Bubbles and Tunnels: Unveiling the Secret Structure of Our Galaxy

Title: The SRG/eROSITA diffuse soft X-ray background: I. The local hot bubble in the western Galactic hemisphere

Authors: Michael C. H. Yeung, Gabriele Ponti, Michael J. Freyberg, Konrad Dennerl, Teng Liu, Nicola Locatelli, Martin G. F. Mayer, Jeremy S. Sanders, Manami Sasaki, Andy Strong, Yi Zhang, Xueying Zheng, Efrain Gatuzz

First Author’s Institution: Max Planck Institute for Extraterrestrial Physics, Garching, Germany

Status: Published in Astronomy & Astrophysics [open access]

Hot Single Bubbles In Your Area!

While it may not seem like it, there’s a lot of stuff in between stars. Dust and gas, all interacting and evolving in a complex system we call the interstellar medium. Even near our solar system, the interstellar medium is doing some weird and interesting things, as explored in today’s paper. 
Ranging from 50-300 parsecs from the sun, the area around our solar system seems to be missing something. Specifically, it’s missing the dust that is widespread in our galaxy and its relatively cool neutral gas. This emptied-out region has come to be known as the local hot bubble (LHB). It’s thought that this region was cleared out by supernovae (and potentially stellar winds) that push material out of the way. However, this bubble is not completely empty. The ‘hot’ element in its name comes from a very diffuse, hot plasma that fills the void left by the absence of cooler gas and dust. This material, due to its temperature, emits in the X-ray band, making it the perfect candidate for study by X-ray survey instrument, eROSITA. 

A Sky Full of X-rays

eROSITA has generated a map of the full sky in X-ray wavelengths, but when studying this map it’s important to remember that there’s a lot more than just the LHB that’s generating x-ray emission. This team divided the sky into 2000 regions and looked at the spectral information in each area. They were able to isolate the LHB emission by fitting the spectrum to a model that took into account emission from the Milky Way’s circumgalactic medium, the cosmic x-ray background, absorption by cooler gas, instrumental background, and of course the bubble itself. Thus they were able to extract the spectral components coming from the LHB and use that information to study it in detail.

Temperature Variations Cause Questions

The first interesting result to come from this was the discovery of a temperature gradient. The LHB is significantly cooler when looking in the northern galactic hemisphere than the southern (see Figure 1). There is some uncertainty in what could be causing this variance. It could be that there were some recent off-center supernovae in the bubble, heating the gas unevenly. Or maybe the medium was not uniform before the LHB was developed, and those variations persisted over time. The true reason, as of now, remains unclear.

Secret Tunnel?

The second thing researchers were able to do with this data is create a three-dimensional map of the LHB, as shown in Figure 2. The bubble is kind of weird looking, with many irregularities. However, there are some explainable trends. For one, the bubble extends out further when not in the plane of the Milky Way’s disk. This is likely because the material in the disk is denser, making it more difficult for the bubble to expand in that direction. Some elements of the bubble’s shape also point to a larger structure that has been postulated since the 1970s–a tunnel network of low density hot gas, carved out by supernovae and worming across the entire galaxy! 

You see, the LHB isn’t the only bubble we’ve found. Those found around the galaxy are usually called “superbubbles”, but they’re in essence the same as the LHB-cavities with low density hot material. Figure 3 shows how one of the “fingers” of the LHB reaches towards the Gum Nebula, which is itself connected to a larger superbubble. Another tunnel was discovered here, this one reaching towards the constellation Centaurus. Beyond these two tunnels, these results also point to the possibility that the LHB is connected to several other nearby supernova remnants. 

All combined, these results demonstrate eROSITA’s ability to map out diffuse X-ray emission with never-before seen detail. The LHB is only one of many structures generating X-rays in these wavelengths, so this data is sure to yield even more results in the future!

Astrobite edited by Jessie Thwaites

Featured image credit: Max Planck Institute for Extraterrestrial Physics, Yeung et al. 2024