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image of an ultra-diffuse galaxy

Ultra-diffuse galaxies are as large as the Milky Way but contain only a smattering of stars and gas, scarcely enough to fill a dwarf galaxy. Observations suggest that these oddly faint galaxies are common, leaving astronomers puzzling over how they form. To disentangle the history of the ultra-diffuse galaxy UGC 9050-Dw1, Catherine Fielder (Steward Observatory) and collaborators tracked down its globular clusters: spherical collections of hundreds of thousands of stars that can hold clues to a galaxy’s star formation and merger history. The team identified 52 globular clusters (cyan circles) in new observations from the Hubble Space Telescope, shown above. This is an exceptionally large number of globular clusters for a galaxy of UGC 9050-Dw1’s brightness; 20% of the galaxy’s light comes from globular clusters! The clusters are all roughly the same color, which suggests that they are composed of stars of the same age and metallicity. This hints that UGC 9050-Dw1’s globular clusters formed in a single burst of star formation, which could have been triggered by a merger. Fielder and collaborators favor the merger of dwarf galaxies as an explanation for UGC 9050-Dw1’s unusual properties, adding yet another possible formation mechanism for ultra-diffuse galaxies to the list. To learn more, be sure to check out the full research article linked below.

Citation

“The Disturbed and Globular-Cluster-Rich Ultradiffuse Galaxy UGC 9050-Dw1,” Catherine E. Fielder et al 2023 ApJL 954 L39. doi:10.3847/2041-8213/acf0c3

X-ray image of sources in NGC 4214

An X-ray view of the dwarf starburst galaxy NGC 4214 created from Chandra X-ray Observatory Data. [Adapted from Lin et al. 2023]

Ten million light-years from Earth, a tiny galaxy that glitters with new stars houses a rare kind of binary system. The binary, cataloged as CXOU J121538.2+361921 and referred to simply as X-1, is the brightest X-ray point source in its home galaxy. The system’s bright X-ray light is the result of a star having its atmosphere stolen by a compact companion, like a black hole or a neutron star, creating an extremely hot accretion disk. In a recent research article, a team led by Zikun Lin (Key Laboratory of Optical Astronomy, Chinese Academy of Sciences; University of Chinese Academy of Sciences) used data from the Chandra X-ray Observatory and the Hubble Space Telescope to learn more about this unusual system. The Chandra data, shown to the right, allowed the team to confirm that the binary components eclipse each other every 3.6 hours. The Hubble data, shown above, allowed them to identify the optical counterpart of the X-ray binary for the first time: a blazingly hot blue star that is likely a massive, evolved star with powerful winds and a metal-rich atmosphere. In some tens of millions of years, this star and its companion — the team suspects a black hole, though a neutron star can’t yet be ruled out — will lose their orbital energy to gravitational waves and coalesce in a spectacular cosmic explosion.

Citation

“On the Short-period Eclipsing High-mass X-Ray Binary in NGC 4214,” Zikun Lin et al 2023 ApJ 954 46. doi:10.3847/1538-4357/ace770

map of the surface of the minor planet Vesta, with the surface subdivided into dozens of different regions based on origin and composition

hybrid map of Vesta's surface

The final hybrid map, showing Vesta’s polar regions in the top two views and equatorial regions at the bottom. Click for high-resolution version. [Yingst et al. 2023]

In 2011, the Dawn spacecraft began its 14-month survey of the minor planet Vesta, giving us the clearest look yet at Vesta’s surface. Dawn collected data on Vesta’s surface color, roughness, and composition, and mapped its large-scale surface features. While these observations presented an excellent opportunity to study one of the largest objects in the asteroid belt, it also presented a challenge: how do you combine different types of data to maximize the amount of information presented? Using data from the Dawn mission, Aileen Yingst (Planetary Science Institute) and collaborators first made two separate maps that divided Vesta’s surface into dozens of regions based on 1) the color of the surface and 2) large-scale surface features like craters. To combine these maps into one hybrid map, shown above and to the right, the team used a decision tree to determine how to combine the different data sets to characterize each region. The resulting map includes 18 types of terrain, distinguished by both the type of landform (craters, highlands, and more) and the color. For more details about this mapping method and what it revealed about Vesta’s surface, be sure to read the full research article linked below.

Citation

“A Geologic Map of Vesta Produced Using a Hybrid Method for Incorporating Spectroscopic and Morphologic Data,” R. Aileen Yingst et al 2023 Planet. Sci. J. 4 157. doi:10.3847/PSJ/acebe9

images of the newly discovered Einstein cross system

As the light from a distant galaxy travels toward us, it sometimes encounters a region of spacetime warped by a massive galaxy in its path. If the alignment between the foreground galaxy and the background galaxy is just right, we’re treated to a spectacular sight: multiple images of the background galaxy arrayed around the foreground galaxy — a phenomenon called gravitational lensing. If the foreground galaxy is elliptical, the images of the background galaxy form a cross known as an Einstein Cross, as is the case with a system newly confirmed by Aleksandar Cikota (Gemini Observatory/NSF’s NOIRLab) and collaborators. To confirm the gravitationally lensed nature of the system, the team demonstrated via spectroscopy that the four images were of the same galaxy. The golden central galaxy is an elliptical behemoth whose light has been traveling to us for nearly 6 billion years, while the lensed starburst galaxy is far more distant, giving us a glimpse of when the universe was just 18.5% of its current age. To learn more about this new addition to the short list of known Einstein Crosses, be sure to check out the full article linked below.

Citation

“DESI-253.2534+26.8843: A New Einstein Cross Spectroscopically Confirmed with Very Large Telescope/MUSE and Modeled with GIGA-Lens,” Aleksandar Cikota et al 2023 ApJL 953 L5. doi:10.3847/2041-8213/ace9da

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.

Bonus

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.

Citation

“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.

Citation

“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.

Citation

“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.

Citation

“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.

Citation

“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.

Citation

“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

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