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representative-color image of the galactic plane showing gravitational wave sources

representative-color images of the galactic plane showing gravitational wave sources

Representative-color gravitational wave images of the Milky Way created with simulated data from LISA (top) and AMIGO (bottom). Click to enlarge. [Szekerczes et al. 2023]

What would our galaxy look like if we could see gravitational waves? A recent research article by Kaitlyn Szekerczes (NASA’s Goddard Space Flight Center) and collaborators explores that question, using simulations of data from upcoming gravitational wave observatories to create representative-color images of the Milky Way. So far, all of the gravitational wave sources detected and identified by our current facilities gave been located outside the Milky Way. However, planned and proposed observatories such as the Laser Interferometer Space Antenna (LISA) and the Advanced MilliHertz Gravitational-wave Observatory (AMIGO) will observe lower-frequency gravitational waves, cluing us in to the steady winding-down hum of ultra-compact binary systems containing black holes, neutron stars, white dwarfs, and supergiant stars. The images above and to the right show 1,000 simulated sources drawn from a detectable population of more than ten thousand ultra-compact binaries, with the amplitude and frequency of each source’s gravitational waves represented by the intensity and color, respectively, of the data points. From these images, it’s clear that the advent of low-frequency gravitational waves will gives us a whole new way to study our home galaxy.


“Imaging the Milky Way with Millihertz Gravitational Waves,” Kaitlyn Szekerczes et al 2023 AJ 166 17. doi:10.3847/1538-3881/acd3f1

Two Hubble images of a section of the Cygnus Loop, superimposed atop one another

Hubble images of a section of the Cygnus Loop supernova remnant

Top: H-alpha image of a segment of the supernova remnant in 2001. The numbers along the top indicate the proper motion, in milliarcseconds per year, of that section of the shock front. Bottom: H-alpha images from 2020 (red) and 2001 (cyan). Click to enlarge. [Sankrit et al. 2023]

If we could see the Cygnus Loop supernova remnant without the aid of a telescope, it would span an area of the sky six times as wide as a full Moon. The Cygnus Loop is a wispy, rapidly expanding shell of gas that marks the grave site of a massive star that exploded some 20,000 years ago. In a recent research article, Ravi Sankrit (Space Telescope Science Institute) and collaborators present new Hubble Space Telescope observations of a small portion of this famous supernova remnant. Paired with previous Hubble observations from 2001 and 1997, the new images clearly demonstrate how the remnant’s shock front has expanded over time. In the images above and to the right, red and cyan mark the position of the shock front in 2020 and 2001, respectively. By analyzing the shock’s location, Sankrit’s team found that the shock hasn’t slowed at all over the past 22 years, speeding into interstellar space at 240 kilometers each second. While this seems incredibly fast, it’s actually on the slow end for a supernova shock wave. To learn more about these new observations of the Cygnus Loop, be sure to check out the full article linked below.


“Third Epoch HST Imaging of a Nonradiative Shock in the Cygnus Loop Supernova Remnant,” Ravi Sankrit et al 2023 ApJ 948 97. doi:10.3847/1538-4357/acc860

two images of a galaxy observed with the Hubble Space Telescope

To track down galaxies in the early universe, astronomers search for Lyman-alpha emission, which is generated by electrons in hydrogen atoms sliding down to their lowest energy level. Although this method is commonly used to find galaxies, it can be difficult to link the properties of the Lyman-alpha emission to those of the galaxy because the photons are are absorbed, scattered, and re-emitted as they travel from their birthplaces in the surroundings of hot, young stars to our telescopes. To understand how Lyman-alpha emission reflects the properties of distant galaxies, Jens Melinder (Stockholm University) and collaborators surveyed Lyman-alpha-emitting galaxies in the nearby universe. The team observed 45 nearby galaxies with the Hubble Space Telescope and used models to determine their properties. The images above show one galaxy from the sample in two ways: the left-hand image shows the stellar continuum emission captured by Hubble’s broad filters, while the right-hand image shows a combination of ultraviolet stellar emission and narrow emission lines (including Lyman alpha in blue) from glowing hydrogen gas. Using these observations, the team determined that the dustier the galaxy, the less Lyman-alpha emission makes it to our telescopes, and the same may be true for galaxies containing more stars. To learn more about the results of this survey, be sure to read the full article linked below.


“The Lyα Reference Sample. XIV. Lyα Imaging of 45 Low-redshift Star-forming Galaxies and Inferences on Global Emission,” Jens Melinder et al 2023 ApJS 266 15. doi:10.3847/1538-4365/acc2b8

still images showing the results of two computer simulations of black hole accretion

We know that the supermassive black holes at the centers of galaxies can ensnare nearby gas and consume it; we see the doomed gas glow brightly as it advances toward the black hole. But exactly how a black hole’s meal makes its way toward the waiting gravitational maw isn’t clear. Are small gas clumps plucked at random from larger gas clouds, or does gas assemble into an orderly disk before falling into the black hole? In a recent research article, a team led by Minghao Guo (郭明浩) from Princeton University used fluid dynamics simulations to explore how gas accretes onto the supermassive black hole at the center of the massive elliptical galaxy Messier 87. The images above each show a region 6,500 light-years across that is centered on the supermassive black hole, with a zoomed-in 650-light-year region shown in the corner. The images show the two main pathways of cold gas accretion: chaotic accretion (left), which occurs only 10% of the time, and disk accretion (right), which is the dominant way for cold gas to be accreted. To learn more about the dynamics of gas accretion near a black hole, be sure to read the full article linked below.


“Toward Horizon-scale Accretion onto Supermassive Black Holes in Elliptical Galaxies,” Minghao Guo et al 2023 ApJ 946 26. doi:10.3847/1538-4357/acb81e

Images of dust emission from four nearby galaxies

Cosmic dust makes up just a small fraction of a galaxy’s mass, but it provides a useful way to study the interstellar medium: the gas and dust from which new stars and planets form. Using maps created from Herschel Space Telescope data presented in an earlier article, a collaboration led by Christopher Clark (Space Telescope Science Institute) studied the dust in the Large Magellanic Cloud, the Small Magellanic Cloud, the Andromeda Galaxy, and the Triangulum Galaxy (from left to right; not to scale). The three-color images above show hydrogen gas (red), cool dust (green), and warm dust (blue), highlighting how the dust density and temperature varies from galaxy to galaxy. The team used the new maps to study the galaxies’ dust-to-gas ratio — an important descriptor of the interstellar medium — and found that the ratio increased as the density of the interstellar medium increased. This trend might suggest that dust grains can bulk up more readily by nabbing material from the surrounding gas when the interstellar medium is denser. To learn more about the evolution of dust in our neighboring galaxies, be sure to check out the full article linked below.


“The Quest for the Missing Dust. II. Two Orders of Magnitude of Evolution in the Dust-to-gas Ratio Resolved within Local Group Galaxies,” Christopher J. R. Clark et al 2023 ApJ 946 42. doi:10.3847/1538-4357/acbb66

simulations of neutron stars merging

When a massive star ends its life in a supernova explosion, it can leave behind a tiny, dense remnant called a neutron star. Sometimes, two neutron stars end up locked in a gravitational embrace, emitting gravitational waves as they dance toward each other over millions of years. When the pair finally meets, their collision lights up the electromagnetic spectrum and creates heavy elements like gold and platinum. In a recent research article, Luciano Combi (Argentine Institute of Radio Astronomy, Perimeter Institute for Theoretical Physics, and University of Guelph) and Daniel Siegel (Perimeter Institute for Theoretical Physics, University of Guelph, and University of Greifswald) simulated the nuclear reactions and electromagnetic radiation produced after the merger of a pair of neutron stars. The image above illustrates four stages of their simulation, from the moment before the neutron stars meet, when their mutual gravity stretches them into teardrop shapes, to the merger aftermath, when an accretion disk feeds the sole remaining star. To learn more about the simulations described above, be sure to check out the full article linked below!


“GRMHD Simulations of Neutron-star Mergers with Weak Interactions: r-process Nucleosynthesis and Electromagnetic Signatures of Dynamical Ejecta,” Luciano Combi and Daniel M. Siegel 2023 ApJ 944 28. doi:10.3847/1538-4357/acac29

simulation of the formation of the first stars

grid showing the simulation results for the particle number density under different values of rotation rate

Simulation results showing the number density of gas particles for 16 simulations with different values of β, which increases as the rotation rate of the cloud increases. Notice that the scale of the images changes as β increases. Click for high-resolution version. [Raghuvanshi and Dutta 2023]

When the first stars in the universe formed, ending millions of years of darkness, what masses did they have? This question is more than a matter of idle curiosity: if any of the first stars formed with masses less than 0.8 solar mass, they would still exist today. In a recent research article, Shubham Raghuvanshi and Jayanta Dutta (both from the Harish-Chandra Research Institute in India) performed hydrodynamic modeling to test how the rotation of a primordial gas cloud affects the resulting masses of the first stars. The images above and to the right show the results of their simulations after 50 solar masses of gas had been collected by the newly forming protostars. Ultimately, Raghuvanshi and Dutta found that in the fastest-spinning clouds, 10–12% of young stars might be ejected before they can grow past 0.8 solar masses. This suggests that if early gas clouds spun fast enough, some of the most ancient stars might still exist in modern galaxies, waiting to be found. To learn more about how the team modeled the making of the first stars, be sure to check out the full article linked below!


“Simulating the Collapse of Rotating Primordial Gas Clouds to Study the Possibility of the Survival of Population III Protostars,” Shubham P. Raghuvanshi and Jayanta Dutta 2023 ApJ 944 76. doi:10.3847/1538-4357/acac30

When a gas cloud collapses and fragments into stars, what masses do those stars have? The mass distribution of newborn stars in a cluster is called the initial mass function. While the initial mass functions of different star clusters appear to be similar, it’s still unclear as to whether certain factors, such as the abundance of elements heavier than helium (what astronomers call metals), can affect the mass distributions of stars when they’re born. To test the possibility that the abundance of metals affects the initial mass function, Chikako Yasui (National Astronomical Observatory of Japan) and collaborators searched the outskirts of the Milky Way for star clusters poor in metals, close enough that individual stars can be studied, and young enough that the stellar population still reflects the cluster’s initial mass function. The image above shows the team’s near-infrared observations of two suitable star clusters, which contain ~350 and ~1,500 stars each and are just 3 and 5 million years old, respectively. The team’s results suggest that these clusters might contain more high-mass stars than clusters rich in metals do, but further work is necessary to confirm this result. To learn more about this study, be sure to check out the full article linked below!


“Mass Function of a Young Cluster in a Low-metallicity Environment. Sh 2-209,” Chikako Yasui et al 2023 ApJ 943 137. doi:10.3847/1538-4357/ac94d5

composite X-ray, optical, and radio image of the galaxy NGC 253

The Silver Coin Galaxy, also known as NGC 253, is one of the nearest examples of a starburst galaxy — one that forms new stars faster than typical galaxies. In visible light, the nearly edge-on Silver Coin looks like a bright, narrow ellipse mottled with dark dust clouds. X-ray data tell a different story, though, as the image above shows. While the optical emission (H-alpha; green) is confined to the galaxy’s tilted disk, the X-ray emission (blue) extends perpendicular to the disk, tracing immense outflows powered by the galaxy’s fervent star formation. Millimeter emission (red) rounds out the three-color image. Using images and spectra from the Chandra X-ray Observatory, Sebastian Lopez (The Ohio State University) and collaborators investigated the physical properties of the galaxy’s outflows, finding that the galactic winds expel roughly 6 solar masses of gas each year. Spectral analysis revealed that the innermost region of the outflows are chemically enriched, providing a potential source for the metals found in the sparse gas between the Milky Way and its galactic neighbors. For more details about this windy starburst galaxy, be sure to check out the full article linked below!


“X-Ray Properties of NGC 253’s Starburst-Driven Outflow,” Sebastian Lopez et al 2023 ApJ 942 108. doi:10.3847/1538-4357/aca65e

composite optical and X-ray image of the Guitar Nebula

Roughly 2,700 light-years from Earth, a pulsar streaks across space at a blistering 765 kilometers per second. Its rapid motion through the interstellar medium creates a variety of structures, including the aptly named Guitar Nebula — an ever-expanding bow shock that billows in the pulsar’s wake, resembling the narrow head, long neck, and wide body of a guitar — and a filament that shines in X-ray light. In a recent publication, Martijn de Vries (Stanford University) and collaborators presented new Hubble Space Telescope and Chandra X-ray Observatory images of the pulsar and its surroundings, gaining a fresh perspective on a region that has been observed for more than 25 years. The new optical and X-ray data, combined with observations from the Panoramic Survey Telescope and Rapid Response System into the composite image shown above, allowed the team to analyze the subtle evolution of the Guitar Nebula. Among other changes, the team observed a ridge of bright emission moving through the X-ray filament, which they attributed to an injection of electrons and positrons decades ago. To learn more about the latest investigation of the Guitar Nebula — and to see the full version of the image above — be sure to check out the article below!


“A Quarter Century of Guitar Nebula/Filament Evolution,” Martijn de Vries et al 2022 ApJ 939 70. doi:10.3847/1538-4357/ac9794

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