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illustration of a black hole

It sounds like something out of science fiction: a miniature black hole barrels through the densely packed remnant of a dead star, consuming it from the inside. Strange though it may seem, this scenario might hold the key to discerning between stars composed of different kinds of exotic matter.

Strange Stars, Indeed

illustration of a neutron star on a map of manhattan

Comparison of the size of a neutron star to the size of Manhattan. A strange star of the same mass would be slightly smaller. [NASA/Goddard Space Flight Center]

When a massive star expires, its outer layers explode away from its collapsed core. This core is so dense that electrons and protons are squished together, forming a star composed of neutrons. However, a neutron star’s evolution might not stop there: it’s possible that the extreme conditions inside the star could convert the neutrons into elementary particles called strange quarks. The resulting strange star would be only slightly smaller than the original neutron star, making it difficult to distinguish between them observationally. Since we can’t peer into their interiors, how can we tell if a compact stellar remnant is a neutron star or a strange star?

In new article, Ze-Cheng Zou (邹泽城) and Yong-Feng Huang (黄永锋) from Nanjing University, China, propose an intriguing way to detect strange stars: by watching them be consumed by a tiny black hole. Specifically, Zou and Huang posit that we can differentiate between the gravitational waves emitted as a neutron star or strange star encounters a planet-mass primordial black hole — one that formed spontaneously from a dense spot in the universe less that one second after the Big Bang.

plot of black hole mass over time

Demonstration of the how the mass of the black hole changes as it accretes matter from the neutron star or strange star. [Zou & Huang 2022]

Insights from Inspiraling

Zou and Huang modeled the outcome of a clash between a neutron star or strange star and a planet-mass primordial black hole and predicted the gravitational waves that would be emitted as a result. The authors consider black holes from slightly more massive than Mercury to as massive as Jupiter — with radii of 0.3 to 300 centimeters — and strange stars and neutron stars of 1.4 solar masses.

As the black hole tunnels into the star, its progress gradually slows as a result of accreting stellar material and emitting gravitational waves. Though the modeled neutron star and strange star have the same mass and similar radii (12.6 and 11.0 km, respectively), they are theorized to have different internal structures, which alters the rate at which the black hole spirals inward. This distinction is imprinted on the emitted gravitational-wave signal — but can we tell the difference?

Revealed by Gravitational Waves

Amplitude of gravitational-wave signals as a function of frequency and black hole mass (left) and distance (right). Sensitivities of existing and future detectors are shown for comparison. Click to enlarge. [Adapted from Zou & Huang 2022]

Though the two scenarios produce gravitational waves with similar amplitudes, the shapes of the signals are different — and detecting them is not outside the reach of our current gravitational-wave observatories. The Laser Interferometer Gravitational-Wave Observatory (LIGO) should be able to detect these events up to 32,000 light-years away, depending on the black hole’s mass, while future detectors should be sensitive to events 10,000 times more distant.

Observing these curious collisions might tell us more than just the composition of the dense stellar remnants — it also might tell us something about the composition of dark matter. If we observe primordial black holes interacting with neutron stars — or strange stars — it could help constrain the abundance of primordial black holes and determine what fraction of the total mass of dark matter they comprise. With strange stars, exciting results are clearly the norm!

Citation

“Gravitational-wave Emission from a Primordial Black Hole Inspiraling inside a Compact Star: A Novel Probe for Dense Matter Equation of State,” Ze-Cheng Zou and Yong-Feng Huang 2022 ApJL 928 L13. doi:10.3847/2041-8213/ac5ea6

illustration of a binary star system

How do binaries in circular orbits come to be, especially for pairs of solar-type stars? Using stellar models, a researcher has brought us closer to understanding how the loss of energy in a system eventually leads it to circularize. 

Good Things Come in Pairs 

When you look out into the night sky, you may just see a bunch of singular points of light, but it is estimated that up to 85% of all stars are in multi-star systems — binaries, triples, or even quadruple systems. Around ~50% of stars like our Sun are actually in pairs. Binaries have been observed in all different shapes and sizes, from very elliptical orbits where the orbital period is on the order of multiple years to very close circular binaries that have orbital periods of days. Circular orbits could be caused by a number of physical mechanisms that gradually reduce a binary’s eccentricity over time, but we still don’t fully understand which mechanisms are at work or how. Adrian Barker (University of Leeds) has looked at circular solar-type binary systems to try to help solve the mystery. 

Lines that all start out together but diverge at different ages, some going vertical and some staying horizontal

The age of a star plotted against the quality factor, an inverse measure of dissipation efficiency. This shows that inertial wave dissipation is very efficient during the pre-main-sequence phase when the star is young but becomes less efficient later on in the stars’ lifetime. The different colored lines represent the different star masses in solar masses. [Barker 2022]

Making Waves in the Field of Binary Evolution 

One possible way that the orbits of binary stars could become circular over time is through tidal dissipation: the loss of energy in a binary system due to gravitational distortions of the member stars. Barker tested this theory by modeling solar-type and low-mass stars in binary systems with the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code. For the modeled binary systems, Barker studied the convection zones of the stars and worked out the tidal dissipation due to inertial waves — oscillations that take place within the star — in the systems. He also explored relations such as how stellar radius evolves with age and how the circularization period (the maximum orbital period in which binary orbits are perfectly circular) evolves with time.  

 

Multiple lines showing the different masses starting out as horizontal lines and all going vertical at different ages.

Age of the system plotted against the maximum binary period that circularizes, showing how energy dissipation from the inertial waves is capable of circularizing orbits out to 10-day periods before a few million years. The different colored lines represent the different star masses. [Barker 2022]

Coming Full Circle 

Previous theoretical work on this subject found that inertial wave dissipation in binary systems could help circularize binary orbits, but it didn’t seem strong enough to account for full circularization of solar-type main-sequence star binaries. While other studies searched for other mechanisms to explain it, this study concludes that tidal dissipation due to inertial waves is sufficient to explain the circularization of binary orbits and the synchronization of the spins of the stars. Barker also found that the maximum period that circularizes due to this mechanism increases with binary age, consistent with what observations of main-sequence binary systems have shown. 

While this study focused on a highly simplified model and involved proof-of-concept calculations, the author hopes that other studies will look at tidal dissipation in more detail with more sophisticated calculations while also focusing on the dynamical evolution of stellar populations.  

Citation 

“Tidal Dissipation Due to Inertial Waves Can Explain the Circularization Periods of Solar-type Binaries,” Adrian J. Barker 2022 ApJL 927 L36. doi:10.3847/2041-8213/ac5b63 

artist's impression of an earth-like planet orbiting a star

What determines whether a planet is dotted with continents or has oceans as far as the eye can see? Astronomers turn to models to understand how bulk planetary properties affect surface oceans on rocky, water-rich exoplanets.

Land Ho?

If a planet’s surface is perfectly smooth, water will wash across it to form a global ocean of constant depth. If instead the surface is roughened with mountains and valleys, the planet can host more water before its surface is fully covered. But what factors control whether a planet’s surface is flat and featureless or textured with topography, and thus how much water it can hold before becoming an ocean world?

cartoon of dynamic topography

A schematic showing the generation of dynamic topography [Adapted from Guimond et al. 2022]

A new article led by Claire Marie Guimond (University of Cambridge, UK) explores how the distribution of dry land and oceans on exoplanetary surfaces might be impacted by dynamic topography. Dynamic topography arises when a planet’s core is hotter than its surface, causing the material in between — the mantle — to undergo a slow churning on million-year timescales. This convection applies pressure to the planet’s crust, inducing upwellings and depressions up to a kilometer in size. Unlike plate tectonics, which is largely responsible for the diversity of topography on Earth but doesn’t occur on all planets, dynamic topography is expected to be a nearly universal feature of rocky planets.

plots of topography in meters as a function of planet age, mass, core mass fraction, and uranium and thorium abundances relative to solar

Plots of the change in a planet’s root-mean-square topography as a function of four potentially observable quantities. The quantities held fixed in each set of simulations are listed in the upper right corner of each panel. Click to enlarge. [Adapted from Guimond et al. 2022]

Modeling Mantle Convection

Guimond and collaborators first used complex two-dimensional numerical models to understand how a planet’s dynamic topography scales with factors like the thickness and composition of the mantle. These trends formed the backbone of less computationally expensive models, with which the team explored how a planet’s mass, age, radioactive element abundance, and core mass impact the size of its surface features.

The team found that the typical height of surface features increases with increasing planetary age and core mass and decreases with increasing planetary mass and radioactive element abundance. Planetary mass and radioactive element abundance had the strongest effect, suggesting that these factors are key to predicting a planet’s topography. The modeled surface features tended to be in the hundred-meter range, though the most massive planets had subtle hills and valleys of only a few tens of meters.

Weighty Water Worlds and Low-Mass Land Planets

plot of basin capacity, planet mass, and surface water fraction

Basin capacity as a function of planet mass. If a given surface water mass fraction exceeds the basin capacity, the planet will have a global ocean (labeled “water planets”). If the water mass fraction is less than the basin capacity, the planet will have dry land (labeled “land planets”). [Adapted from Guimond et al. 2022]

How much water can a planet hold before all its surface features are submerged beneath a global ocean? This quantity depends on the maximum height of the planet’s surface features, since a planet with an Everest-sized mountain will require more water to be fully covered than one with small rolling hills. Intriguingly, the authors found that the most massive planets can support the least water before turning into water worlds since they have the shallowest surface features.

Aside from dynamic topography, planets are likely to have many other surface processes at play, like impacts from asteroids and comets, volcanism, and plate tectonics, all of which might boost the amount of high-and-dry land available on a planet. The authors hope that further modeling as well as observations of exoplanets and Earth itself can increase our ability to predict the surface conditions of distant planets.

Citation

“Blue Marble, Stagnant Lid: Could Dynamic Topography Avert a Waterworld?,” Claire Marie Guimond et al 2022 Planet. Sci. J. 3 66. doi:10.3847/PSJ/ac562e

photograph of the solar corona during a total solar eclipse

How does the temperature of the solar atmosphere leap from 6000K to more than a million kelvin over a distance equal to just 0.3% of the Sun’s radius? Today’s article explores the coronal heating conundrum with a model that has earned its “awesome” moniker.

Heating Up the Solar Atmosphere

illustration of the Sun's layers

An illustration of the different layers of the Sun. The Sun’s atmosphere gets progressively warmer with increasing distance from the core, from the photosphere at 6000K, to the chromosphere at 10000K, to the corona at 1 million kelvin. [NASA]

Understanding how the Sun’s rarefied upper atmosphere, or corona, reaches its scorching million-kelvin temperature is one of the most important problems in solar physics. The corona is the site of explosive solar activity like solar flares and coronal mass ejections, as well as the launching-off point for the diffuse solar wind that suffuses the solar system.

Solar physicists have identified a wide array of processes that could potentially heat the corona. Undulating plasma waves that carry energy outward from the dense solar surface to the tenuous corona are one potential source of heating, but it’s unclear when or where this process might dominate over other heating methods. In a new publication led by Tong Shi (University of Michigan), a team of scientists has explored coronal heating by plasma waves with a high-resolution solar model.

plot of radial magnetic field strength for the Sun

The team elected to model a solar active region that appeared on July 13, 2018. The active region can be identified by the paired dark red and blue spots at a longitude of approximately 300. The color scale shows the strength and direction of the magnetic field in the radial direction in Gauss. Click to enlarge. [Shi et al. 2022]

Wave Simulations

The Alfvén Wave Solar Model (AWSoM) simulates heat transport by Alfvén waves — a type of plasma wave in which oscillating ions and magnetic fields pass energy back and forth. AWSoM has been used to model a variety of solar phenomena, including predicting the conditions the Parker Solar Probe would experience on its first close approach to the Sun.

Now, Shi and collaborators have applied AWSoM to a solar active region for the first time. AWSoM has performed well in its previous applications, but reproducing the conditions in and around an active region — an area of the Sun’s surface with particularly strong magnetic fields — is a critical test. Since AWSoM simulates the entire Sun rather than just a small area, attempting to capture the fine details of a complex active region promised to push the boundaries of AWSoM’s computational abilities.

From Global to Local

example images of the solar surface based on model output

Extreme-ultraviolet and X-ray images of the Sun from the Solar Dynamics Observatory (left column) and corresponding images based on model output (center and right columns). [Shi et al. 2022]

Shi and collaborators used AWSoM to model plasma densities and temperatures and produce outputs like spectra and ultraviolet images of the Sun’s disk that could be compared to spacecraft observations. Overall, the team’s results show that the Alfvén wave heating processes incorporated in their model are able to heat the solar corona to roughly the temperature we observe, even in the intricate magnetic loops that arc over the active region.

The authors estimate that by incorporating more levels of refinement in their model grid, they achieved their highest resolution ever, even approaching the levels seen in ultra-detailed images from spacecraft like the Solar Dynamics Observatory and Hinode. However, even AWSoM’s unparalleled resolution may not be sufficient to understand the physics of the transition region — a narrow range of altitudes between the solar surface and the corona over which the temperature abruptly increases. With further development, AWSoM should allow us to refine our understanding of coronal heating even more.

Citation

“AWSoM Magnetohydrodynamic Simulation of a Solar Active Region with Realistic Spectral Synthesis,” Tong Shi et al 2022 ApJ 928 34. doi:10.3847/1538-4357/ac52ab

illustration of an X-ray binary system

A new X-ray source in a Milky Way star cluster has puzzled astronomers. What’s the most likely explanation for this source’s unusual properties?

Things That Go Bump in the Night Sky

plot of X-ray flux for two time periods as well as model fits and residuals

NuSTAR X-ray spectra of MAXI J1848-015 during the outburst (blue) and as the outburst was fading (gold). Model fits are shown as dashed lines. [Pike et al. 2022]

In December 2020, the Monitor of All-sky X-ray Image (MAXI) spotted a new X-ray source from its vantage point on the International Space Station. Though the source — MAXI J1848-015 — was too close to the Sun for many telescopes to observe it, the Nuclear Spectroscopic Telescope Array (NuSTAR) managed to make follow-up observations a few days after the source was discovered, and again a week later.

The observations revealed that the source suddenly flared into view before beginning to fade just five days later. What kind of astrophysical object was responsible for this brief outburst? A team led by Sean Pike (California Institute of Technology) has performed an in-depth analysis of the object’s X-ray spectra to find out.

An X-ray Exploration

Pike and collaborators found that during the outburst, the source’s emission was dominated by soft, low-energy X-rays, peaking around 5 kiloelectronvolts. A week later, as the outburst was fading, harder, higher-energy X-rays surged, and though the source was fainter overall, its emission peaked between 20 and 30 kiloelectronvolts. This transition from soft and bright emission to hard and faint emission is typical for X-ray binaries — systems containing a compact object like a neutron star or black hole that is accreting material from a stellar companion.

illstration of a black hole with an accretion disk and a jet

This artist’s impression shows an outflowing jet of energetic particles and a bright corona, which appears just above an accreting black hole. [NASA/JPL-Caltech]

The authors modeled the spectra obtained during the soft and hard emission states and found further evidence for the X-ray binary scenario. Specifically, the team found that models containing a close-in accretion disk that reflects X-rays from a hot corona near the source fit the data well. Intriguingly, the models also showed evidence that the accretion disk changed as a result of the outburst — the innermost edge of the disk appears to have moved away from the accreting object in the later observations.

 

Neutron Star or Black Hole?

infrared image of a star cluster

The Spitzer Space Telescope discovered GLIMPSE-C01, the star cluster where MAXI J1848-015 resides, in 2004. [NASA/JPL-Caltech/H. Kobulnicky (Univ. of Wyoming)]

The observations are consistent with a binary system containing a star partnered with either a neutron star or a black hole. But which is it? Pike and collaborators considered several pieces of evidence:

  • Spin: The source spins almost as fast as is physically possible for an object of its size and angular momentum — far faster than even the most quick-whirling neutron stars.
  • Luminosity: Archival X-ray observations of the star cluster where the source is located show that when the object is in a quiescent state, its luminosity is roughly 1,000 times fainter than a typical neutron star.
  • Outburst length: The outburst is short compared to those from most — but not all — other accreting black holes.

Based on these findings, the authors rule that MAXI J1848-015 is most likely an accreting black hole, though some pieces of the puzzle aren’t a perfect fit. One lingering mystery is how an accreting black hole with a close-in accretion disk can have such a low luminosity. Catching another outburst in the act should help illuminate the nature of this intriguing source!

Citation

“MAXI and NuSTAR Observations of the Faint X-Ray Transient MAXI J1848-015 in the GLIMPSE-C01 Cluster,” Sean N. Pike et al 2022 ApJ 927 190. doi:10.3847/1538-4357/ac5258

illustration of a planet orbiting two stars

Can planets form in a tilted disk around a pair of stars? New simulations explore what happens in off-kilter planetary systems.

Seeking Skewed Systems

radio observations of the disk around a protostellar binary system

Radio observations revealed that the young binary system L1551 NE has a misaligned disk. [Adapted from Takakuwa et al. 2017]

Thanks to exoplanet-hunting spacecraft like Kepler, we’ve discovered more than a dozen planets orbiting two stars rather than one. Though all of the circumbinary planets discovered so far orbit close to the same plane as their host stars, theory suggests they could be found perpendicular to the plane of the binary system, and observations of tilted disks around young stars suggest that a wide range of inclination angles is possible.

The lack of planets seen in highly misaligned orbits might mean these planets are rare, but it could also mean that these planets are simply difficult to detect. With misaligned circumbinary systems still hard to come by, we must turn to models to explore what happens in these systems — what determines whether a planet orbiting two stars does so in the same plane as the binary system, perpendicular to it, or somewhere in between?

Modeling Misalignment

Anna Childs and Rebecca Martin (University of Nevada, Las Vegas) approached this question by modeling how the eccentricity of a binary system and the initial tilt of the surrounding disk affect the final orbital parameters of a planetary system. Childs and Martin investigated binary systems with perfectly circular orbits (e=0) and disks inclined by 30 and 60 degrees, as well as binary systems with very elongated orbits (e=0.8) and disks tilted by 60 degrees. In each case, the team initiated their simulations late in the process of planet formation, when the planetary systems are mostly free of gas and contain a host of Moon- and Mars-sized planetesimals.

example simulation results

Simulation results at three points in time for a circular binary system with a 30° inclined disk (top) and an eccentric binary system with a 60° inclined disk (bottom). Click to enlarge. [Childs & Martin 2022]

Their results showed that certain initial configurations generate only coplanar planets, while others churn out only perpendicular planets. In circular binary systems, disks that are tilted by 30 degrees tend to form planets that stick close to the plane of the binary, but if the initial tilt of the disk is cranked up to 60 degrees, collisions and gravitational interactions kick out 84% of the planet-forming material. In contrast, in eccentric binary systems, planets can still form in an extremely tilted disk — but in this case, the resulting planets orbit perpendicular to the binary system.

Giant Planets in the Mix

plot of simulation results

Degree of misalignment from a coplanar or polar orbit for simulations without (left) or with (right) giant planets introduced. The simulations shown are circular and inclined by 30 degrees (C30), circular and inclined by 60 degrees (C60), and eccentric and inclined by 60 degrees (P60). The symbol size and color varies with the planet mass. Click to enlarge. [Adapted from Childs & Martin 2022]

The authors also introduced Jupiter and Saturn analogs into some of their simulations to understand how the presence of giant planets affects the formation of planets in a misaligned disk. Giant planets tended to increase the amount of material kicked out of the planetary systems, but they also increased the rate at which the planetesimals collided, so the few planets remaining at the end of the simulations tended to be more massive.

Notably, almost all of the simulated planets fell into coplanar or perpendicular orbits. If future observations reveal planets at intermediate inclinations, this might mean that those planets followed a different formation pathway from the one explored in this study. Hopefully, it’s just a matter of time before we detect circumbinary planets in misaligned orbits and put our theories to the test!

Citation

“Misalignment of Terrestrial Circumbinary Planets as an Indicator of Their Formation Mechanism,” Anna C. Childs and Rebecca G. Martin 2022 ApJL 927 L7. doi:10.3847/2041-8213/ac574f

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

Does the spin of a dark matter halo align with the spin of the galaxy it’s situated around? And what can this tell us about the early universe? Hydrodynamical simulations of galaxies in the early universe might help us answer these questions. 

Bullet cluster (two galaxy clusters colliding)

The Bullet Cluster of galaxies. X-rays are shown in pink and the gravitational lensing is shown in blue. This cluster is considered one of the smoking guns for the presence of dark matter. [X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.]

No Longer in the Dark About Dark Matter 

Though the field of dark matter is expansive and exciting these days, astronomers didn’t even know dark matter existed until the 1980s. Now, dark matter can be used to probe everything from the force of gravity to galactic structure and evolution. Studying dark matter also has implications for cosmology and can help us better understand the initial conditions of the universe. Specifically, the alignment between the spins of a galaxy and its dark matter halo can help constrain the dark matter equation of state (which can tell us about the mass of the galaxy and help with predictions of its dynamics).

Taking Dark Matter for a Spin 

Galaxy with arrows on it showing how the galaxy is spinning.

Illustration of the spin of a galaxy. The velocity stays constant with radius, which shows that dark matter is present in the halo. [Adapted from ESO/L. Calçada; CC BY 4.0]

There are two main questions: first, how well do the observable spins of galaxies align with the spins of their dark matter halos, which can’t be measured? Studies exploring this question using numerical simulations have concluded that galaxies and their surrounding dark matter halos can be substantially misaligned. The second, potentially more important question is what this misalignment implies — if the spins are not aligned, does this mean that the motions of a galaxy’s stars are decoupled from changes in the background gravitational field? If so, this implies that we can no longer use the spin alignment of visible matter to probe the background cosmology. A team led by Jounghun Lee at Seoul National University aims to address this second question using hydrodynamical simulations to probe different scenarios. 

Illustrating What’s Happening Inside Galaxies 

Galaxies placed along filaments in the cosmic web

Simulation of the cosmic web. Every point of light is a galaxy and those galaxies are placed along filaments. [Illustris Project]

The team used the IllustrisTNG suite of simulations to model galactic dynamics. This software takes into account everything from star formation rate of the galaxy to feedback from supernovae and growth of black holes to model the physics going on. When the universe was ~9 billion years old, the luminous matter and various forms of stellar feedback like supernovae occurred along filaments in the cosmic web, which coupled the galaxy and dark matter halo spins, allowing us to probe early cosmology. However, going back to when the universe was ~5 billion years old, those filaments hadn’t yet formed and the matter density was fairly uniform, so these matter processes occurred randomly and didn’t have any structure to follow. This led to the uncoupling of the galaxy spin and the dark matter halo spin.

Lee and collaborators also find that properties such as black hole-to-stellar mass ratio, specific star formation rate (rate of creation of stars per unit stellar mass), and average metallicity either correlate or anticorrelate with the angle between the galaxy stellar and dark matter spins. 

Future work will involve finding direct evidence for the scenario of decoupling between the spins of the galaxy and its dark matter halo earlier in the universe’s history, modeling it, and exploring its connection to the initial conditions of the universe.

Citation 

“How Do the Galaxy Stellar Spins Acquire a Peculiar Tidal Connection?,” Jounghun Lee et al 2022 ApJ 927 29. doi:10.3847/1538-4357/ac4bda

hubble image of a galaxy cluster

How can we trace the formation of structure in the early universe? A new article surveys the environments around massive galaxies that existed less than two billion years after the Big Bang to learn more.

Questing for Quasars

image of a distant quasar

A cloud of gas surrounds the distant quasar SDSS J102009.99+104002.7 in this image from ESO’s Very Large Telescope. The name “quasar” is a shortening of “quasi-stellar radio source”, though we now know that only a small fraction of quasars are radio-loud. [ESO/Arrigoni Battaia et al.; CC BY 4.0]

One of the best ways to understand the conditions in the early universe is by studying quasars — extremely bright centers of young galaxies where supermassive black holes are accreting material. Looking back billions of years into the past, quasars appear not to be distributed randomly throughout space, which suggests that the massive galaxies they inhabit might be tracers of underlying dark matter structures. If this is the case, non-quasar-hosting galaxies in the early universe should also be found preferentially close to quasars.

Past studies have explored this hypothesis, but the results have been conflicting. Some studies have found that quasars have an abundance of galaxies in their vicinity, while others have found that there are no more galaxies than to be expected if they were randomly scattered throughout space. There are many potential reasons for this disagreement, including the possibility that dust hides these distant galaxies from the searching eyes of optical telescopes. In a new article, a team led by Cristina García-Vergara (Leiden Observatory, The Netherlands) approached this question in a new way — by using a massive array of radio telescopes to peer through the dust.

ALMA emission line maps and spectra

Sample emission line maps (left column) and spectra extracted from each source’s brightest pixel (right column) for the sources detected in this work. Click to enlarge. [Adapted from García-Vergara et al. 2022]

Long Wavelengths and Large Distances

García-Vergara and collaborators observed the areas surrounding 17 quasars with redshifts of z ~ 4 (roughly 1.6 billion years after the Big Bang) with the Atacama Large Millimeter/submillimeter Array (ALMA) — a collection of 66 radio telescopes working together as one. The team sought emission from a particular spectral line of carbon monoxide, which can signal the presence of a galaxy even if it is so shrouded in dust that it would be invisible at optical wavelengths.

Using a search algorithm, García-Vergara and collaborators identified all the sources of carbon monoxide emission in each quasar’s vicinity, finding a total of nine carbon monoxide-line-emitting galaxies among the 17 fields they searched. As is often important when studying sources billions of light-years away, the team also assessed the likelihood that the sources they detected are actually galaxies in the local universe masquerading as galaxies in the early universe and found it unlikely.

Galactic Neighbors

plot of the sources in relation to their central quasars

Distribution of the nine carbon monoxide sources (filled symbols) for all 17 quasar fields combined. The black diamond at the center indicates the location of the quasar, and the empty symbols indicate the locations of Lyman-alpha emitting galaxies from a previous study. [García-Vergara et al. 2022]

The team then used their observations to estimate how many more galaxies are present in the vicinity of quasars than would be expected if the galaxies were distributed randomly throughout space. Based on this analysis, the team found that there were 17.6 times more carbon monoxide-line-emitting galaxies in the areas surveyed than predicted by a random distribution.

Not only are there more galaxies than expected, they’re also tightly clustered around the quasars. These two pieces of evidence strongly support the idea that quasars are tracers of massive structures forming early in the universe, though the authors note that the statistical significance of their result could be improved by pushing the observations deeper or wider — hopefully the future will bring new observations and a fresh perspective on galaxies in the early universe!

Citation

“ALMA Reveals a Large Overdensity and Strong Clustering of Galaxies in Quasar Environments at z ∼ 4,” Cristina García-Vergara et al 2022 ApJ 927 65. doi:10.3847/1538-4357/ac469d

photograph of the rubin observatory telescope mount assembly as of april 2021

photograph of a mountain ridge with two telescopes

Rubin Observatory will sit atop the Cerro Pachón ridge, joining Gemini south and the Southern Astrophysical Research Telescope. [LSST Project Office; CC BY 4.0]

First light for the Vera C. Rubin Observatory atop Cerro Pachón in Chile is fast approaching — the eagerly awaited facility is slated to begin observations in 2024. The observatory’s 8.36-meter Simonyi Survey Telescope will carry out the Legacy Survey of Space and Time (LSST), a 10-year endeavor that will map the entire sky visible from its location every three days.

Rubin Observatory and LSST are poised to revolutionize astronomy in ways both anticipated and unexpected; through repeated observations, LSST will enable us to detect and monitor transient events like supernovae and gamma-ray bursts, map the Milky Way, probe the nature of dark energy and dark matter, and expand our catalogs of solar system objects by more than an order of magnitude. A new focus issue of the Astrophysical Journal Supplement Series explains how astronomers used science to guide the development of the upcoming survey.

map of the LSST footprint

LSST footprint showing the number of visits as a function of position on the sky. The acronyms refer to the surveys that fall under the LSST umbrella. Click to enlarge. [Bianco et al. 2022]

The survey — which is actually a combination of multiple surveys with specific goals — is notable not only for its breadth and depth, but also its design, which incorporated input from the astronomy community at every step. The articles in the new focus issue, six of which are already published, detail the scientific rationale behind the path the survey will take across the night sky, known as the observing cadence, and explore the potential science gains to will achieve. A few of the topics explored in this focus issue include:

  • Standard candles: RR Lyrae-type variable stars show correlations between the period of their variation, their luminosities, and their colors. These relationships allow RR Lyrae stars to be used as standard candles for determining the distances to other galaxies, so precisely determining the period of their variation is critical. LSST potentially occupies the sweet spot for studying RR Lyrae stars: wide enough coverage to allow for population-wide studies of these important stars, but with short enough cadence to capture subtle changes in the period and amplitude of their variations, the cause of which is still unknown.
  • illustration of a blazar

    Artist’s impression of the jet from an active galactic nucleus directed toward Earth. [NASA/Goddard Space Flight Center Conceptual Image Lab]

    Blazar variability: Blazars — relativistic jets pointed toward Earth that emanate from distant supermassive black holes that are actively accreting material — can vary in brightness on timescales from minutes to years. LSST is expected to observe thousands of known blazars while potentially discovering thousands more, enhancing our understanding of blazar variability as well as the environments they live in.
  • The unknown: Possibly even more enticing than the science advances we expect are those that we can’t even begin to imagine. How can a survey be optimized to discover something that we know nothing about? This issue is tackled by estimating how complete the survey will be in terms of the volume of space explored, the wavelengths covered, and other factors. They key to discovery might be switching things up; survey strategies that vary exposure time and time between observations find more novel sources than those that don’t.

Rubin Observatory is notable not only for the science data it will collect during the planned 10-year duration of LSST — roughly 300 petabytes’ worth (that’s 300,000,000,000,000,000 bytes) — but also what it plans to do with it. Each night, the observatory will release data related to millions of transient and variable objects to enable immediate follow-up of intriguing targets. The 20 terabytes of data collected nightly will be processed, stored, and shared with all scientists in the United States and Chile, as well as anyone affiliated with a number of participating institutions. Two years after its collection, the data will be made available to all. A decade of exceptional data, shared widely with the community and the world? It doesn’t get much better than that!

Citation

Articles in the Rubin LSST Survey Strategy Optimization Focus Issue will be collected here.

“Optimization of the Observing Cadence for the Rubin Observatory Legacy Survey of Space and Time: A Pioneering Process of Community-focused Experimental Design,” Federica B. Bianco et al 2022 ApJS 258 1. doi:10.3847/1538-4365/ac3e72

magnetar

New observations have captured pulses of radiation from a magnetic stellar remnant called a magnetar. What do these observations tell us about how magnetars and other neutron stars generate their beams of emission?

Rare Stellar Remnants

x-ray image of the vela pulsar

The Vela pulsar — seen emitting jets of fast-moving particles in this image — was the first pulsar to be detected at submillimeter wavelengths. Now, the first neutron star to pulsate in this wavelength range has been discovered. [NASA/CXC/Univ of Toronto/M. Durant et al.]

Neutron stars — ultra-dense, city-sized remnants of stars that exploded as supernovae — come in many flavors. Those that emit narrow beams of radio waves that sweep past Earth like the beacon of a lighthouse are called pulsars. Those that have extremely strong magnetic fields — 100 million times more intense than the strongest magnet ever made — are called magnetars. In rare cases, a neutron star can be both a pulsar and a magnetar!

It’s not yet clear how pulsars generate their beams of radio emission. One way to probe the generation mechanism is by studying the emission across a wide range of wavelengths, since some models predict that the emission should increase at a “turn-up” point somewhere between radio and infrared wavelengths. Previous observations have found tantalizing hints of this feature, but it has never been detected definitively. Can a new search at submillimeter wavelengths find the elusive turn-up point?

plot of submillimeter emission

Detection of XTE J1810−197 on 27 February 2020 at a wavelength of 0.85 mm. The panels show the target during a pulse (left) and not during a pulse (right) as well as the difference between the two states (bottom). Click to enlarge. [Adapted from Torne et al. 2022]

A Submillimeter Signal

A team of astronomers led by Pablo Torne (Institute of Millimeter Radio Astronomy, Spain; East Asian Observatory; and Max Planck Institute for Radio Astronomy, Germany) searched for signs of the turn-up point in observations of XTE J1810-197, one of only six neutron stars categorized as both a pulsar and a magnetar.

Torne and collaborators used telescopes across the globe to observe XTE J1810-197 over the course of 15 months. They detected a beam of emission swinging by for a few hundred milliseconds once per rotation period (5.54 seconds) at wavelengths ranging from 0.85 millimeters to 5.0 centimeters, marking the first time pulses from a neutron star have been detected at submillimeter wavelengths. However, they didn’t detect the pulses at 0.45 mm — the shortest wavelength searched in this study. What do these observations imply about the location of the turn-up point?

Where Will the Turn-up Turn Up?

plot of flux density versus frequency

Power-law (top) and broken power-law (bottom) spectral fits for XTE J1810−197. The downward pointing arrows indicate upper limits. [Torne et al. 2022]

Torne and collaborators found that XTE J1810-197’s emission is mostly flat across the wavelength range surveyed, with a potential downturn at longer wavelengths — and no sign of the turn-up point. If a turn-up is present, it might lie in the infrared or in the unexplored 0.37–3.00 cm (10–80 gigahertz) range.

However, searching these wavelength ranges may not be as simple as pointing the right kind of telescope at the target; magnetars are complicated, ever-changing objects that exhibit extremely energetic outbursts at short wavelengths and day-to-day variability across all wavelengths. This high level of variability can make determining the true shape of a neutron star’s spectrum challenging, since it might not be possible to compare measurements made at different times. (For example, astronomers have observed XTE J1810-197 in the infrared once before, but those observations were made when the magnetar was undergoing an explosive outburst.) However, with a little planning, simultaneous observations from radio to infrared could help us track down the turn-up point.

Citation

“Submillimeter Pulsations from the Magnetar XTE J1810-197,” Pablo Torne et al 2022 ApJL 925 L17. doi:10.3847/2041-8213/ac4caa

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