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projection of the milky way

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

On a Hunt for Dark Matter

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

Illustration of a galaxy and its dark matter halo (shown in blue). [ESO/L. Calçada; CC BY 4.0]

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

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

Measuring Midpoints

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

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

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

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

Capable Spacecraft

plot of a simulated eclipse

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

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

Citation

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

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

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

Fantastic, Radiant, Baffling 

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

2D map of sources on the sky

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

Searching for Signals of a Coincident Companion

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

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

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

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

Mysterious Magnetars or Fantastic Flares? 

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

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

Citation 

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

illustration of rocky material bombarding the young Earth

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

A Crash Course in Earth History

plot of elemental abundances in Earth's crust

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

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

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

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

Plentiful Planetesimals

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

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

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

Ample Earth-Like Material

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

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

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

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

Citation

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

photograph of the lupus 3 star-forming region

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

Stymied Star-Formation or Mistaken Models?

map of the milky way's star forming clouds

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

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

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

A Milky Way Mystery

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

plot of collapse efficiency as a function of the virial parameter

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

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

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

Closing In on a Solution

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

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

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

Citation

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

photograph of the asteroid bennu

photographs of laboratory experiments compared to a photo of the moon's surface

Top: A Moon lander observed a glow on the lunar horizon, possibly caused by suspended particles. Middle and bottom: Lab experiments show that electrostatic lofting can lift particles several centimeters. [NASA/CU-Boulder/LASP]

Spacecraft observations have revealed particles skittering across the surfaces of airless asteroids and moons. Without wind or water, what makes these particles move?

Ejection Events

When the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft arrived at nearby asteroid Bennu in 2018 after a two-year journey, it observed rocky particles up to 10 centimeters in size being ejected from the asteroid’s surface.

Closer to home, laboratory experiments have found that the repulsion between particles with similar electrical charges can cause them to levitate. Could the same process be responsible for the particles seen escaping from Bennu?

diagram of the model setup

Diagram depicting the simplified model surface particles (circles) and the forces at play. The red areas emit electrons, which are collected by blue areas. Under certain conditions, the electrostatic force (FES) between areas with like charge can overcome the forces of gravity (Fg) and cohesion (FCO) and allow the particle to escape. [Adapted from Hartzell et al. 2022]

Charging Up an Asteroid’s Surface

In a new publication, a team led by Christine Hartzell (University of Maryland) investigated whether this process of electrostatic lofting could cause the particle ejection events seen by OSIRIS-REx. In order for particles to be lifted in this way, they must build up enough electrical charge — from impinging solar photons as well as electrons and ions of the solar wind — to overcome the combined forces of gravity and cohesion, which work to keep the particles stuck to the asteroid’s surface.

Using a simplified model composed of particles of identical shape and size, the team found that electrostatic lofting can raise small particles off Bennu’s sunlit surface. The size of the lofted particles was strongly dependent on how cohesive the surface material is; if the cohesion is low, particles a few tenths of a millimeter in size could escape, while only micron-size particles could depart if the cohesion is high.

From Day to Night

The team also considered whether electrostatic lofting could explain the particles seen lifting off from Bennu’s night side. On the night side, the particles are not exposed to sunlight, so they mainly become charged when electrons from the solar wind curl around from the opposite side of the asteroid and strike the surface, knocking electrons free.

simulations of electric fields on bennu

Top: Electric field needed to lift particles on Bennu’s dayside for low (S=0.1; black) and high (S=0.75; blue) cohesion, compared to the expected electric field (red). Bottom: Required and present electric field strengths on the night side for low cohesion values and different densities of the solar wind. Click to enlarge. [Adapted from Hartzell et al. 2022]

The resultant electric field is about a thousand times smaller on the asteroid’s night side than it is on the day side, meaning that electrostatic lofting is unlikely to cause particles to escape from Bennu’s night side — even if the cohesion between particles is very low. The authors identified one exception: if an asteroid passes through a region with lots of high-energy electrons — such as the wake of a magnetized planet — the particles may be able to get the electrical oomph they need to leave the asteroid’s surface.

Ultimately, the team concluded that electrostatic particle lofting is unlikely to be responsible for ejecting the large particles seen by OSIRIS-REx, but it may selectively remove small particles from Bennu’s surface. However, the case isn’t fully closed: further work may show that the asteroid’s irregular shape causes strong electric fields to develop in certain areas, which could be the sites of larger particle ejections. What’s more, the planned return of a sample of Bennu’s surface in 2023 should also give scientists more information about the surface material and allow for more accurate modeling of electrostatic lofting — stay tuned!

Bonus

Curious about the fate of particles ejected from Bennu’s surface? Check out the video below to follow the trajectories of particles ejected in the four largest events observed by OSIRIS-REx. [M. Brozovic/NASA/JPL-Caltech/University of Arizona]

Citation

“An Evaluation of Electrostatic Lofting and Subsequent Particle Motion on Bennu,” Christine Hartzell et al 2022 Planet. Sci. J. 3 85. doi:10.3847/PSJ/ac5629

optical image of the northern region of the vela supernova remnant

Some supernovae unexpectedly increase in brightness in ways that our current models can’t explain. Can a new model that combines shock waves and sound waves make sense of this mystery?

Late to the Party

time series of supernova remnant SN 1987A from 1994 to 2016

This time series of Hubble images shows the expansion of supernova remnant SN 1987A. As shock waves interacted with the ambient gas, the supernova remnant brightened dramatically. [NASA, ESA and R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation) and P. Challis (Harvard-Smithsonian Center for Astrophysics)]

When a massive star explodes as a supernova, it briefly outshines its host galaxy before gradually fading from view. Sometimes, expanding supernova shock waves collide with nearby gas — such as circumstellar material ejected by the star before it went supernova — which causes a temporary increase in brightness. When this happens, the material being swept up is compressed, heated, and ionized, causing new emission lines to appear in the supernova’s spectrum. However, some supernovae brighten without showing new emission lines — what causes this behavior?

New work by Eric Coughlin (Syracuse University) and Jonathan Zrake (Clemson University) suggests that delayed brightening might not always be evidence for a new interaction with circumstellar material — but rather, an echo of a previous perturbation.

A Sound Solution

Coughlin and Zrake used linear perturbation theory — a way to mathematically describe the properties of a system in terms of a slowly varying background and a small perturbation in that background — to explore the scenario in which a supernova shock wave expanding into circumstellar material encounters an especially dense region of gas.

plot of the shock luminosity as a function of distance

The shock luminosity relative to the luminosity obtained when there is no density enhancement present (solid line) is plotted as a function of the shock’s position. The scaled density is shown by the dotted line. [Adapted from Coughlin & Zrake 2022]

In this scenario, Coughlin and Zrake predict that the supernova will brighten twice. As the shock wave expands into the circumstellar material, it encounters a denser region of gas and the supernova brightens for the first time and exhibits new emission lines. As the shock collects more and more material, it slows down and the brightness decreases.

So far, this is just a typical interaction between a shock wave and circumstellar material. Here’s where things change: the new model explored by Coughlin and Zrake incorporates a second wave — a slower-moving sound wave — which is launched by the initial collision between the shock wave and the denser circumstellar material. As the shock wave slows down, the sound wave catches up to it and hits it from behind. In the authors’ new framework, it’s the interaction between the initial shock wave and the secondary sound wave that causes the second brightening of the supernova — not a change in the density of the surrounding material. And since no additional material is being swept up and ionized, no new emission lines are produced.

Explaining Outliers

plot of shock luminosity as a function of time

The shock luminosity as a function of time for the density enhancement introduced in the previous figure. The increase in luminosity around 75 days agrees qualitatively with the observations of SN 2019tsf. [Adapted from Coughlin & Zrake 2022]

This model may explain the behavior of supernovae that have brightened long after their initial explosions, like SN 2019tsf, iPTF14hls, and SN2020faa. Should we expect delayed brightening to be a feature of all supernovae? Unlikely, say Coughlin and Zrake — extremely dense circumstellar material could obscure an increase in brightness, and if the density of the material decreases too quickly with distance, the sound wave won’t be able to catch up with the shock wave.

The authors note that there’s much more to explore, since real supernovae expanding into circumstellar gas are far more complex than the framework introduced in this article. Hopefully, future work will help us understand the wide variety of supernova behaviors seen so far!

Citation

“A Physical Model of Delayed Rebrightenings in Shock-interacting Supernovae without Narrow-line Emission,” Eric R. Coughlin and Jonathan Zrake 2022 ApJ 927 148. doi:10.3847/1538-4357/ac4033

Illustration of the sun setting off a coronal mass ejection headed to Earth and it hitting Earth's magnetosphere

Are there any indications as to when monster solar storms are going to happen? A team of astronomers used simulations combined with observations of our home star to show that we may have another way to tell when these massive events are coming toward us. 

An Active Atmosphere of a Seemingly Serene Star 

Space weather, particularly solar flares and coronal mass ejections (CMEs), have a direct impact on life here on Earth. Though our world is protected by the blanket of Earth’s magnetic field, if a solar storm is energetic enough, it can damage weather satellites, GPS satellites, and knock out power grids. Therefore, understanding these events is crucial to maintaining daily life in a world dependent on these technologies.  

Diagrams showing the brightening and dimming events on two separate days

Two dimming/brightening events with the difference between the base image and the observation shown by the color scale (dimmer than the original event shown in black and brighter shown in red). [Adapted from Jin et al. 2022]

Though the Sun may look like a steadily glowing sphere in the sky, it’s actually very dynamic. Sunspots grow and shrink, solar flares build and erupt, and the Sun’s outermost layer, or corona, brightens and dims. Recent studies have explored coronal dimmings and have found that the majority of them occur just after coronal mass ejections. They can therefore provide useful information about space weather events. In a new study, a team led by Meng Jin from the Lockheed Martin Solar Astrophysics Laboratory/the SETI Institute combined observations with simulations to study the relationship between coronal dimming events and coronal mass ejections. 

An AWSoM Simulation of the Sun 

The team used the Alfvén Wave Solar Model (AWSoM) to reconstruct the Sun’s corona and the solar wind environment and then simulated CMEs with various characteristics, such as speed, mass, and magnetic energy. These CMEs are initiated by magnetic flux ropes erupting from the surface. Jin and collaborators also looked at observations of the dimming events from the Atmospheric Imaging Assembly and the Extreme Ultraviolet Variability Experiment, which are part of the Solar Dynamics Observatory.  

Diagrams showing brightenings and dimmings, with magnetic field lines coming from one of the diagrams

Two of the authors’ simulations of the Sun. The difference between the base image and the observation is shown by the color scale (dimmer than the original event shown in black and brighter shown in red) and field lines shown in white and green on Panel (d). [Jin et al. 2022]

Magnetospheric Modeling and Coronal Construction 

The authors’ models were able to reproduce many features observed in the coronal dimming that followed a CME eruption, which shows that dimmings can help us understand characteristics of the associated CMEs such as their energy, mass, and the magnetic configuration of the flux ropes that caused them. Using that knowledge, the team deduced that transient brightening and dimming patterns are related to plasma heating processes in the corona, while long-lasting core and remote dimmings (which are away from the source region) are caused by mass loss from CMEs. They also found that the coronal dimming patterns are significantly influenced by the interaction between the erupting flux ropes with different orientations and the solar corona as a whole. 

Diagram showing that CMEs emanate from core dimming spots and another showing core dimming and remote dimming with a topological connection with open/quasi-open field lines coming from both dimming areas

A cartoon showing the location of CMEs and magnetic field lines in relation to dimming spots. [Adapted from Jin et al. 2022]

 

This knowledge about the relationship between coronal dimmings and solar CMEs could also help us detect CMEs on stars outside of the solar system! Distinguishing between stellar flares and CMEs is difficult but important when considering habitability because CMEs are more likely to erode planetary atmospheres. With knowledge that CMEs are associated with stellar dimmings, we can get a better handle on which stars have CMEs, and therefore better judge which planets may be habitable. 

Citation 

“Coronal Mass Ejections and Dimmings: A Comparative Study Using MHD Simulations and SDO Observations,” Meng Jin et al 2022 ApJ 928 154. doi:10.3847/1538-4357/ac589

simulation of matter spiraling around a pair of black holes

When galaxies merge, the supermassive black holes at their centers spiral around each other and eventually coalesce into a single black hole. How can we track down these massive mergers?

The Hunt for Massive Mergers

illustration of an active galactic nucleus emitting a jet

Modeling the emission from supermassive black hole binaries may help us to distinguish them from active galactic nuclei powered by single black holes. [NASA/JPL-Caltech]

Since the first detection of gravitational waves from a pair of stellar-mass black holes in 2015, gravitational waves have been a powerful tool to study merging black holes. However, detecting the extremely long-wavelength gravitational waves from merging supermassive black holes — with wavelengths of up to tens of light-years! — is beyond our current capabilities. What other methods can we use to detect supermassive black holes in the midst of merging?

One possibility is to track down the electromagnetic radiation produced by the hot plasma that surrounds the black holes as they draw closer. If we can detect this radiation, we can study supermassive black holes as they merge as well as potentially identify the small fraction of active galactic nuclei that are actually powered by black hole binaries rather than by a single black hole — a population that has never been definitively detected.

plots of simulated surface brightness

Simulated surface brightness of the accretion disks around merging black holes with a total mass of one million solar masses. Results are shown for three different wavelengths (left to right: 45, 12, 0.3 nanometers) and for spinning (top row) and non-spinning (bottom row) black holes. In all simulations, a gap opens between the circumbinary disk and the mini-disks — a feature that is not present in models of accretion disks around single black holes. Click to enlarge. [Gutiérrez et al. 2022]

Verging on Merging

A team led by Eduardo Gutiérrez (Argentine Institute of Radio Astronomy and Rochester Institute of Technology) used general relativistic magnetohydrodynamics simulations to model the electromagnetic radiation generated as two supermassive black holes approach a merger.

To predict the light emitted by the system, Gutiérrez and collaborators first modeled the motion of the superheated plasma surrounding the black holes. As the black holes circle around each other, the surrounding material forms a disk that envelops both black holes as well as mini-disks that circle each black hole. A dense region called the “lump” develops on the inner edge of the larger disk, periodically feeding material to the mini-disks.

The team then simulated the winding path that photons would take through the superheated plasma and warped spacetime to reach an observer on Earth. The resultant spectrum is mainly composed of emission from the disk around the binary, the mini-disks, and the streams of material that connect the larger disk to the mini-disks.

Seeing Double

plot of simulated spectral energy distributions

Spectral energy distributions derived from simulations of accreting black holes. In the simulations, the mass of the single black hole is equal to the sum of the masses of the binary components. [Adapted from Gutiérrez et al. 2022]

Gutiérrez and coauthors found that the radiation from merging supermassive black holes should be detectable, and there are significant differences in the emission from merging black holes and a single black hole. Specifically, a binary system emits less energy than a single black hole, and its emission peaks at a lower frequency and decreases less sharply at frequencies above the peak. And unlike a single black hole, emission from black hole binaries should show semi-periodic behavior; because the lump that feeds material to the mini-disks has a slightly elliptical orbit, the accretion rate — and therefore the strength of the emission — increases when the lump passes closest to the mini-disks.

The authors predict that the signal from a black hole binary with a total mass of a billion solar masses would vary with periods of ~20 and ~150 days, while the emission from a million-solar-mass binary would vary on shorter timescales. Repeated X-ray observations should be able to detect this variation, determine whether the cause of the emission is one black hole or two, and give us the first-ever look at behemoth black holes moving toward a merger.

Citation

“Electromagnetic Signatures from Supermassive Binary Black Holes Approaching Merger,” Eduardo M. Gutiérrez et al 2022 ApJ 928 137. doi:10.3847/1538-4357/ac56de

representative color infrared image of the spiral galaxy messier 81

An infrared survey of nearby galaxies revealed outbursting objects that defied existing classifications. Now, new optical observations with Hubble investigate whether these unusual objects are more than meets the (infrared) eye.

A Spirited Survey

infrared image of the rho ophiuchi cloud complex

Young stellar objects in dusty nebulae, like those in the Rho Ophiuchi cloud complex shown here in an image from the Wide-field Infrared Survey Explorer (WISE), are a potential source of the SPRITEs discovered by SPIRITS. [NASA/JPL-Caltech/UCLA]

From 2014 to 2019, a team of astronomers carried out the first large-scale survey of variable objects at infrared wavelengths in nearby galaxies. The SPitzer InfraRed Intensive Transients Survey (SPIRITS) discovered a curious class of outbursting objects, dubbed eSPecially Red Intermediate-luminosity Transient Events (SPRITEs), which are more luminous than novae but less luminous than supernovae and have outbursts that last anywhere from days to years.

Unlike other types of infrared transients, which flare into view at optical wavelengths during an outburst, SPRITEs remain hidden or extremely faint at wavelengths shorter than infrared even when they’re at their brightest. What might these SPRITEs be — supernovae cloaked in clouds of dust, young stars in natal nebulae, or something else entirely?

light curves for three objects targeted

Spitzer light curves for three of the objects studied. Blue symbols indicate 4.5-micron measurements and red symbols indicate 3.6-micron measurements. The remaining symbols indicate upper limits. Click to enlarge. [Adapted from Bond et al. 2022]

Diving Deep with Hubble

In a new publication led by Howard Bond (Penn State University and Space Telescope Science Institute), a team of astronomers performed deep optical and near-infrared imaging with the Hubble Space Telescope to take a closer look at 21 SPIRITS targets, most of which were classified as SPRITEs. Their goal was to detect optical counterparts for these infrared transients and characterize their environments in order to identify the source of the outbursts.

Many of the targets were still not detected in the Hubble observations, but observations of their surroundings yielded new insights into their identities. What’s more, the team continued to monitor the targets with Spitzer, leading to entirely new classifications for many of the objects: of the 21 supposed transients, only three retained that moniker, while a dozen others were discovered to have periodic behavior, and six were found to vary irregularly. How has this affected our understanding of the SPIRITS targets and SPRITEs specifically?

From Transient to Periodic

images of the site of SPRITE 17fe

Hubble images of the site of SPRITE 17fe, which is marked with a red cross in the top image. The middle row shows a zoomed-in view of the site years before the outburst in three different wavelength bands. The bottom row shows the same field during an outburst — 0.8 years after the peak — in two near-infrared bands. [Bond et al. 2022]

Two of the three remaining SPRITEs were located in star-forming regions, suggesting that they are associated with massive stars, though one was found in a region with predominantly old stars and is likely a classical nova shrouded in dust. Thus, SPRITEs can have many causes; merging massive stars, dusty supernovae, and eruptions of bright variable stars are all possibilities.

The objects found to vary periodically did so over extremely long timescales (from 670 to more than 2,100 days) and tended to be found near star-forming regions. Comparison with known types of variable stars indicates that these slowly varying sources are likely dusty, evolved giant stars with masses in the 5–10 solar-mass range. The irregular variables detected in Hubble images might be pulsating red supergiants, while the redder variables undetected at Hubble wavelengths might be associated with mass-loss events of extremely dusty luminous blue variable stars.

There’s still much to learn about the dusty objects identified by SPIRITS. Infrared spectroscopy is likely to illuminate the nature of these sources and luckily, JWST and future observatories like the Roman Space Telescope should be more than equal to the task. For a detailed discussion of each target studied by Bond and collaborators, be sure to check out the full article cited below!

Citation

“Hubble Space Telescope Imaging of Luminous Extragalactic Infrared Transients and Variables from the Spitzer Infrared Intensive Transients Survey,” Howard E. Bond et al 2022 ApJ 928 158. doi:10.3847/1538-4357/ac5832

hubble image of spiral galaxy messier 83

Many galaxies seem to have far less visible matter than expected. In a new article, astronomers have taken the search for missing matter to the outskirts of spiral galaxies.

Mysterious Matter

hubble image of the spiral galaxy messier 51

Messier 51, the Whirlpool Galaxy, is one of the many recognizable galaxies investigated in this work. The galactic halos studied in this article likely extend hundreds of thousands of light-years into space — far larger than the galaxies’ starry disks. Messier 51’s disk is 76,000 light-years in diameter. [NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)]

Just 15% of all the matter in the universe is thought to be visible matter — the kind we interact with on a daily basis — with dark matter making up the remaining 85%. Though dark matter is certainly the more elusive of the two, visible matter isn’t without its mysteries — after adding up the masses of the visible components of galaxies like stars and gas clouds, most galaxies appear to have less visible matter than expected based on observations of galaxies early in the universe. Milky Way-like galaxies seem to lack about 70% of their mass while low-mass galaxies can be missing all but a few percent.

Where might this missing matter be hiding? One possibility is that much of the mass of galaxies lies in extended halos that stretch hundreds of thousands of light-years beyond the bright, starry regions that make up the main body of a galaxy. This gas is difficult to detect because of its low density and interference from intervening gas within our own galaxy. How, then, can we weigh this halo gas?

global projection of the cosmic microwave background temperature fluctuations

A map of temperature fluctuations in the cosmic microwave background from WMAP. This map shows temperature deviations of up to 200 microkelvin. [NASA / WMAP Science Team]

A Sunyaev–Zeldovich Stack

A team led by Joel Bregman (University of Michigan) searched for missing matter in galactic halos by looking for evidence of the Sunyaev–Zeldovich effect — the process through which low-energy photons from the cosmic microwave background are kicked up to higher energies through interactions with extremely hot gas. The magnitude of this effect is proportional to the mass and temperature of the gas, making it a useful probe of hot, diffuse halos that might otherwise be impossible to spot.

Bregman and collaborators used this method to investigate the halos around 12 spiral galaxies located 10–33 million light-years away — close enough to determine the spatial extent of the hot halo gas. Most of the halos were too faint to be detected individually, so the team stacked the observations from 11 of the 12 galaxies (one galaxy showed significant differences and was analyzed separately) to extract a signal and determine the average properties of the galaxies in the sample.

Luminous Matter at Large

plot of sunyaev-zeldovich signal strength and signal to noise ratio

The integrated Sunyaev–Zeldovich signal from the stack of 11 galaxies as a function of radius (left) and the signal to noise ratio (right). Click to enlarge. [Bregman et al. 2022]

Bregman and coauthors found that out to a radius of 815,000 light-years (250 kiloparsecs), each galaxy contains 98 billion solar masses of gas. Each galaxy is expected to contain about 310 billion solar masses of visible matter, so this constitutes about 30% of the galaxies’ total mass. The galaxies’ stars, star-forming gas, and cooler halo gas make up a further 30%, meaning that the remaining 40% of the visible matter in these galaxies likely resides at even larger distances.

In order to search for gas even farther out, the team hopes to stack observations from more galaxies and develop new algorithms to reduce uncertainties. Proposed cosmic microwave background detectors like the Probe of Inflation and Cosmic Origins may also aid the search for missing matter, helping us understand where present-day galaxies hide their mass.

Citation

“Hot Extended Galaxy Halos around Local L* Galaxies from Sunyaev–Zeldovich Measurements,” Joel N. Bregman et al 2022 ApJ 928 14. doi:10.3847/1538-4357/ac51de

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