Title: On the origin of the diffuse radio emission in Abell 85 – insights from new GMRT observations
Authors: Majidul Rahaman, Ramij Raja, Abhirup Datta, Jack O Burns and David Rapetti
Institution of first author: Department of Astronomy, Astrophysics and Space Engineering, Indian Institute of Technology Indore, Indore, India
Status: Accepted for publication in the Monthly Notices of the Royal Astronomical Society [closed access]
Deep in the far reaches of space, a phoenix could be brought back to life! Galaxy clusters are among the largest and most turbulent objects in the universe, containing many hundreds of galaxies that are gravitationally connected. These galaxies are surrounded by hot, turbulent plasma – so hot that we can see X-rays from its thermal emission. The plasma has had time to expand and cool, but interactions between galaxies in the cluster or with other objects can create bursts of supersonic material in the plasma. These collisions compress the plasma, reactivating strong magnetic fields and re-accelerating electrons in these regions, causing them to emit synchrotron radiation in the form of radio waves. We call these regions of reactivated radio emission Funk phoenixesand you can show us the distribution and properties of electrons that have been mostly unchanged in the cluster for an extremely long time.
This paper presents new observations of Abell 85, a galaxy cluster containing a galaxy with one of the largest black holes we have ever measured! In addition to reviewing archived X-ray data from the Chandra X-ray Observatory, the authors of this article present a new high-resolution map of the cluster in radio waves taken with the Giant Metrewave Radio Telescope (GMRT) in India. This new map shows that the radio phoenix in Abell 85 has a complex structure of long, thin filaments (see Figure 2 for a spoiler), and the spectrum (or brightness as a function of observation frequency) of the emission is very steep. Previous studies and simulations of radio phoenixes have shown steep spectra and a complex filament structure, characteristics that make this radio phoenix quite standard. Additionally, using the X-ray data to create a temperature map of the cluster allowed the authors to search for these turbulent shocks, and they found one that matched the Radio Phoenix very closely!
The left panel of Figure 1 shows the X-ray brightness map of the galaxy cluster, with the red spot corresponding to the cluster’s bright center and two subclusters to the south and southwest of the main cluster. The right panel shows the temperature map generated from the brightness map after accounting for the absorption of the interstellar medium, the spectral properties of the plasma, and the distinction between the source and background objects in the image. This temperature map shows that the cluster does indeed have a cool center, with hot spots denoted by red ellipses. These two panels map the same region – so the hot regions BE on the right match very well with the edges of the two subclusters in the left panel, proving the idea that these smaller structures are actively merging with the main cluster and mixing the Heat up electrons and heat up the plasma to reactivate this radio phoenix!
But the star of the show is the new map of the galaxy cluster’s radio emission taken by GMRT (Figure 2)! This map is the most high-resolution radio image of the region ever recorded and showed the filiform shape of the radio phoenix. Radio relics that aren’t phoenixes tend to exhibit a more “lumpy” and less filamentous structure, so this stream of brighter emission helps solidify that this is indeed a phoenix being reactivated.
Using the X-ray and radio maps together, the authors created a program that would scan these maps for shocks. By looking for temperature discontinuities between pixels in the X-ray map and changes in the spectral index of the radio emission, they were able to translate these into a calculation of the Mach number at that point – a measure of how quickly the shock penetrates the medium, with a Mach number greater than 1, which means the shock is traveling faster than the speed of sound!
The authors found compelling evidence of a shock perfectly localized along with the Radio Phoenix and the southwestern subcluster (see Figure 3). The Mach numbers are low enough, albeit with some uncertainty, that we would expect them to be caused by fusion shocks – and since this shock is in the same region as the subcluster, the authors conclude that the most likely scenario is the subcluster is merged with the main cluster and creates a shock that reactivates radio emission in the region and raises the radio phoenix from the ashes.
Despite all these signs that seem to point directly to this object being a radio phoenix, the authors point out some inconsistencies and a possible alternative explanation. While there have not been many well-documented radio phoenixes, this one is much larger and farther from the center of the cluster than average. A competing theory about radio phoenixes says that as gas sloshes around in the galaxy cluster, it could push radio-emitting electrons around and shape them into a filamentous structure that looks like a phoenix without really being one. In previous studies, a large gas slosh arm was identified in A85, with the Radio Phoenix right at the end; In addition, these new radio images revealed two long-tailed radio-emitting galaxies within the Radiophoenix. There is no conclusive evidence either way, but it is plausible that these energetic tails could inject radio-emitting electrons into the galaxy cluster, where they are slung around by this gas sloshing arm to form a fraudulent radio phoenix. Whatever the outcome, the astronomical community will be on the lookout for more cosmic birds to flap their wings!
Astrobite edited by Jessie Thwaites and Roan Haggar
Selected photo credits: this paper and Niki Vandermosten
About Evan Lewis
Evan is a third-year graduate student in astronomy at West Virginia University. His research focuses on transient radio sources, including pulsars, magnetars, and fast radio bursts. Outside of research he enjoys playing the drums, hugging dogs, baking and playing video games!
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