Title: Cosmic Rays Masquerading as Hot CGM Gas: An Inverse-Compton Origin for Diffuse X-ray Emission in the Circumgalactic Medium
Authors: Philip F. Hopkins, Eliot Quataert, Sam B. Ponnada, Emily Silich
First Author’s Institution: TAPIR, California Institute of Technology, California, USA
Status: Submitted to the Open Journal of Astrophysics, available on arxiv
The circumgalactic medium (CGM) is the invisible atmosphere that surrounds galaxies. It has a complex structure and plays a large role in the evolution of galaxies through the expulsion, accretion and recycling of gas. While there are many approaches to studying the CGM (you can explore some in the bites here, here, and here!) a growing tool in the last decade is X-ray observations. With recent data releases from the SRG-eROSITA mission, there have been several papers exploring X-ray emission in the CGM, which has been commonly attributed to thermal emission from hot gas. But today’s paper throws some doubt on this interpretation and suggests that some of the X-ray emission might be coming from a more unexpected source.
Not Feeling the Heat
Before delving into that, let’s talk about why the thermal emission interpretation has been called into question in the first place. One element is slight disagreements between observations and simulations of galaxy evolution, in which simulations predict a steeper radial profile (how the X-ray surface brightness changes as a function of distance from the galaxy) and lower X-ray luminosities than what’s being observed around galaxies with masses similar to the Milky Way and Andromeda. Additionally, the observed X-ray luminosities of galaxies and their abundance of hot gas seem inconsistent; given measurements of their luminosities, it seems there should be a greater abundance of hot gas in the CGM. These issues are mass dependent: for bigger galaxies found in large groups and galaxy clusters the observations are well explained by thermal emission. However, at lower masses, the relationship between X-ray luminosity and stellar mass changes and the other issues enumerated above start to arise. This paper suggests that these discrepancies point to another source for the X-ray emission observed around lower mass galaxies.
Cosmic Rays to the Rescue?
Alright enough suspense…what is this elusive alternative explanation? To put it technically, the X-ray emission could be coming from the inverse Compton scattering of cosmic microwave background photons by GeV cosmic ray electrons. In other words, really fast electrons generated by active galactic nuclei and supernovae are giving some of their energy to the background radiation of our universe. This boosts that radiation up to X-ray status, in a way that as it turns out looks pretty much exactly like the emission that is being observed around these smaller galaxies! Figure 1 shows a comparison between models of this cosmic ray-based emission and eROSITA observations of different mass galaxies, and as you can see they line up pretty dang well.
They back this idea up in a few different ways, including using cosmic ray detections around our own solar system to confirm the availability of these high energy cosmic rays, comparing against other cosmic ray signatures, and comparing against simulations with cosmic rays, again shown in Figure 1. Somewhat unfortunately, the spectral shape of this cosmic ray X-ray emission is pretty much identical to that of thermal emission with the sensitivity of modern telescopes, so there’s no smoking gun we could find in spectroscopic data right now that would definitively point to where these X-rays are coming from. And the success of these cosmic ray models doesn’t mean there isn’t any hot gas emitting in the CGM of these smaller galaxies. More observations will be crucial to help us better understand the CGM and what is responsible for the X-ray emission we’re seeing. But for now, this is another interesting piece of the puzzle that is the structure and behavior of the CGM.
Astrobite edited by Abbe Whitford
Featured image credit: MPE, J. Sanders for the eROSITA consortium
