smaller than the scattering caused within the inner disk of our galaxy (MW Inner Galaxy). [Ocker et al. 2021]

From their work, Ocker and collaborators were able to place an upper limit on the amount of scattering contributed by the Milky Way’s halo to the FRBs that they explored. The authors then compared these results to data from FRB signals that passed through additional, intervening galaxy halos on their way to us. They found that the scattering contributions from other halos are consistent with the upper limits set on the Milky Way’s halo.

Ocker and collaborators’ study suggests that galaxy halos have only a very small impact on the scattering of light in FRBs. While additional FRB and pulsar data will be helpful in constraining more lines of sight, this work provides a valuable step in disentangling different reservoirs of ionized gas to ultimately probe the density fluctuations across our universe.

Citation

“Constraining Galaxy Halos from the Dispersion and Scattering of Fast Radio Bursts and Pulsars,” Stella Koch Ocker et al 2021 ApJ 911 102. doi:10.3847/1538-4357/abeb6e

composite photo of a dwarf galaxy.

Using a new technique, scientists have identified a supermassive black hole lurking in a low-mass, low-metallicity galaxy. Could this discovery be just the tip of the iceberg? 

Hunting for Seeds

How did the first supermassive black holes — black holes of millions or billions of solar masses — form?

early quasar

Artist’s illustration of a primordial galaxy dominated by the supermassive black hole in its center. [NASA/ESA/ESO/Wolfram Freudling et al. (STECF)]

Today, we know that giant black holes lie at the heart of most galaxies. Many of them have grown substantially since they first formed, via galaxy mergers and accretion of mass around them. But did they start out as large stars? Or collapse directly from molecular clouds? Or build up rapidly from the merger of smaller black holes?

To identify the seeds of supermassive black holes and address these questions, we need to explore the least-disturbed supermassive black holes that we can find today. Small, low-metallicity galaxies — those that have had a peaceful cosmic history, devoid of the mergers that drive significant black-hole growth — are thus the perfect targets to search for the relics of supermassive black hole seeds.

The catch? These are precisely the environments in which it’s difficult to spot black holes! 

AGN

Artist’s depiction of the active nucleus of a galaxy, including an accretion disk spiraling around the supermassive black hole and jets of material flung out from both poles. [NASA/Dana Berry/SkyWorks Digital]

A New Approach

The easiest black holes to detect are those that are actively feeding, known as active galactic nuclei (AGNs). But the typical method for identifying an AGN — which relies on specific signatures in the source’s optical spectrum — is biased against low-metallicity and relatively merger-free galaxies, missing the precise population we want to find! Only a handful of AGNs have been identified in dwarf galaxies, and most of these lie in high-metallicity environments. So how do we find our seed relics?

According to a team of scientists led by Jenna Cann (George Mason University), it’s time for a different approach. Instead of relying on optical signatures, Cann and collaborators focus on finding coronal lines — near-infrared emission lines produced by ions that are excited by high-energy radiation. The presence of these lines can reveal a hidden AGN, even when a galaxy shows no sign of an AGN in optical emission.

Discovery of a Relic

the near-infrared spectrum of a dwarf galaxy, showing a small emission line.

The near-infrared spectrum of J1601+3113, captured using the GNIRS instrument at Gemini North, shows the presence of the [Si VI] coronal line, the tiny orange bump on the right of the spectrum. This provides evidence of an AGN. [Adapted from Cann et al. 2021]

In a recent study, Cann and collaborators demonstrate that their unique method works: they detected a coronal line in J1601+3113: a nearby, low-metallicity galaxy that’s only a tenth of the mass of the Large Magellanic Cloud! The authors’ detection is consistent with the presence of a supermassive black hole of roughly 100,000 solar masses, opening a window onto precisely the relic black hole seeds we’re hoping to find.

Cann and collaborators’ discovery marks the first time that an AGN has been identified in a low-mass, low-metallicity galaxy with no optical signs of AGN activity, underscoring how the coronal-line technique can help us find AGNs that might otherwise go undetected.

And with the James Webb Space Telescope scheduled to launch this year, we’ll (hopefully!) soon be collecting infrared spectra with unprecedented sensitivity. With any luck, we’re about to have access to a remarkable new population of lightweight AGNs hiding in small, low-metallicity galaxies — and with it, valuable insight into how these objects were born.

Citation

“Relics of Supermassive Black Hole Seeds: The Discovery of an Accreting Black Hole in an Optically Normal, Low Metallicity Dwarf Galaxy,” Jenna M. Cann et al 2021 ApJL 912 L2. doi:10.3847/2041-8213/abf56d

NGC-1559

Star formation in galaxies appears to be highly regulated by the flow of gas into and out of galaxies. We still haven’t pinned down the specifics of these flows, but we can learn a lot about them by studying galaxies during “cosmic noon”, when star formation rates across the universe were at their highest. 

Star formation rates versus redshift (lower axis) and lookback time (upper axis). The star formation rates were determined from infrared and ultraviolet observations. The peak around redshifts 2 and 3, or “cosmic noon”, is evident. [Madau & Dickinson, 2014]

The Ins and Outs at Cosmic Noon

“Cosmic noon” corresponds to redshifts of z = 2–3, when the universe was roughly between 2 to 3 billion years old (coincidentally). Over this relatively short period, galaxies formed about half of their current stellar mass. This makes cosmic noon an ideal time to examine mechanisms of star formation.

Stars form from gas, and gas is constantly flowing in and out of galaxies. Specifically, gas flows between the intergalactic and the interstellar mediums, passing through the circumgalactic medium (CGM) as it does. So, the CGM is a record-keeper of what sort of gas has flowed in and out of a given galaxy.

Since stars convert lighter elements into heavier elements, we’d expect that the gas flowing into a galaxy is dominated by light elements while the gas flowing out contains more heavy elements. However, we haven’t observed this effect at low redshifts, likely due to several intervening factors like dust and gas mixing. But could it be more apparent at higher redshifts, like at cosmic noon?

A recent study led by Nikole Nielsen (Swinburne University of Technology/ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions, Australia) presents the first results from the “CGM at Cosmic Noon with Keck Cosmic Web Imager” program, which aims to study gas flows from galaxies during cosmic noon. Using data taken by the Keck Cosmic Web Imager (KCWI), the Hubble Space Telescope, and the Very Large Telescope, Nielsen and collaborators studied the properties of one particular galaxy at ~ 2 in great detail.

Absorbing as Much Information as Possible

The Hubble Space Telescope’s view of the focus galaxy in this study (G) along with the quasar that lights up its absorber (QSO, or “quasi stellar object”). [Adapted from Nielsen et al. 2020]

The CGM at Cosmic Noon with KCWI program specifically aims to obtain “absorber–galaxy pairs” suitable for studying gas flows. “Absorbers” are bodies of material that are backlit by quasars, extremely bright, active galaxies. As light from a quasar passes through an absorber, the quasar light is altered by the absorber in ways unique to the contents of the absorber. Nielsen and collaborators were especially interested in absorbers that showed signatures of magnesium and carbon, since those elements are easily detectable during cosmic noon and can be used to trace metal-enriched, ionized gas. The relevant absorbers in this study were observed by the Very Large Telescope.

We don’t take it for granted that an absorber is associated with a galaxy, which is where the KCWI and Hubble data come in. The KCWI data can be used to find the characteristic hydrogen emission from a galaxy, while the Hubble images allow for galaxy shape to be determined. The focus galaxy of this study appears edge-on to us, with the quasar shining down its narrower axis.

The left plot is the KCWI view of the quasar (QSO) and focus galaxy (G); the upper right plot is the Hubble image of the focus galaxy. The middle and lower right plots show the hydrogen emission from the galaxy and the absorption signature in the quasar spectrum respectively. Click to enlarge. [Nielsen et al. 2020]

Likely on Its Way Out

Based on features of our own galaxy and the focus galaxy’s likely orientation, Nielsen and collaborators assumed that the gas flows associated with magnesium are outflows. If this is the case, then the authors estimate that gas is flowing out of the galaxy at a rate of perhaps 50 solar masses per year. The CGM of the focus galaxy does appear to be more enriched with heavy elements than average, but not so enriched that it’s completely dominated by outflows.

Nielsen and collaborators noted that there are many viable interpretations of these data, so no absolute conclusions can be made based on this one galaxy. More detailed models that account for the effects of different elements and galaxy orientations will be useful in the future. However, this study is an excellent demonstration of what can be done by combining data from different instruments. So, on to the next galaxy!

Citation

“The CGM at Cosmic Noon with KCWI: Outflows from a Star-forming Galaxy at z = 2.071,” Nikole M. Nielsen et al 2020 ApJ 904 164. doi:10.3847/1538-4357/abc561

Illustration of a star surrounded by a bright, extended disk of dust and gas.

How does material move through an accretion disk to the young star at its center? Surprising detections from a fortuitously angled disk have now provided new insights.

Driving Inflow

When stars are born from the collapse of a dense molecular cloud, they spend their early stages surrounded by circumstellar disks: disks of gas and dust that we understand to be accreting onto the young stars at their centers.

Taurus Molecular Cloud complex

Herschel infrared view of the Taurus Molecular Cloud complex, the home of GV Tau. [ESA/Herschel/NASA/JPL-Caltech; acknowledgement: R. Hurt (JPL-Caltech)]

How do we know that the disk matter is flowing onto the stars? Evidence for accretion comes from the high-energy light emitted when inflowing material strikes the surface of young stars, producing accretion shocks. But, though these observations provide evidence that accretion is occurring, they don’t tell us much about the mechanisms that drive these flows within the disk.

For material to move inwards within a disk, it must first lose angular momentum — but where does that momentum go? What processes remove or redistribute it? In a new study led by Joan Najita (NSF’s NOIRLab), a team of scientists presents high-resolution observations of an unusual disk — one that happens to be angled in such a way as to help us answer these questions.

Lucky Alignment

Two-panel diagram illustrating disk geometry.

Schematics representing the likely observing geometry of GV Tau N; the observer is on the right. Top: The line of sight to the disk continuum (orange) passes through the warm molecular atmosphere at larger radii (pink), producing absorption. Bottom: View of the molecular gas velocities. The combination of rotation (blue arrows) and inflow (green arrows) produces net redshifted (red arrows) absorption velocities. [Adapted from Najita et al. 2021]

Najita and collaborators used the TEXES spectrograph on the Gemini North 8-m telescope to conduct mid-infrared observations of GV Tau N, a young star surrounded by a nearly edge-on circumstellar disk. The authors’ observations revealed rare molecular absorption lines, a result of the nearly edge-on inclination of the disk.

The unique viewing angle for GV Tau N means that our sightline passes through the disk atmosphere in the inner few au of the disk — the region where planet formation is thought to occur. The molecules in this gas absorb some of the continuum light emitted by the interior disk, leaving signatures in the spectrum that provide valuable insight into the composition and motions of the gas at the surface of the inner disk.

Caught in the Act

Najita and collaborators found evidence for a variety of molecular species in the disk: acetylene (C2H2), hydrogen cyanide (HCN), water (H20), and even ammonia (NH3), which has never before been detected in an inner accretion disk. But the especially interesting result is that these molecules’ absorption lines are redshifted, lying at longer wavelengths than expected if the gas were moving in a stable circular orbit.

Spectrum showing ammonia absorption.

Spectrum showing various ammonia absorption lines from GV Tau N. [Adapted from Najita et al. 2021]

This redshift is an indication that the gas observed is flowing rapidly (about 1 au per year) inward along the disk surface — direct evidence for accretion in action. The authors show that their observations match expected mass accretion rates for active T Tauri stars: roughly a few to a few tens of Earth masses per year. The observations fit neatly with a disk accretion model in which angular momentum is redistributed within the disk, causing surface gas to flow in and accrete while the midplane of the disk spreads outward.

GV Tau N is a lucky break — its orientation allowed us to make these unique measurements. But it’s surely not alone! With more observations of systems like GV Tau N, we’ll be able to further deepen our understanding of disk accretion.

Citation

“High-resolution Mid-infrared Spectroscopy of GV Tau N: Surface Accretion and Detection of NH3 in a Young Protoplanetary Disk,” Joan R. Najita et al 2021 ApJ 908 171. doi:10.3847/1538-4357/abcfc6

Simulation showing two black holes in the process of merging. A star field makes up the background.

Theory predicts that gravitational-wave detectors should be able to observe a population of huge black holes. A new study explores what we’ll learn from these mysterious objects and when we can hope to find them.

A Preferred Size

Stellar graveyard Nov 2020

A recent version of the rapidly expanding “stellar graveyard”, a plot that shows the masses of the different components of observed compact binary mergers. Click to enlarge. [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

So-called stellar-mass black holes — the black holes probed by gravitational-wave detectors like LIGO/Virgo — can theoretically span a broad range of sizes, from just a few solar masses to hundreds of times the mass of the Sun.

The LIGO/Virgo gravitational-wave detectors have discovered signals from dozens of black-hole binaries completing their final death spirals and merging. So far, these observed primary black holes have primarily fallen into a mass range below ~45 solar masses, indicating a precipitous drop in the population of binary black holes above this mass.

supernova

Artist’s impression of a supernova. Progenitor stars of a certain mass are susceptible to pair-instability supernova, preventing the formation of a black hole. [ESO/M. Kornmesser]

Avoiding an Unstable End

Why the dearth of heavier black holes? Theorists have an explanation: the pair-instability supernova mass gap. Based on our understanding of stellar evolution, black holes in a certain mass range — roughly 50–120 solar masses — shouldn’t be able to form. This mass gap arises because the progenitor stars needed to produce black holes of this size are predicted to undergo a runaway process, eventually exploding as violent supernovae that prevent remnant black holes from forming.

The formation of black holes above ~120 solar masses, however, should still be possible — so we’d expect a population of enormous far-side-of-the-mass-gap black holes to be lurking in our galaxy and beyond. In a new study, University of Chicago scientists Jose María Ezquiaga and Daniel Holz dig further into this prediction.

Hunt on the Far Side

Ezquiaga and Holz use the statistics of past black-hole binary detections and predictions of the capabilities of current and future gravitational-wave detectors to estimate what’s in store for us in terms of far-side black holes.

First, the authors show that these heavyweights would be the most massive sources detectable by LIGO/Virgo, and — if they exist — we should be able to spot up to tens of them during LIGO/Virgo’s next two observing runs (O4 and O5).

Plot of the number of detections of black holes expected as a function of mass.

The estimated maximum number of black-hole-binary mergers detected per year for various current and upcoming ground-based gravitational-wave detectors, and in 4 years for LISA. [Adapted from Ezquiaga & Holz 2021]

What’s more, far-side binaries should also lie in the observing band of LISA, the upcoming space-based gravitational-wave mission. They may dominate the population of binaries that can be observed by both LIGO/Virgo and LISA, providing valuable information about how the merger rate for black-hole binaries changes over time.

Finally, Ezquiaga and Holz show that observations of far-side binaries with LISA, LIGO/Virgo, and the Einstein Telescope (a next-generation detector) will provide an independent measure of the expansion of the universe at different redshifts: z ~ 0.4, 0.8, and 1.5, respectively. By exploiting the upper edge of the mass gap, far-side black holes can act as standard sirens and enable precision cosmology.

Soon To Be Found?

So what’s the upshot? The outlook is good for far-side black holes!

If these heavyweights exist, we should spot them within the next couple years and they’ll be able to provide us with valuable insight into a variety of science questions. If we don’t observe any within this time frame, that also provides a powerful statement about black-hole formation, demanding new theories to explain the dearth.

Citation

“Jumping the Gap: Searching for LIGO’s Biggest Black Holes,” Jose María Ezquiaga and Daniel E. Holz 2021 ApJL 909 L23. doi:10.3847/2041-8213/abe638

Photograph of a large, bright, elliptical galaxy in the midst of a broader galaxy field.

In the outer reaches of galaxies, stars don’t move quite how they should. Is this deviation due to mysterious dark matter? Or is something else at work? In a recent study, scientists turn to elliptical galaxies in search of new clues.

Weirdness in Galactic Fringes

Animated gif that shows two galaxies rotating with different behaviors.

Two rotating galaxies are shown with their rotation curves in this animated gif. Based on the distribution of visible matter, we would expect to see inner stars moving fast and outer stars moving slowly, producing a rotation curve like what’s seen on the left. Instead, we see stars moving with the same speed, producing the flat rotation curve seen on the right. [Ingo Berg]

Usually, things that orbit do so in predictable ways. In our solar system, for instance, planets orbit the Sun following predictable laws of gravitation: close-in planets speed along quickly, whereas planets farther out move more slowly.

You might think that galaxies would work the same way. Since most of a galaxy’s visible mass is concentrated at its center — just like most of our solar system’s mass (the Sun) is at its center — we would expect stars near the center of a galaxy to orbit quickly, and stars in the galaxy’s outermost fringes to orbit very slowly. Instead, we see that stars throughout galaxies move at roughly the same speeds: galaxies have flat rotation curves.

Dark matter halo

Artist’s illustration of the distribution of dark matter in the halo surrounding the visible disk of the Milky Way Galaxy. [ESO/L. Calçada]

Dark Matter? Or a Gravitational Misunderstanding?

What drives this weird behavior? The most widely accepted explanation is dark matter: the idea that there’s a lot of matter in a galaxy that isn’t concentrated in its center — we just can’t see it. This dark matter is distributed in a wide halo around the galaxy, and its gravitational tug causes outer stars to orbit faster than they would under the influence of the visible matter alone.

But dark matter isn’t the only possible explanation for galactic rotation curves: an alternative, modified Newtonian dynamics (MOND), was first introduced nearly 40 years ago. The MOND hypothesis contends that dark matter isn’t needed to explain galaxy rotation curves — because our understanding of gravity is wrong.

According to MOND, normal Newtonian gravitation applies in regions where acceleration is large — say, in the case of the Moon orbiting the Earth, planets orbiting a solar system, or in the inner regions of a galaxy. But gravity behaves slightly differently in regions where acceleration is small — like in the outer reaches of galaxies.

Critical Acceleration

How can we test MOND as a theory? One clue is whether individual galaxies stray from expected gravitational behavior at a consistent acceleration scale — some fundamental value of acceleration that effectively marks the transition from the Newtonian regime to the MOND regime. If, instead of a universal acceleration scale, we find a large amount of scatter in how galaxies deviate, that would rule out the MOND hypothesis.

Plot of the individually fitted g† for the authors' 15 elliptical galaxies.

The authors’ elliptical galaxy sample is consistent with a universal acceleration scale of g = 1.2 x 10-10 m/s2 — which is consistent with the acceleration scale previously found for spiral galaxies. [Chae et al. 2020]

A universal acceleration scale has, in fact, been found in previous studies that explored the observed radial acceleration of spiral galaxies. Now, in a study led by Kyu-Hyun Chae (Sejong University, Republic of Korea), scientists have demonstrated that this same scale appears to extend to a sample of elliptical galaxies as well.

Chae and collaborators’ study is important because it shows that both galaxies supported by rotation (spirals) and those supported by pressure (ellipticals) behave the same way. These two independent measures underscore the universality of how galaxies’ acceleration deviates from what’s predicted from visible matter alone — and suggest that MOND isn’t out of the running just yet.

Citation

“On the Presence of a Universal Acceleration Scale in Elliptical Galaxies,” Kyu-Hyun Chae et al 2020 ApJL 903 L31. doi:10.3847/2041-8213/abc2d3

active galactic nucleus

Active galactic nuclei are exactly what they sound like — central regions of galaxies that emit enormous amounts of energy. Typically, they consist of a supermassive black hole surrounded by a hot disk of material being accreted onto the black hole. Hardly the most hospitable environment, but stars can still live in these surroundings!

Centaurus A

This composite image reveals Centaurus A, a galaxy with an active nucleus spewing fast-moving jets into its surroundings. [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)]

Actively Hostile Environments

It would be hard to overstate how energetic active galactic nuclei (AGN) are. Some can outshine the rest of their host galaxy at almost all detectable wavelengths! Spectra of material near the central black hole have shown that AGN environments contain a higher abundance of heavier elements than the environment of our Sun. So it’s possible that those heavier elements were produced in the accretion disk and then swept closer in towards the black hole.

But what produces heavier elements? Stars! Stars can be found near the central supermassive black holes of galaxies, like the Milky Way’s Sagittarius A*, but AGN have far more extreme environments than our placid central black hole. So what sort of stars live in AGN environments? A recent study led by Matteo Cantiello (Flatiron Institute/ Princeton University) dives into this question.

The mass and brightness of an AGN star over time. This star was modeled under specific AGN conditions. LEdd stands for Eddington luminosity, which is the maximum brightness a star can have when it has balanced its outward radiative pressure with its inward gravitational contraction. Mass loss starts roughly around when the star’s luminosity reaches the Eddington luminosity. [Cantiello et al. 2021]

What Massive Stars Make

Cantiello and collaborators were especially interested in how the evolution of stars in AGN environments differs from stellar evolution in calmer environments. To get where they are, AGN stars have to either form in accretion disks or get captured and pulled into the disks. Both models are viable and supported by looking at stellar populations around central black holes that were previously active, like Sagittarius A*.

Once in the disk, stars can rapidly accrete material and become hundreds of times more massive than the Sun. Massive stars experience more internal mixing than less massive stars, so the contents of a massive star are evenly distributed within the star’s interior. This is very different from stars like our Sun, where the outer layers of the star contain lighter elements like hydrogen and helium while inner layers are dominated by heavier elements.

However, massive stars are also unstable and can lose mass quickly as they teeter between expansion and collapse. Their sheer bulk also means that they will end their lives through core collapse — forming heavier and heavier elements through fusion until they run out of material to fuse and collapse onto themselves. The bottom line is that AGN stars are good at producing heavy elements and sending those elements out into the accretion disk.

A schematic showing stellar evolution in the accretion disk of an AGN. Low mass stars can be formed in or accreted by the disk, where they gain mass and eventually evolve to leave behind compact remnants near the center of the disk. [Cantiello et al. 2021]

Signs of Stellar Life and Death

So Cantiello and collaborators identified two signatures of AGN stars: high abundances of heavy elements and compact stellar remnants left behind from core collapse. There are studies showing evidence for the first signature, and interestingly, this abundance of heavy elements doesn’t seem to depend on redshift.

The second signature is a bit trickier to tease out. Before gravitational-wave observatories, our best bet would be to search for the explosions associated with core collapse in the accretion disk of an AGN. Now, we can also look for the gravitational-wave signatures of the mergers of dense objects, with an expectation of how often these mergers would occur.

Sagittarius A* is a good proving ground for the findings of this study, since our galaxy’s nucleus may approximate the aftermath of an AGN. With predictions in hand, it’s now time to observe!

Citation

“Stellar Evolution in AGN Disks,” Matteo Cantiello et al 2021 ApJ 910 94. doi:10.3847/1538-4357/abdf4f

Photograph of the rocky surface of an asteroid.

Samples from the near-Earth asteroid (162173) Ryugu recently arrived at Earth, ready for laboratory analysis. In the meantime, ground-based measurements of Ryugu’s surface are helping us to complete our picture of this nearby, rocky body.

Observations on Earth and from Earth

Illustration of a spacecraft with two sets of solar panels in the foreground, in front of a gray, rocky body.

Artist’s illustration of the Hayabusa2 spacecraft. [JAXA]

In December 2020, the Japanese spacecraft Hayabusa2 completed a daring 6-year mission, successfully landing on near-Earth asteroid Ryugu and returning a sample of this body’s material to Earth. Laboratory analysis of the sample is sure to provide valuable new insight into the structure and composition of the surface of this carbonaceous asteroid. But while we’re waiting for those results, there’s more to be learned from ground-based observations!

One way to study the surfaces of nearby objects is by making polarization measurements, which track the orientation of light waves reflected off of an object. In the case of an airless body like Ryugu, the amount of polarization measured from different viewing angles can tell us about the surface texture of the object.

In a new study led by Daisuke Kuroda (Kyoto University, Japan), scientists report the first polarization measurements of Ryugu, captured by four different observatories based in Japan and South Korea.

Plot of linear polarization vs. phase angle for 7 different objects.

Phase angle–polarization dependence of near-Earth asteroids and cometary nuclei. The solid red circles represent the polarimetric data for Ryugu, marking the highest polarization measured for such an object thus far. [Kuroda et al. 2021]

Breaking Records

Kuroda and collaborators used data gathered between September and December 2020 to measure the linear polarization of light scattered off of Ryugu as the angle between the Earth, Ryugu, and the Sun changed. The authors’ measurements covered a range of this phase angle spanning 28° to 104°.

The authors find that Ryugu exhibits the highest polarization degree ever measured for an asteroid or comet: as much as 53% of the light from Ryugu was linearly polarized at a phase angle of 100°!

This large degree of polarization is consistent with the asteroid’s low albedo. Why? Light wave orientations are scrambled by repeated reflection and refraction, resulting in lower polarization. For dark objects with poor reflectivity, like Ryugu, the reduced scattering results in higher polarization.

A Grainy Surface

Ryugu surface

Color photo of the surface of Ryugu, taken by the MASCOT lander deployed by Hayabusa2. [MASCOT/DLR/JAXA]

The authors’ next step was to use observations of space-rocks we can analyze up close and in person — in this case, meteorites found on Earth — to better understand observations of Ryugu. By comparing the polarimetric data for Ryugu to those measured in the lab for different meteorites, Kuroda and collaborators inferred that Ryugu’s surface layer is dominated by grains of submillimeter-order size.

When we complete the analysis of the actual surface material returned from Ryugu by Hayabusa2, those results will provide valuable context for the polarimetric observations presented here — and vice versa! The combination of these data will help us to learn more from future observations of our near-Earth rocky neighbors.

Citation

“Implications of High Polarization Degree for the Surface State of Ryugu,” Daisuke Kuroda et al 2021 ApJL 911 L24. doi:10.3847/2041-8213/abee25

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