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simulations of radio jets

Simulated radio intensity of jets at various viewing angles

Simulated radio intensity of jets at various viewing angles. Click for high-resolution version. [Nolting et al. 2023]

At the centers of many galaxies, supermassive black holes accrete gas from their surroundings and shoot out a pair of immense jets of plasma. In some cases, these jets extend in opposite directions from the galaxy’s center, such as in the classic example of Hercules A. Jets aren’t always so well behaved, though, and researchers have found examples of jets that snake and zigzag into S, X, and Z shapes. Using fluid dynamics simulations, Chris Nolting (Los Alamos National Laboratory and College of Charleston) and collaborators explored possible causes of this alphabet soup of shapes, focusing on precession: slow changes in the axis of rotation of a spinning object. For the case of radio jets, precession could be caused by accretion instabilities or by a black hole companion to the central supermassive black hole. The simulation results, some of which are presented in the images above and to the right, show that a single jet configuration can appear vastly different when viewed from different angles. Unexpectedly, the team also found that jets can sometimes take sharp turns that can mimic the effects of outside forces, like shocks or winds.


The team’s simulations are animated here, showing how the shapes of the radio jets change with time as well as with the viewing angle.


“Simulations of Precessing Jets and the Formation of X-shaped Radio Galaxies,” Chris Nolting et al 2023 ApJ 948 25. doi:10.3847/1538-4357/acc652

an ultra-thin galaxy seen edge on, with a potential satellite galaxy indicated off to the side

Though the ultra-thin galaxy UGC 11859 looks perfectly flat in the image above, close analysis has revealed warps and flares in its disk. These imperfections provide clues to the galaxy’s history, as the imprints of past gravitational interactions take billions of years to fade from the disk’s faint outer regions. Luis Ossa-Fuentes (University of Valparaíso and Valencian International University) and collaborators observed UGC 11859 with the 10.4-meter Gran Telescopio Canarias, aiming to study the galaxy’s structure. They found that the galaxy’s brightness doesn’t decrease smoothly from its center to its outskirts, but instead drops off suddenly about 78,000 light-years from the center. On top of that, the left side of the galaxy is tipped upward, and the distribution of stars flares out above and below the plane of the galaxy toward either side. While it remains to be seen if these features are related, it’s clear that there’s more to this galaxy than meets the eye. To learn more about the subtle structure of UGC 11859, be sure to check out the full article linked below.


“Flares, Warps, Truncations, and Satellite: The Ultra-thin Galaxy UGC 11859,” Luis Ossa-Fuentes et al 2023 ApJ 951 149. doi:10.3847/1538-4357/acd54c

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

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