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MWA 1.28 MHz observations

Look closely. What do you see in the dead center of this 4° x 4° stacked radio image? If your answer is “not much”, you’re absolutely right — and that’s super interesting! The observations above were made by the Murchison Widefield Array (MWA) as it shadowed the pointings of the Australian Square Kilometre Array Pathfinder (ASKAP). During the time the two instruments synchronized their pointings, ASKAP detected several fast radio bursts — extremely energetic and brief flashes of radio emission — including one that should have appeared in the center of the MWA image above. But MWA spotted nothing but weak candidate sources (circled in green) that were later discarded.

Why did MWA turn up nothing? A primary difference between the arrays is that ASKAP scans at a higher radio frequency than MWA — 700 MHz to 1.8 GHz, compared to MWA’s 80 to 300 MHz. This null result in MWA’s observations therefore has important implications for understanding mysterious fast radio bursts: it means that either the bursts don’t emit below a certain radio frequency (which raises the question: why not?), or that something is blocking the lower-frequency radio signal on its way to Earth (which raises the question: what?). To learn more about the team’s findings, check out the article below.

Citation

“No Low-frequency Emission from Extremely Bright Fast Radio Bursts,” M. Sokolowski et al 2018 ApJL 867 L12. doi:10.3847/2041-8213/aae58d

core collapse simulation

Modeling the collapse of a stellar core and the supernova explosion that follows — with the inclusion of all of the complex physics involved in these processes — is notoriously difficult. Even more difficult: doing this in three dimensions. In a new study, Evan O’Connor (Stockholm University, Sweden) and Sean Couch (Michigan State University) present the results of an extensive set of 3D core-collapse supernova simulations that include the physics of transporting neutrinos in multiple dimensions, high-resolution hydrodynamics, and approximate general relativistic gravity. These simulations show that capturing the 3D behavior is critical: the large-scale aspherical motion in the star’s silicon and oxygen shells aids the expansion of the shock and brings the star closer to exploding. The figure below (click for a closer look) shows the difference between two models that do (top row) and don’t (bottom row) include aspherical perturbations in these shells, as the star’s entropy evolves in time from left to right. For more information and images, check out the original study linked below.

core-collapse evolution simulation

Citation

“Exploring Fundamentally Three-dimensional Phenomena in High-fidelity Simulations of Core-collapse Supernovae,” Evan P. O’Connor and Sean M. Couch 2018 ApJ 865 81. doi:10.3847/1538-4357/aadcf7

CHIME

The Canadian Hydrogen Intensity Mapping Experiment, or CHIME, is a novel radio telescope originally intended to map features in hydrogen gas to measure dark energy. It has an additional mission now, however: CHIME will search the sky for signs of new fast radio bursts (FRBs). FRBs — energetic transient radio pulses that last only a few milliseconds — were first discovered about a decade ago, and though we’ve only observed ~30 of them so far, some estimates suggest they occur at a rate of several hundred to a few thousand per day across the sky! CHIME’s large field of view, high sensitivity, and wide bandwidth will help us hunt for these explosive events. In a new report by the CHIME/FRB collaboration, the team details this unique telescope, located in British Columbia. CHIME is made up of four 20-m x 100-m semicylindrical paraboloid reflectors, giving it its unusual appearance. The team expects that when CHIME begins science operations, it will detect FRBs at a rate of 2–42 FRBs per sky per day. For more information, check out the article below!

Citation

“The CHIME Fast Radio Burst Project: System Overview,” The CHIME/FRB Collaboration et al 2018 ApJ 863 48. doi:10.3847/1538-4357/aad188

circumnuclear disks and rings

These dramatic simulated images (click for the full view) reveal some of the circumnuclear gas structures that can form from the tidal disruption of molecular clouds in the nucleus of a galaxy. In a study led by Alessandro Trani (The University of Tokyo, Japan; International School for Advanced Studies, Italy; INAF-Astronomical Observatory of Padua, Italy), a team of scientists has conducted a series of simulations exploring what happens to gas in a galactic nucleus consisting of a supermassive black hole and a nuclear star cluster. Their work shows that the gas can be drawn into extended disks or compact rings, depending on whether the black hole’s influence is stronger than that of the nuclear star cluster. To read more about their outcomes, check out the paper below.

Citation

“Forming Circumnuclear Disks and Rings in Galactic Nuclei: A Competition Between Supermassive Black Hole and Nuclear Star Cluster,” Alessandro A. Trani et al 2018 ApJ 864 17. doi:10.3847/1538-4357/aad414

M100

These side-by-side images (click for a closer look) show the spiral galaxy Messier 100 in two views: the image on the right is taken with the Very Large Telescope in optical bands, and the image on the left is an infrared view captured by the Spitzer space telescope. In a new study led by Bruce Elmegreen (IBM Research Division, T.J. Watson Research Center), a team of scientists has further analyzed Spitzer’s observations of M100. The authors focus on the regularly spaced infrared-bright, star-forming clumps that lie along the dusty filaments — which, while clearly visible in infrared, often can’t be seen in the optical. The regularity of the spacing and size of these clumps suggest that star formation within the spiral arms of M100 occurs as a result of gravitational instabilities in gas that was accumulated by spiral density waves moving through the galaxy. For more information, check out the original article below.

Citation

“Regularly Spaced Infrared Peaks in the Dusty Spirals of Messier 100,” Bruce G. Elmegreen et al 2018 ApJ 863 59. doi:10.3847/1538-4357/aacf9a

SDSS J0924+0510

How can we hunt down so-called dual active galactic nuclei (AGN) — pairs of accreting supermassive black holes that are likely headed for a merger after the collision of their host galaxies? The above Hubble image (broader ~40” x 40” view on the left, 8” x 8” zoom-in on the right) reveals two separate stellar bulges lying at the center of the minor galaxy merger SDSS J0924+0510. In a new study led by Xin Liu (University of Illinois at Urbana-Champaign), a team of scientists further explore this merger with Hubble to demonstrate that the two stellar bulges contain two spatially distinct regions showing [O III] emission — a strong indication that there are two obscured AGN independently ionizing gas at the heart of this merger. The authors show that the dual AGN are separated by only ~3,000 light-years — just a hair’s-breadth in cosmic distances! For more information, check out the original article below.

Citation

“Hubble Space Telescope Wide Field Camera 3 Identifies an rp  = 1 Kpc Dual Active Galactic Nucleus in the Minor Galaxy Merger SDSS J0924+0510 at z = 0.1495,” Xin Liu et al 2018 ApJ 862 29. doi:10.3847/1538-4357/aac9cb

MWC 758

This image, captured with the Very Large Telescope SPHERE adaptive optics in Chile, reveals the large-scale spiral arms visible in the MWC 758 protoplanetary disk, located less than 500 light-years away. Such arms are thought to be triggered by one of two mechanisms: gravitational instability, or a companion orbiting within the disk. A team of scientists led by Bin Ren (The Johns Hopkins University) has recently used observations of these arms over a decade-long baseline to track the speed of rotation of the arms. Since companion-driven arms corotate with their drivers, this exercise which could reveal the location of a planetary-mass, unseen companion that drives the arms. Ren and collaborators find that the most likely location for such a planet to orbit in this disk is at 89 AU, just outside of the visible spiral arms. For more information, check out the article below.

Citation

Bin Ren et al 2018 ApJL 857 L9. doi:10.3847/2041-8213/aab7f5

What would the Milky Way look like if the supermassive black hole at its center was a little more active? This stunning HST/WFC3 image of NGC 6744, spanning 160” x 160” (click for the whole view), may provide us with a reasonable guess! NGC 6744 is a nearby galaxy that’s morphologically very similar to our own — with the exception of the presence of an apparent low-luminosity active galactic nucleus at its center. The image to the right is a 10” x 10” zoom-in on the core of this galaxy, which was recently studied with the Gemini South Multi-Object Spectrograph by a team of scientists led by Patrícia da Silva (University of São Paulo, Brazil). The authors’ observations suggest that this galaxy’s nucleus was more luminous in the past — perhaps as a result of a merger — and has now settled down. For more information, check out the article below.

Citation

Patrícia da Silva et al 2018 ApJ 861 83. doi:10.3847/1538-4357/aac6e3

supernova Refsdal

The insets in this beautiful Hubble image of the MACS 1149 cluster shows the well-known supernova Refsdal, which appears as multiple copies of the same supernova due to strong gravitational lensing by a foreground galaxy. A new study led by Claudio Grillo (University of Milan, Italy and University of Copenhagen, Denmark) has now used the time delay between the multiple images of supernova Refsdal as a means of measuring the Hubble constant, a fundamental cosmological parameter that defines scales like the universe’s size, expansion rate, and geometry. By calculating the Hubble constant using time delays in a lens galaxy cluster, Grillo and collaborators confirm the possibility of an approach independent from techniques previously used to measure the constant. To learn more about their study, check out the article below.

Citation

C. Grillo et al 2018 ApJ 860 94. doi:10.3847/1538-4357/aac2c9

Are you planning to watch 4th of July fireworks tonight? Here’s a little preview on a cosmic scale! The images above — roughly 8’ across and captured by the Canada–France–Hawaii Telescope (CFHT) on the left and the Isaac Newton Telescope on the right — show the stunning planetary nebula NGC 6543 in all its large-scale glory. This may look a little different from images you’re used to seeing of NGC 6543, however: the most commonly seen view is of just the inner ~45” of this nebula (shown in the top image third from the left in the grid below). In a new study led by Xuan Fang (The University of Hong Kong and the National Astronomical Observatories, NAOC, in China), a team of scientists has used the CFHT to explore the extended molecular hydrogen structures of 11 planetary nebulae. The team’s work help us to better understand how these nebulae spread out into their surroundings after being expelled from dying, low-mass stars, and how the gas of the nebulae interacts with the interstellar medium. For more information — and lots of spectacular images of planetary nebulae — check out the article linked below!

planetary nebulae

The inner regions of just a few of the planetary nebulae the authors explore in this study. [Adapted from Fang et al. 2018]

Citation

Xuan Fang et al 2018 ApJ 859 92. doi:10.3847/1538-4357/aac01e

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