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A projection of the night sky and La Silla observatory

Chile’s Coquimbo region is renowned for its starry skies and is home to some of astronomy’s most powerful and productive telescopes. In a recent article, researchers quantified the impact of artificial lighting on sites across the Coquimbo region for the first time and showed that we must actively preserve dark skies in remote areas and urban corridors alike.

Measuring the Impact of Light Pollution

illustration of the number of stars seen at different levels of light pollution

An illustration of the Bortle scale value, which describes the degree of light pollution experienced in different settings. Click to enlarge. [ESO/P. Horálek, M. Wallner; CC BY 4.0]

Is the Milky Way visible from where you live? If so, consider yourself lucky — light pollution prevents a third of earthlings from seeing the Milky Way, including nearly 80% of North Americans and 60% of Europeans. The luckiest few are getting fewer, with less than 1% of residents in North America and Europe enjoying pristine night skies.

While city dwellers might make do with a dwindling number of visible stars, professional astronomical observatories rely on exquisitely dark skies. To understand the impact of light pollution on professional observatories as well as cities, Rodolfo Angeloni (Gemini Observatory) and collaborators assessed the degree of light pollution at four sites across Chile’s Coquimbo region: 1) Fray Jorge National Park, a certified Starlight Reserve; 2) Las Campanas Observatory, a professional astronomical observatory atop a mountain in the Atacama Desert; 3) Collowara Astrotourism Observatory, located near a city of 11,000 people; and 4) La Serena, a city of 450,000 people.

Angeloni’s team used ground-based all-sky imaging sensors to measure the brightness of the night sky at these locations on cloudless, moonless nights. By comparing the all-sky images to models of sky brightness and cross-referencing them with the positions of artificial light sources as seen from space, the team estimated the contribution of artificial light to the night sky brightness in each area.

Map of Chile's Coquimbo region showing the locations of the four sites studied in this work

Locations of Las Campanas Observatory (LCO), La Serena (LA-CQ), Collowara Astrotourism Observatory (CAO), and Fray Jorge National Park (FJNP) on a map of artificial night sky brightness. Click to enlarge. [Angeloni et al. 2024]

From Light to Dark

The measurements confirmed that Fray Jorge National Park is an exceptional dark sky site, with just 4% of the night sky brightness coming from artificial lights. (Scattered starlight, scattered sunlight, and airglow are natural sources of night sky brightness.) Fray Jorge National Park is one of the darkest measured sites in the world, and this study highlights the immediate need to protect this site from light pollution.

At Las Campanas Observatory, the future home of the 25-meter Giant Magellan Telescope, artificial lights contributed about 11% of the observed sky brightness, with the largest contributions coming from the cities of La Serena and Vallenar (117 and 49 kilometers away, respectively). Luckily, the impact of light pollution on the observatory is currently small, but the growth of nearby cities and the ongoing Pan-American Highway project could brighten the skies at Las Campanas.

Collowara Astrotourism Observatory, situated near the 11,000-person city of Andacollo, has artificial sky brightness comparable to that of Flagstaff, AZ, a much larger city. Another way of looking at this finding is that Flagstaff, named the world’s first Dark Sky Community in 2001, has night sky brightness comparable to that of a much smaller city — in other words, active efforts to reduce light pollution work, and cities worldwide can take proven steps to address the issue.

plot of the sky brightness at Las Cumbres Observatory with major contributors of natural and artificial sky brightness labeled

The sky brightness surrounding Las Campanas Observatory, with the major sources of natural and artificial sky brightness labeled. The Milky Way (MW) dominates at about 150 degrees, with La Serena (LS-CQ) just slightly less bright but extending to a lower altitude. The city of Vallenar is also a source of sky brightness. [Angeloni et al. 2024]

The skies surrounding La Serena are overwhelmingly brightened by artificial sources, and the impact of the bright city lights was felt far away; La Serena was the largest source of artificial sky brightness at the other three sites monitored. This result makes it clear that it’s important to reduce light pollution in overly bright areas like La Serena, as these bright regions impact the sky brightness in distant locales, and it’s critical to preserve the few truly dark sites that remain. Angeloni’s team plans to continue their efforts in the region by continuously monitoring the sky brightness in La Serena and installing 40 more sensors in dark sky sites.

Bonus

Check out the video below to see a time-lapse of the night sky conditions at Collowara Astrotourism Observatory.

Citation

“Toward a Spectrophotometric Characterization of the Chilean Night Sky. A First Quantitative Assessment of ALAN Across the Coquimbo Region,” Rodolfo Angeloni et al 2024 AJ 167 67. doi:10.3847/1538-3881/ad165c

An icy surface overlaid by a liquid water ocean. Beams of light are streaming through the water from the ocean's top towards the ice below.

One would be forgiven for thinking that it’s easy to tell different types of planets apart. Unfortunately, the most interesting worlds look awfully like comparatively boring ones to our telescopes. A well-studied planet, LHS 1140b, nicely illustrates this tension: after a re-analysis of archival data, it’s unclear if the planet has a potentially habitable surface ocean, or just a thin layer of hydrogen over a rocky surface.

Previous Work

A flux-vs-time plot showing a clean dip in the center.

Observations of four transits of LHS 1140b collected by the Spitzer Space Telescope. [Cadieux et al. 2024]

The star named LHS 1140 has become quite popular among exoplanet astronomers in the last seven years, and for good reason. While not quite as famous as its spotlight-stealing sibling TRAPPIST-1, LHS 1140 shares many of the same properties that make the former such a magnet for telescopes. At just under 50 light-years away, it’s nearby and fairly bright; at just a fifth the mass and radius of the Sun, it’s lightweight enough to be tugged around by tiny planets and small enough that those planets block a large fraction of its light when they transit. All of these characteristics combined make it an excellent host star for scientists looking to make sensitive measurements.

Consequently, LHS 1140 has been observed with a battery of instruments since the discovery of its two accompanying planets in 2017 and 2019. When analyzed one-by-one these datasets suggest that the two worlds, creatively christened in the typical exoplanet fashion as LHS 1140b and LHS 1140c, are a little bigger and heavier than our home world but basically made of the same combination of rock and metal. However, all of these observations hadn’t before been analyzed in a single, joint study.

Another Look

Charles Cadieux, University of Montreal, took up this challenge and led a team to reprocess nearly every byte of information collected on these worlds in an attempt to better constrain their masses and radii. They succeeded: as the authors put it, “the LHS 1140 planets are [now] among the best-characterized exoplanets to date, with relative uncertainties of only 3% for the mass and 2% for the radius.”

However, the team wasn’t aiming to shrink the old error bars just for precision’s sake. Instead, their ultimate aim was to constrain the composition of each planet. Although their analysis confirmed that LHS 1140c is likely a generic super-Earth, their new measurements placed LHS 1140b in a strange corner of parameter space.

A mass-vs.-radius plot. The oval-shaped contours denoting the uncertainty of LHS 1140 b parameters from this study lie between two underlying contours denoting different compositions.

The mass and radius of the LHS 1140 planets, along with contours belonging to different compositions. LHS 1140b falls just above the rocky line, but one the very edge of the gas-envelope region. [Cadieux et al. 2024]

They found that LHS 1140b must have a bulk density less than that of Earth but still much higher than those of the giant planets. After further modeling, the team was left with two very different scenarios for how a planet could arrive in this in-between gray zone. Either LHS 1140b has a very light, puffy atmosphere of hydrogen and helium overlaying a rocky surface, or, more exotically, LHS 1140b is a “water world,” likely covered in ice with a pocket of liquid water.

A Path Forwards

While it’d be thrilling to have a potentially habitable planet in our galactic backyard, our current data, even when collected with our best instruments and processed with the most cutting-edge techniques, is frustratingly unable to distinguish between the two scenarios. There is technically a path forward to resolving the uncertainty: the authors boldly advocate for an 18-transit observational campaign with JWST to confirm a thick, water-friendly atmosphere. However, with many planets to point to and galaxies galore to observe, there’s no guarantee that the community will choose to dedicate so much time to one target. We may have to live with just the hint that there’s a nearby alien ocean: a bitter, but awe-inspiring, ambiguity.

Citation

“New Mass and Radius Constraints on the LHS 1140 Planets: LHS 1140 b Is either a Temperate Mini-Neptune or a Water World,” Charles Cadieux et al 2024 ApJL 960 L3. doi:10.3847/2041-8213/ad1691

disk of hot gas swirling around a black hole

Sometimes, stars and black holes can happily coexist. Other times, the star gets ripped apart. In a recent research article, a team explored what can happen in between those two extremes.

Challenging Companions

Hubble Space Telescope image of the Milky Way's nuclear star cluster

Dense star clusters, like the Milky Way’s nuclear star cluster depicted at the center of this image, provide opportunities for black holes and stars to get dangerously close. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)]

Across the universe, stars are living precariously close to black holes. This is true everywhere from densely packed star clusters, where stars and stellar-mass black holes are linked together in binary systems, to the centers of galaxies, where stars circle supermassive black holes at dizzying speeds.

When stars and black holes meet, it can be spectacular: a black hole’s tidal forces can pull apart a star, resulting in a tidal disruption event that powers bright emission across the electromagnetic spectrum. Tidal disruption events can fully or partially disrupt a star, and they can occur around supermassive or stellar-mass black holes (the latter events are called micro tidal disruption events). Tidal disruption events take place when stars approach black holes on parabolic or highly elliptical orbits — what happens when the star’s orbit is nearly circular, instead?

Tidal Peeling Events

Chengcheng Xin (Columbia University) and collaborators used smoothed-particle hydrodynamics simulations to model the interaction between a star and a black hole in a close binary system. They modeled 96 combinations of starting parameters, changing the eccentricity of the system from a perfect circle to a slightly squashed one, varying the mass of the star from 1 to 15 solar masses, and exploring different values of the penetration factor: the ratio between 1) the distance from the black hole at which an object would be tidally disrupted and 2) the pericenter distance.

spiral gas structure created in a tidal peeling event

Example of the spiral gas structure created in a tidal peeling event. [Adapted from Xin et al. 2024]

Xin’s team showed that for certain orbital conditions — namely, stars on nearly circular orbits with low penetration factors, or stars on slightly more elongated orbits with penetration factors around one — the black hole gradually ensnares material from the star over the course of several orbits, leading to the star being partially or entirely disrupted. The authors dub these events tidal peeling events. Tidal peeling events leave behind a spiral of gas that can escape into space, form a disk around the black hole, or re-encounter the star as it orbits, forming a shock.

A New Kind of Transient Phenomenon?

From our distant vantage point, would we be able to distinguish a tidal peeling event from a micro tidal disruption event? Both phenomena exhibit intense electromagnetic radiation powered by accretion of stellar gas onto a black hole. The rates of accretion are similar between tidal peeling events and micro tidal disruption events, but their timescales and time evolution are different. Tidally peeled stars are stripped apart over the course of several short orbits — likely a few to a few tens of hours — while micro tidal disruption events can be slower to play out.

As this study is an initial exploration of tidal peeling events, there is still much to learn about their physical and observational characteristics. Future avenues for investigation include the impact of outflowing jets and winds, shocks that form when the star collides with its own tidal debris, and what changes might arise when simulations are started sooner, long before the star and the black hole are already positioned close together and poised for the peeling to begin.

Citation

“‘Tidal Peeling Events’: Low-eccentricity Tidal Disruption of a Star by a Stellar-mass Black Hole,” Chengcheng Xin et al 2024 ApJ 961 149. doi:10.3847/1538-4357/ad11d3

a narrow ribbon of gas that is part of the Cygnus Loop supernova remnant

A supernova is a spectacular way for a star to die. Massive stars meet this fate when the outward pressure exerted by core nuclear fusion can no longer hold off the gravity of the star’s outer layers, and the remnants of lower-mass stars can attain this honor through accretion or collisions. Today we’ll introduce four research articles that examine various aspects of supernova science, from attempts to determine how lightweight a supernova progenitor star can be to exploring why some massive stars don’t produce supernovae at all.

Probing the Smallest Supernova Stars

stellar evolution schematic

The main stellar evolution pathways. It’s not yet clear exactly which stars explode as supernovae and which become white dwarfs. Click to enlarge. [ESA]

Where is the dividing line between stars that end their lives in core-collapse supernovae and those that are fated to become white dwarfs? The least massive stars that undergo core collapse lie somewhere in the 8–12-solar-mass range, and refining the estimate further requires researchers to track down the faintest, most rapidly evolving supernovae.

Luckily, increasing coverage by transient-hunting surveys has generated a growing sample of these faint, fast events. Kaustav Das (California Institute of Technology) and coauthors studied nine supernovae detected by the Zwicky Transient Facility that were found to have certain chemical abundance ratios that hint at the progenitor stars being low mass. These supernovae are calcium-rich Type IIb supernovae, which have much larger [Ca II]/[O I] ratios than typical core-collapse supernovae.

Das’s team used spectra of each of these supernovae to measure the amount of oxygen present, which can be used in theoretical models to estimate the mass of the star that exploded. The mass estimates for all stars in the sample were less than 12 solar masses, suggesting that this type of supernova tends to arise from stars near the low-mass end of the progenitor mass range. The current sample of known calcium-rich Type IIb supernovae is still small, but future detections should allow researchers to refine models and improve estimates.

supernova remnant W49B

Not all massive stars end their lives as supernovae, leaving behind a supernova remnant like W49B shown here. [X-ray: NASA/CXC/MIT/L.Lopez et al.; Infrared: Palomar; Radio: NSF/NRAO/VLA]

Focusing on Failed Supernovae

When massive stars extinguish their core nuclear fusion and collapse, do they always generate luminous supernovae? Both observations and theory suggest that the answer is no, with some would-be supernovae forming a black hole with no accompanying supernova. Up to a third of massive stars might fail to generate a supernova!

Eric Coughlin (Syracuse University) performed a mathematical exploration of failed supernovae, focusing on the creation and propagation of a shock between the collapsing core and the outward-moving outer layers of the star. Coughlin showed that while some dying massive stars don’t produce supernovae per se, they still undergo an explosion that marks their impending demise. The strength of the explosion from a failed supernova depends on the properties of the star and how much of its mass is lost in the form of neutrinos: chargeless, nearly massless subatomic particles that rarely interact with matter.

In addition to the mathematical solutions that described the explosions, the equations also permitted a solution in which the matter settles near the central object. While we’ll have to wait for future work for a full examination of these solutions, it’s possible that they’ll apply to smaller stellar outbursts that do not destroy the star.

Prospects for Detecting Supernova Neutrinos

When a massive star’s core collapses, it can form a black hole or a neutron star: an extremely dense, rapidly spinning, city-sized sphere made almost entirely of neutrons. As protons and electrons are crushed into neutrons in the star’s core, the transformation produces neutrinos that push the star’s collapsing outer layers outward. While most of the star’s outer layers escape, forming the glowing, complex structures of a supernova remnant, a small fraction of the material falls back onto the protoneutron star, generating even more neutrinos.

calculated neutrino event rates for two detectors and two neutrino mass hierarchies

The event rates for a 1.98-solar-mass protoneutron star with various accretion rates as seen by Super-Kamiokande (left) and DUNE (right) and for normal (top) and inverted (bottom) neutrino mass hierarchies. [Akaho et al. 2024]

Ryuichiro Akaho (Waseda University) and collaborators calculated the likelihood of detecting the neutrinos that are produced when material rebounding from the collapsed stellar core falls back onto the core. Using detailed neutrino radiation–hydrodynamics simulations, the team modeled the fluxes and flavors of the neutrinos produced about ten seconds after the supernova occurs.

Akaho’s team found that the mass of the protoneutron star and the rate at which it gathers material from its surroundings both have an impact on the output neutrino luminosity and the average energy of the neutrinos. For a supernova happening about 33,000 light-years away, the neutrino flux should rise above the background measured by the existing Super-Kamiokande and under-construction Deep Underground Neutrino Experiment (DUNE) detectors. The exact strength of the signal depends on several factors, including neutrino oscillation — the process through which a neutrino born in a certain “flavor” morphs to a different flavor as it travels through space.

Investigating Type Ia Supernova Diversity

Supernovae aren’t always the result of massive stars collapsing. Many arise from white dwarfs, which are the exposed cores of low- to intermediate-mass stars that have finished fusing hydrogen in their cores and lost their outer layers. When a white dwarf accretes gas from a companion star, the white dwarf gains mass and heats up, eventually triggering a supernova. Alternatively, the collision of two white dwarfs can generate a supernova. Supernovae arising from white dwarfs are called Type Ia or thermonuclear runaway supernovae.

illustrations of the two main Type Ia supernova pathways

Illustrations of the two main Type Ia supernova pathways: the single-degenerate model (top) and the double-degenerate model (bottom). [Both images from NASA’s Goddard Space Flight Center Conceptual Image Lab]

Observations show that Type Ia supernovae have substantial variety in their light curves and properties, leading Mao Ogawa (Kyoto University) and collaborators to investigate the origins of this diversity. Ogawa’s team focused on the division between normal-velocity and high-velocity supernovae, which are differentiated by the velocity of their ejecta.

The team selected a sample of 14 Type Ia supernovae for which spectra were collected within one week of the explosion being detected at Earth. The sample included high-velocity supernovae, normal-velocity supernovae, and some that were similar to the peculiar supernova SN 1999aa. The team then used radiative transfer modeling to model the spectra and extract the properties of the supernova ejecta. Ultimately, they found that the supernovae fell into two groups: one with high-density, carbon-poor ejecta, which makes up the high-velocity sample and some of the normal-velocity sample and one that has low-density, carbon rich ejecta, which makes up the remaining normal-velocity sample and those like SN 1999aa. While more work remains to be done, the team suspects these two groups might be the result of different formation mechanisms.

Citation

“Probing the Low-Mass End of Core-Collapse Supernovae Using a Sample of Strongly Stripped Calcium-Rich Type IIb Supernovae from the Zwicky Transient Facility,” Kaustav K. Das et al 2023 ApJ 959 12. doi:10.3847/1538-4357/acfeeb

“The Division Between Weak and Strong Explosions from Failed Supernovae,” Eric R Coughlin 2023 ApJ 955 110. doi:10.3847/1538-4357/acf313

“Detectability of Late-Time Supernova Neutrinos with Fallback Accretion onto Protoneutron Star,” Ryuichiro Akaho et al 2024 ApJ 960 116. doi:10.3847/1538-4357/ad118c

“Systematic Investigation of Very-early-phase Spectra of Type Ia Supernovae,” Mao Ogawa et al 2023 ApJ 955 49. doi:10.3847/1538-4357/acec74

ultraviolet image of the spiral galaxy Messier 81

By studying the formation and evolution of galaxies in the early universe, researchers seek to test the predictions of our leading theory of cosmology. New research suggests that the ultraviolet luminosity of low-mass galaxies just a few hundred million years after the Big Bang may provide a way to differentiate between cosmological models.

Supersonic Flows in the Early Universe

snapshot of simulation output showing the dark matter and gas in two galaxy clusters

Simulation output showing the distribution of dense dark matter (orange), diffuse dark matter (blue), and high-velocity gas (white) in and surrounding two galaxy clusters. [TNG Collaboration]

The leading theory of cosmology, called ΛCDM, describes a universe in which the matter that we see and touch every day (so-called “normal” or baryonic matter), makes up just 15% of all matter. The remaining 85% is dark matter, the nature of which is still unknown. While dark matter doesn’t have much of an impact on daily life today — a little dark matter probably passed through you while you read this sentence, and I’m guessing you didn’t notice! — it played a leading role in the early universe, dictating when, where, and at what masses the first galaxies formed.

While galaxies today are nestled within dark matter halos and arrayed along strands of dark matter scaffolding, dark matter and luminous matter weren’t always so aligned; ΛCDM predicts that in the very early universe, just after protons and electrons came together to form atoms, there were places where normal matter moved relative to dark matter with immense speed — faster than the local speed of sound. This supersonic relative motion complicated the formation of the first stars and galaxies, potentially altering early galaxy formation in a measurable way.

simulation results with and without a difference in the velocities of dark and normal matter in the early universe

Comparison of the gas and stellar structures that form when the relative velocity between dark and normal matter is supersonic (left) and when there is no velocity difference between dark and normal matter (right). Click to enlarge. [Williams et al. 2024]

Suppression of Small-Scale Structure

Claire Williams (University of California, Los Angeles) and collaborators set out to understand the impact of this supersonic relative motion on the formation of low-mass galaxies. Using high-resolution fluid dynamics simulations, Williams’s team modeled the evolution of galaxies from a redshift of 200 to a redshift of 12. Their simulations tackled the cooling and condensing of molecular clouds, the ignition of the first stars, and the assembling of galaxies and star clusters.

The team found that having dark matter and normal matter moving at supersonic velocities relative to one another makes it harder for small galaxies to form; an order of magnitude fewer galaxies formed with stellar masses less than about 3 million solar masses when a velocity difference was present. Larger galaxies formed in the same numbers regardless of the velocity difference.

Clues from Ultraviolet Photons

measured and predicted ultraviolet luminosity functions

Modeled ultraviolet luminosity functions in the case of a supersonic velocity difference (pink line) and no velocity difference (blue line). The remaining colored data points are values derived from observations and the gray and black lines are model predictions. [Adapted from Williams et al. 2024]

What effect do these findings have on quantities that we could potentially measure? The velocity difference affects the simulated galaxies’ star formation, which impacts their ultraviolet luminosity. This in turn determines the number of galaxies present at each ultraviolet luminosity, otherwise known as the ultraviolet luminosity function.

Williams’s team found key differences in these quantities at a redshift of 12 in areas with the velocity difference and areas without. In regions without the supersonic flow, small galaxies and star clusters form readily and begin to churn out stars. In regions with the supersonic flow, small galaxies are uncommon, and the gas that would have gone toward the smallest galaxies is instead captured by and spurs star formation in slightly more massive galaxies, in the 10-million-solar-mass range. This means that the ultraviolet luminosity function is lower for the smallest, faintest galaxies but is boosted for slightly brighter and more massive galaxies at a redshift of 12.

Because ultraviolet photons emitted in the distant past are shifted to longer wavelengths when they reach us today, the ultraviolet luminosity function is potentially measurable at infrared wavelengths with JWST. Ultra-deep JWST observations such as the NGDEEP survey may reach ultraviolet magnitudes as faint as −14, potentially revealing the small galaxies that can probe early-universe cosmology.

Citation

“The Supersonic Project: Lighting Up the Faint End of the JWST UV Luminosity Function,” Claire E. Williams et al 2024 ApJL 960 L16. doi:10.3847/2041-8213/ad1491

artist's impression of colliding black holes

When researchers scour the detections of merging black holes made by gravitational wave observatories, they use models and statistics to make careful inferences about the population of black holes in our universe. In a recent article, researchers explored whether an emerging trend in gravitational wave data is real or an artifact of previous analysis methods.

A New Window on the Universe

Illustration of the first black hole merger detected by LIGO

Illustration of the first black hole merger detected by LIGO. [Aurore Simmonet (Sonoma State University)]

The detection of gravitational waves from merging black holes in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) gave scientists a new way to investigate black holes. By analyzing the spacetime ripples from colliding black holes, researchers hope to understand how the black holes formed (through the collapse of massive stars or the successive mergers of existing black holes?) and how they came to exist in binary systems (by first belonging to a stellar binary system or by forming solo and linking up with another black hole later?).

One potential result that has emerged from several analyses of gravitational wave signals is that the effective spins and the ratios of the masses of merging black hole binaries appear to be anticorrelated. But as with all results that are extracted delicately, statistically from complex data sets, it’s important to ask if this is a real feature of the data, with real implications for how black hole binary systems are assembled, or if it’s a result of our models or statistical analyses.

a diagram illustrating positive and negative effective spin

Top: The black hole spins are aligned with the system’s orbital angular momentum (positive effective spin). Bottom: The black hole spins are misaligned with the system’s orbital angular momentum (negative effective spin). [AAS Nova/Kerry Hensley]

Statistical Investigation

Christian Adamcewicz (Monash University and OzGrav) and collaborators approached this question by applying a new statistical treatment to detections of black hole mergers. This new treatment features a new model for effective spin and allows for a subpopulation of black hole binaries with zero effective spin, which hasn’t yet been ruled out and might have an impact that hasn’t been accounted for.

The team applied their population model to the third catalog of gravitational wave signals from the LIGO and Virgo detectors and used Bayesian statistical methods to extract the properties of the overarching population of black holes. They found that the previously reported anticorrelation between effective spin and mass ratio is likely real, ruling out the possibility of there being no correlation at 99.7% probability.

More Work, a Paradox, and Astrophysical Possibilities

Adamcewicz and collaborators acknowledge that this work doesn’t provide a final verdict on this question (as they put it, “a modeler’s job is never done”), and that other statistical effects need to be rooted out. One lingering possibility is that this result is due to the amalgamation paradox, which arises when trends present in different factors disappear or flip when the factors are considered together.

If the observed anticorrelation holds up to further statistical scrutiny, a number of astrophysical phenomena could be responsible for this effect. Extensive mass transfer between black hole progenitor stars, stars evolving within a common envelope and accreting matter at a high rate, or even black hole binary systems assembled within the accretion disks of active galactic nuclei should all be investigated with future black hole population models.

Citation

“Evidence for a Correlation Between Binary Black Hole Mass Ratio and Black Hole Spins,” Christian Adamcewicz et al 2023 ApJ 958 13. doi:10.3847/1538-4357/acf763

A photograph of a concrete tube extending to the horizon into low shrubs and swamp.

Joseph Giaime, a Professor of Physics at Louisiana State University, doesn’t sleep as soundly as he used to. The cause is obvious: every so often, his phone will emit a random, loud ping, regardless of the hour and without care for his desire to rest. He could turn these notifications off, but he purposefully chooses not to. In fact, he’s grateful for the recent disturbances: that’s because Giaime isn’t being woken up by a mindless news alert or a sleepless colleague, but by an automated system within the Laser Interferometer Gravitational-Wave Observatory (LIGO) informing him that it has just spotted a collision at the edge of eternity.

Giaime, like many physicists and astronomers, had to wait many decades to get these notifications, and now savors each one. What was once a just-out-of-reach dream has morphed into a steady stream of discoveries and phone alerts. Every few days LIGO reports that it heard the “sounds” of another merging pair of black holes, or occasionally a set of neutron stars that just smashed each other apart.

To better understand these extreme phenomena and how humanity got so good at detecting them, the AAS Press team led a group of reporters to the LIGO Livingston facility in southern Louisiana after the recent 243rd meeting of the AAS in New Orleans. Here, we provide a summary of that tour and what we learned from the dedicated scientists and support staff that worked for so long to hear the universe’s most distant rumbles.

Ripples in Reality

The “O” in LIGO denotes that the facility is an observatory, but unlike the vast majority of astronomical facilities throughout the world and in space, LIGO does not look for electromagnetic light. Instead, the massive 4-km-long arms that make up the Livingston facility were constructed to detect a different kind of wave: ripples in spacetime itself.

Two white spheres against a black background surrounded. by a purple spiral.

An artist’s rendition of two neutron stars spiraling towards each other and emitting gravitational waves. [ESA, CC BY-SA 3.0 IGO]

These wrinkles were first predicted by the theory of General Relativity nearly 100 years ago, though scientists were not audacious enough to go looking for them until the middle of the 20th century. The mathematics suggest that they are emitted when two massive objects collide and that they should travel at the speed of light. They also suggest that they should be extremely tiny, and therefore difficult to detect. There is an extreme contrast between the violence of the collisions and the strength of the signal detected here on Earth: after two objects both more massive than the Sun slam into each other at relativistic speeds, scientists can measure the lengths of their laser beams changing by only a fraction of a proton’s width.

The Road to Detection

Today’s reality where LIGO regularly hears these signals (one was detected just three days before our visit) is very unlike the previous several decades of quiet. LIGO’s origins lie in a proposal to the National Science Foundation (NSF) submitted in the late 1980s. The team broke ground at the two identical facilities in Washington state and Louisiana in 1994 and 1995, respectively, before many of the graduate students currently working on the data analysis were born. Then, from the early 2000s through 2010, both facilities “listened” to the universe, but heard nothing.

A photograph of 6 television monitors, each displaying a graph.

Monitors in the LIGO control room to track the real-time noise contributions of various sources, including “human activity,” “ocean waves,” and “earthquakes.” [Ben Cassese]

The detectors were simply not sensitive enough to pick out a gravitational wave from background sources of noise that could hide a signal, including vibrations in the ground caused by ocean waves rocking against the continent, occasional nearby logging operations, or moderately-sized earthquakes happening anywhere in the world (the recent large and deadly earthquake in Japan temporary halted all observations all the way in Louisiana).

Instead of backing away from their commitment, already one of the largest projects in their history, the NSF doubled down in 2010 and approved another tranche of funding to upgrade the facilities. LIGO went dark for five years as the collaboration overhauled numerous components, implementing both lessons learned during previous observations and new technologies that had been invented in the past decade. In 2015, they were ready to begin testing their new setup, dubbed “Advanced LIGO.”

Immediate and Continuing Payoff

In September, the collaboration was in the middle of an “engineering run,” meaning the detectors were fully operational but the team hadn’t yet transitioned into science operations. As has been well documented elsewhere, the impatient universe did not bother with their commissioning plans, and almost immediately the team recorded the distinctive and amazingly clear “chirp” of two merging black holes. “This [signal]was so big that we didn’t even need theorists” says Giaime, and immediately it became clear that the long wait for a gravitational wave signal was over.

Five printed photographs of technicians in bunny suits interacting with intricate components.

Photographs hung on the walls of the LIGO Livingston site depicting scenes from upgrades and maintenance over the years. Click to enlarge. [Ben Cassese]

Since that first Nobel-prize-winning detection, LIGO has undergone several minor upgrades, including a prolonged shutdown during the COVID-19 pandemic. These have steadily increased its sensitivity, which in turn has increased its rate of discoveries. After years of non-detections and over $1 billion of NSF funding, it stands as one of the US government’s greatest achievements in science funding and has opened a new window through which we can observe the cosmos.

Other facilities have already joined the search for gravitational waves, and have now detected them at other frequencies. The NANOGrav collaboration and their discovery of a gravitational wave background, for example, was the focus of the kickoff plenary session at the 243rd meeting of the AAS. Other proposed projects, such as LISA, should continue exploring yet more frequencies in the coming years. Each of these missions will stand on the shoulders of scientists working near the Louisiana swamps, and the LIGO Livingston facility will rightly be remembered as a historical site in science.

photo of the supernova remnant 30 Doradus B with a pulsar

A pilot survey using the world’s largest radio dish has led to the discovery of four pulsars, two of which are ultra-precise millisecond pulsars. This survey highlights the wealth of pulsars that await discovery at intermediate galactic latitudes.

Small Stars with a Big Impact

artist's impression of a pair of pulsars

An artist’s impression of a pair of pulsars. [Michael Kramer (Jodrell Bank Observatory, University of Manchester)]

When massive stars explode as supernovae, they can leave behind their extremely dense, collapsed cores in the form of neutron stars. Neutron stars spin rapidly and have strong magnetic fields, leading many of them to produce beams of radio emission along their poles. When these beams sweep across our field of view, we see brief, regular pulses of emission and call the objects pulsars.

Several thousand pulsars have been discovered in our galaxy, but there’s a need to find even more: pulsars provide a path to studying stellar evolution, the interiors of neutron stars, and even gravitational waves. Millisecond pulsars — those with the shortest rotation periods, around 10 milliseconds or less — are especially precious, as their pulses are exceptionally regular. By monitoring the arrival times of the pulses from many millisecond pulsars at once, researchers have found evidence for the gravitational wave background, which is thought to be the combined signals of millions of distant supermassive black hole binaries.

A Small FAST Survey

Where and how do we find pulsars? The word pulsar is short for pulsating radio source, and most pulsars are identified in surveys by their characteristic pulses of radio emission. Like most stars, pulsars are concentrated in the thin disk of our galaxy, but interstellar clouds of gas and dust in this region can scatter pulsar signals. Searching the area just above the galactic plane makes for easier pulsar discovery, and current evidence suggests that millisecond pulsars may be more common in these higher-latitude regions.

Plots of the four newly discovered pulsars' average pulse profiles

Pulse profiles of the four newly discovered pulsars. Click to enlarge. [Zhi et al. 2024]

Using the Five-hundred Aperture Spherical Telescope (FAST) — the world’s largest radio dish — Qijun Zhi (Guizhou Normal University) and collaborators searched for pulsars in a small area of the sky about 5 degrees above the galactic midplane. The survey discovered four new pulsars and recovered all seven of the known pulsars in the search area. Of the four newly discovered pulsars, two are of the coveted millisecond variety, with rotation periods of 3.9 and 4.6 milliseconds. One of these two millisecond pulsars especially warrants further study, since it is bright enough to possibly be included in pulsar timing arrays in the future.

More Pulsars to Come

Illustration of how galactic latitude is measured

Illustration of how galactic latitude is measured. [AAS Nova/Kerry Hensley]

The pilot survey described in this study complements the efforts of other pulsar surveys. FAST is currently at work on the Commensal Radio Astronomy FAST Survey and the Galactic Plane Pulsar Survey, both of which aim to find pulsars at galactic latitudes below 10 degrees. These surveys have led to the discovery of roughly 800 pulsars so far, about 200 of which are millisecond pulsars.

Zhi and collaborators expect that many more pulsars await discovery at intermediate galactic latitudes, 5 to 15 degrees above the midplane of the Milky Way. Considering the success of their limited pilot study, the team expects that roughly 900 millisecond pulsars could be found in that region.

Citation

“Discovery of Four Pulsars in a Pilot Survey at Intermediate Galactic Latitudes with FAST,” Q. J. Zhi et al 2024 ApJ 960 79. doi:10.3847/1538-4357/ad0eca

infrared image of stars near the center of the Milky Way

This was a year of superlatives in astronomy — researchers studied the brightest known gamma-ray burst, modeled the strongest material in the universe, and announced the finding of the first compelling evidence for the gravitational wave background. As 2023 draws to a close, let’s take a look back at some of the amazing science we covered on AAS Nova this year. Here are the top 10 most-read posts of 2023:

10. Focusing on the Brightest Gamma-ray Burst of All Time

An X-ray image of GRB 221009A's emission scattering off of dust

An X-ray image of GRB 221009A’s emission scattering off of dust. [Adapted from Williams et al. 2023]

The gamma-ray burst GRB 221009A exploded onto the scene in October 2022 and earned the moniker the BOAT — Brightest Of All Time. A Focus Issue of the Astrophysical Journal Letters highlighted the multifaceted, multinational, and multi-wavelength efforts to study this burst, including the hunt for an accompanying supernova, the search for neutrinos produced during the burst, and an assessment of whether GRB 221009A truly deserves its nickname.

Artist's impression of a pulsar

Artist’s impression of a neutron star, the core of a massive star that has exploded as a supernova. [ESO/L. Calçada; CC BY 4.0]

9. How to Model the Strongest Material in the Universe

The crystalline crust of a neutron star is the strongest material in the universe, and its extreme strength and density pose a challenge for modelers. Using a smoothed-particle hydrodynamics code tuned to include material strength, Irina Sagert and collaborators modeled waves in a neutron star’s crust. These waves might explain some properties of neutron star X-ray flares, and they may have an impact on the gravitational waves produced as neutron stars spiral toward a collision.

8. A New Way to Constrain Dark Energy

Andromeda Galaxy in ultraviolet

This ultraviolet mosaic of our galactic neighbor, the Andromeda Galaxy, is constructed from observations by NASA’s Swift Observatory. [NASA/Swift/Stefan Immler (GSFC) and Erin Grand (UMCP)]

Dark energy is thought to be responsible for the accelerating expansion of the universe. David Benisty, Anne-Christine Davis, and Wyn Evans measured the influence of dark energy in an entirely new way by modeling the orbits of the Milky Way and our neighboring galaxy, Andromeda. This method takes into account the outward pressure that dark energy exerts on the two galaxies as they slowly orbit one another. While the constraint placed by this method isn’t yet particularly stringent, the result agrees with measurements made on much larger scales, and upcoming data should allow the team to refine their results.

Illustration of TRAPPIST-1 and its seven rocky planets

Illustration of TRAPPIST-1 and its seven rocky planets. [NASA/JPL-Caltech/R. Hurt (IPAC)]

7. Monthly Roundup: TRAPPIST-1 Through the Eyes of JWST

In the first entry in the new Monthly Roundup series, we examined five research articles that tackled recent JWST observations of the TRAPPIST-1 system. The M-dwarf star TRAPPIST-1 became a household name when seven Earth-size planets were discovered in orbit around it. This large number of Earth-like, potentially habitable planets — as many as four of the seven may lie in the star’s habitable zone — makes TRAPPIST-1 a tempting target for atmospheric characterization, which is a challenge even for JWST’s gigantic mirror and sensitive instruments. The articles describe studies of JWST spectra of the innermost two planets, modeling of the possible atmospheres of the outer planets, and an exploration into whether we’d be able to detect life on these planets at all, if it exists.

6. First Look at Extragalactic Cepheid Variable Stars with JWST

Cepheid variable stars provide a powerful way to measure the distances to other galaxies. These stars vary in brightness in a predictable way, and how quickly their brightness varies is linked to their intrinsic luminosity. This method has been used to measure the rate of expansion of our universe, but the result disagrees with the value measured through other means, raising a long-standing problem dubbed the “Hubble tension.” Researchers plan to re-observe the Cepheid variable stars previously observed with the Hubble Space Telescope, in the hopes that JWST’s superior infrared capabilities can ease this tension — an effort that will take years. Wenlong Yuan and collaborators got a sneak preview of JWST’s abilities when the telescope observed a galaxy containing multiple Cepheids.

JWST image of the galaxy cluster SMACS

JWST image of the galaxy cluster SMACS J0723.3-7327. [NASA, ESA, CSA, STScI]

5. Update on JWST Observations of Galaxy Cluster SMACS 0723

The first JWST image seen by the public was of the spectacular galaxy cluster SMACS J0723.3–7327 (SMACS 0723). In the months following, astronomers studied the image from every possible angle, examining the structure of the cluster itself as well as the faint, distant galaxies whose light was curved into our line of sight by the cluster. This article introduced five research articles that advanced our understanding of the SMACS 0723 field, including weighing the galaxy cluster, investigating individual star clusters billions of light-years away, and measuring the chemical makeup of distant galaxies.

Wolf–Rayet star

The dusty ejecta of Wolf–Rayet stars makes for fantastic images. These stars will eventually explode as supernovae. [NASA, ESA, CSA, STScI, Webb ERO Production Team]

4. A Cosmic Dust Factory Ramps Up Production

Have you ever wondered where all the dust in the universe comes from? Megan Peatt’s team investigated the dust production in a rare binary system composed of a Wolf–Rayet star — a massive star that has lost its entire hydrogen envelope, leaving behind a scorching hot core wreathed in shells of gas — and an O-type star rotating so fast that it’s losing its grip on its atmosphere. Each time these stars get close, the collision of the Wolf–Rayet star’s fierce stellar winds with the O star’s decretion disk produces dust. The stars are scheduled for their next close approach in 2024, and astronomers have seized the opportunity to collect infrared measurements of the pair that reveal the amount and type of dust being created as production ramps up.

graphic showing the locations of stars in the pulsar timing array

Locations of pulsars (blue stars) in the NANOGrav pulsar timing array relative to the location of the Sun (yellow star). Some pulsar locations are approximate. Click to enlarge. [NANOGrav]

3. First Compelling Evidence for the Gravitational Wave Background

Using a closely monitored collection of rapidly spinning stellar remnants called pulsars, multiple international collaborations including the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, announced their discovery of compelling evidence for the long-sought-after gravitational wave background. This background signal, which is too low in frequency to be detected by gravitational wave observatories on Earth, is thought to be the rumblings of supermassive black hole binaries. The exact source of the gravitational wave background remains undecided, though, and more observations and modeling are needed to rule out the possibility that this background signal is the result of new physics.

2. Sandy, Briny Water on Mars Has a Better Chance of Remaining Liquid

image of the "dragon scale" texture on the surface of Mars

The “dragon scale” texture seen in this Mars Reconnaissance Orbiter image of Mars’s surface is the result of water interacting with bedrock, forming clay-containing rock. [NASA/JPL-Caltech/UArizona]

The presence of persistent liquid water on Mars is a matter of intense debate — it even merited its own round-table discussion at this year’s Division for Planetary Sciences meeting. Recently, laboratory experiments led by Andrew Shumway demonstrated that whether or not water is briny (i.e., salty, in the chemical sense) and mixed with Martian surface material, or regolith, plays a huge role in the conditions under which water remains a liquid. Briny water that seeps into regolith can remain a liquid under colder and drier conditions than pure water alone, suggesting that water could be more widespread on Mars than previously thought.

image of the black hole at the center of the Milky Way

The first image of the supermassive black hole at the center of our galaxy, constructed from data from the Event Horizon Telescope. [EHT Collaboration; CC BY 4.0]

1. Black Holes as the Source of Dark Energy

The most-read article on AAS Nova in 2023 linked two hot topics in astronomy: black holes and dark energy. Duncan Farrah and collaborators found that the supermassive black holes at the centers of galaxies across the universe are growing too quickly than could be accounted for by their galaxies’ supplies of gas and dust. Instead, Farrah’s team proposed that the growth of these black holes is linked to the expansion of the universe — a property belonging to a theorized type of black hole filled with vacuum energy. The expansion-linked growth of these black holes produces outward pressure that accelerates the expansion of the universe or, in other words, produces dark energy.

 

Thank you for joining us for another year of great science — we can’t wait to see the discoveries that 2024 will bring!

location of the X7 gas cloud relative to other sources near the center of the Milky Way

Editor’s Note: In these last two weeks of 2023, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume in January.

The Swansong of the Galactic Center Source X7: An Extreme Example of Tidal Evolution near the Supermassive Black Hole

Published February 2023

Main takeaway:

photographs of X7's thermal dust emission once a year from 2002 to 2021

X7’s thermal dust emission from 2002 to 2021. Click to enlarge. [Ciurlo et al. 2023]

Using two decades of observations, a team led by Anna Ciurlo (University of California, Los Angeles) characterized a galactic center source called X7, a cloud of gas and dust found near the Milky Way’s supermassive black hole. Over time, the cloud has become dramatically elongated, which the authors attribute to tidal forces from the black hole. While the origins and age of the cloud are uncertain, the authors propose that it is the gas ejected in a glancing collision between stars in a binary system.

Why it’s interesting:

The neighborhood surrounding a supermassive black hole is an exciting place: tightly packed stars undergo collisions or near misses, and some even get tidally shredded by the black hole itself. This dynamic environment changes considerably on short timescales, and not just in the astronomical sense — the stars closest to the Milky Way’s central supermassive black hole take scarcely longer than a decade to complete their orbits. Unlike other dusty objects in its neighborhood, like the “G” objects labeled in the header image above, X7 doesn’t appear to be bound to a star and instead looks to be a 50-Earth-mass blob of dusty gas roaming the galactic center.

More details on X7 and its possible age and origin:

The rapid stretching of X7 over the past two decades suggests that the cloud is unlikely to remain intact as it nears the Milky Way’s supermassive black hole. This sets an expiration date for the cloud of 2036, when it will make its closest approach, and it also sets a maximum age for the cloud of around 200 years, given its orbital period. X7 is likely the result of a recent event, then, and the similarity between the orbits of the tip of the X7 gas cloud and a dusty stellar source called G3 raises the intriguing possibility that X7 was ejected when G3 was created in a stellar merger.

Citation

Anna Ciurlo et al 2023 ApJ 944 136. doi:10.3847/1538-4357/acb344

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