Today’s Monthly Roundup is a bit of an astrophysical mishmash, highlighting some of the structures that exist in our universe. From galaxies to planets, we’ll explore where these structures come from and the tools researchers use to study them.
A Ring Galaxy
When gamma-ray telescopes like Fermi survey the sky, they see discrete sources of gamma rays as well as a diffuse background glow. Discerning exactly where these gamma rays come from is no easy task, and roughly 30% of known gamma-ray sources haven’t been identified. Many of these unassociated sources are likely active galactic nuclei (AGN) — accreting supermassive black holes that can launch powerful jets — but some may have more unusual origins.
An ultraviolet image (background color) of the Kathryn’s Wheel system with contours showing the Hα emission. Annotations have been added to show the various components of this system. Click to enlarge. [Adapted from Paliya & Saikia 2024]
Multiwavelength images of Kathryn’s Wheel show bright Hα and ultraviolet emission, both of which signal the presence of hot, massive young stars. Intense star-forming regions are known to emit gamma rays, in part because of the frequent core-collapse supernovae that shock interstellar gas and accelerate cosmic rays. A closer look at the galaxy revealed that its gamma-ray emission is stronger than expected, suggesting either a population of rapidly spinning stellar remnants called pulsars or — as a more mundane explanation — interference from a foreground Milky Way star.
In addition to triggering star formation, galaxy collisions can also activate AGN. While the current data show no signs of gamma-ray variability or relativistic jets — both of which would indicate an AGN — more observations are needed to look into the possibility further.
High-Energy Bubbles
In 2010, scientists discovered that the Milky Way has been blowing bubbles. Observations with the Fermi Gamma-ray Space Telescope revealed bubbles of gamma-ray emission extending 50 degrees above and below the plane of our galaxy. Ten years later, the eROSITA instrument on the Spectrum-Roentgen-Gamma spacecraft spotted a similar but even more extensive structure in X-rays. The origin of these structures, called the Fermi and eROSITA bubbles, is not yet known. Many researchers have suggested that a past period of AGN activity, in which the Milky Way’s central supermassive black hole accreted matter and shot out powerful jets, could be responsible.
Recently, Po-Hsun Tseng (National Taiwan University) and collaborators approached the AGN activity hypothesis from a new angle. Previous work has tested this theory under the assumption that the AGN jets emerged vertically from the plane of the galaxy, matching the orientation of the bubbles. However, this doesn’t have to be the case — the direction of an AGN’s jets is related to the spin of the black hole, which doesn’t have to be parallel to the galactic disk.
Simulated gamma-ray bubbles from jets emerging perpendicular to the disk (top), at a 45-degree angle to the disk (middle), and parallel to the disk (bottom). [Adapted from Tseng et al. 2024]
Ultimately, the team found that under certain conditions, the jets transfer their kinetic energy to the dense layer of gas without plowing through it, and the once-narrow jets instead emerge vertically from the disk as bubbles. While more modeling is needed to understand the origin of the Fermi and eROSITA bubbles, this work shows that the assumption of vertical jets need not apply.
A Chain of Planets
When planets form in the dusty recesses of a protoplanetary disk, their motions within the disk are thought to align the planets in a resonant chain: a setup in which the orbital periods of the planets are integer multiples of one another. For example, a three-planet system with 8-, 16-, and 32-day orbits would be in a resonant chain. If planets do link up in resonant chains when they first form, something — gravitational nudges from passing stars, for example — must break those chains, as only 1% of known planetary systems are in this configuration. It’s critical to identify the small percentage of systems with intact resonant chains since they show the initial state of a planetary system before it’s disrupted.
Cumulative fraction of simulated six-planet systems becoming unstable as a function of time. Nearly all non-resonant systems are unstable by 25 million years, while only a small fraction of six-planet resonant chains go unstable in that same stretch of time. Click to enlarge. [Lammers & Winn 2024]
Lammers and Winn performed N-body simulations to assess the likelihood that the system is arranged in a resonant chain. They found that in order for HD 110067’s six planets to be dynamically stable, the planets almost certainly must be in a chain. Simulated non-resonant systems become unstable within 25 million years — just 0.3% of the current age of HD 110067’s planetary system — and even systems with as many as five planets linked in a chain are unlikely to survive in that configuration to the present day.
Citation
“A γ-Ray-Emitting Collisional Ring Galaxy System in Our Galactic Neighborhood,” Vaidehi S. Paliya and D. J. Saikia 2024 ApJL 967 L26. doi:10.3847/2041-8213/ad4999
“Can the Symmetric Fermi and eROSITA Bubbles Be Produced by Tilted Jets?” Po-Hsun Tseng et al 2024 ApJ 970 146. doi:10.3847/1538-4357/ad50c5
“The Six-Planet Resonant Chain of HD 110067,” Caleb Lammers and Joshua N. Winn 2024 ApJL 968 L12. doi:10.3847/2041-8213/ad50d2