A Rose By Any Other Name Would Be An X-ray Telescope

Title: The SRG/eROSITA All-Sky Survey: Constraints on AGN Feedback in Galaxy Groups

Authors: Y. E. Bahar, E. Bulbul, V. Ghirardini, J. S. Sanders, X. Zhang, A. Liu, N. Clerc, E. Artis, F. Balzer, V. Biffi, S. Bose, J. Comparat, K. Dolag, C. Garrel, B. Hadzhiyska, C. Hernández-Aguayo, L. Hernquist, M. Kluge, S. Krippendorf, A. Merloni, K. Nandra, R. Pakmor, P. Popesso, M. Ramos-Ceja, R. Seppi, V. Springel, J. Weller, S. Zelmer

First Author’s Institution: Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany

Status: Submitted to arXiv (preprint) and Astronomy & Astrophysics

Providing Feedback

Space is gassy. I’ll acknowledge space gas may not be the most romantic topic for Valentine’s Day, but studying the gas that lies in between stars or around galaxies can provide us with a bounty of information that helps us answer questions about chemical composition, star formation, and feedback. But studying this medium can be incredibly difficult due to faint observational signals and the varied impacts of many different astrophysical processes. Of particular relevance to today’s paper are the processes that heat the gas up (see, here’s your Valentine’s link-we’re getting steamy). 

Multi-wavelength image of a spiral galaxy edge on. Jets of material are seen moving out at a nearly perpendicular angle from the disk. The jets expand into larger lobe shapes further from the galaxy. The background of the image shows many small pinpricks of light from foreground stars and background galaxies.
Figure 1: An example of AGN feedback in the galaxy Centaurus A. Material from around the central supermassive black hole is being ejected in galaxy-scale jets. Image credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)

One of the biggest factors is gravity: gas that sits in massive halos can be virialized, or heated to temperatures that directly depend on the mass of their halo. But, crucially, gravity isn’t the only thing that gets gas hot. One of the other major players is Active Galactic Nuclei (AGN) feedback-the process by which accreting matter around a supermassive black hole releases energy back into its host galaxy and beyond. AGN feedback is thought to play a major role in how galaxies evolve over time, but its effects can be hard to study.

Roses in the X-ray

One way to observe the effects of AGN feedback is to look at gas in the X-ray. Gas that has been heated up will emit high-energy X-ray light, and in theory, AGN feedback can be responsible for a lot of that heating. Today’s paper considers the hot gas in galaxy groups, which consist of a few tens of galaxies gravitationally bound together. Galaxy groups are relatively abundant in the universe, and because they are smaller than large structures like galaxy clusters, they make a good target for studying non-gravitational heating effects (read AGN feedback). 

Scatter plot with temperature in keV on the x-axis and entropy in keV cm^2 on the y-axis. The relationship displayed is nearly linear. Data is shown from the eROSITA galaxy groups used in this paper as well as previous work using the Chandra space telescope, and there is strong overlap in the results except at low temperature ranges, where the eROSITA points have higher entropies than Chandra.
Figure 2:  Entropy versus temperature measurements in the IGrM. The colored points represent eROSITA data from this paper, with two different ways of determining error. The grey data comes from a similar work done using the Chandra Space Telescope. Adapted from Figure 14 in Bahar et al. 2024.

The end of January saw the first public data release from the extended ROentgen Survey with an Imaging Telescope Array (eROSITA). eROSITA was launched in 2019 and has been working on an all-sky survey in what we call ‘soft’ X-ray, or the lower energy portion of the X-ray spectrum. It has incredibly high sensitivity and a range of science goals, including mapping out the structure of hot gas in the intergalactic medium, detecting AGN, and studying X-ray sources like supernova remnants and pre-main sequence stars. Today’s paper focuses on identifying galaxy groups in the eROSITA data and extracting many characteristic properties of the gas in between galaxies, what is sometimes called the intragroup medium (IGrM).

Bringing Out the Best

An unprecedented 1,178 galaxy groups were identified in the eROSITA data, but given the faint signals they produced in X-ray wavelengths, the spectral properties of these galaxy groups could not be studied individually. Instead, they were separated into 271 bins, each with ~900 photon counts. The spectra of all the groups were then simultaneously fit to spectrum modeling codes. This process, called co-fitting, allows astronomers to convert individual observations that are too faint to reliably analyze into combined data of many observations with a much higher signal-to-noise. Once the spectra for each bin were generated, the temperatures and entropy of the gas at various radii were calculated. 

The resulting temperature-entropy relations at the mid-regions of galaxy groups are shown in Figure 2. The results are nicely consistent with similar work done using the Chandra X-ray Observatory (shown in grey), although, at low temperatures, eROSITA finds higher entropies, most likely due to differing ways of selecting the galaxy group sample.

Simulation and Observation: Making Each Other Better

An important aspect of much modern astrophysics research is comparing observations to large-scale hydrodynamic simulations. This paper extracts the gas conditions in galaxy groups from three different simulations: Magneticum, MillenniumTNG, and OverWhelmingly Large Simulations (often called OWLS-astronomers really love their ‘clever’ acronyms). Each of these simulations handles its AGN feedback in a different way, so by comparing what we see observationally to the computational predictions, we can perhaps determine which feedback model is the most accurate.

Three line/scatter plots in a row. The x-axis gives temperature in keV and the y-axis gives entropy in keV cm^2. Observational data is shown as scatter plots, with a nearly linear relationship between the two variables. Three lines are overlaid representing data from three different simulations. In the leftmost plot, representing data taken near the cores of groups, the OWLS simulation aligns well with observation, whereas Magneticum and Millenium TNG have too high of entropies. In the central image, Magneticum is best aligned and the other two simulations have lower entropies. In the right image, Magneticum is also the best aligned with higher entropies than the other two, although the three simulations have significant overlap within error bars.
Figure 3:  Comparisons of entropy measurements from eROSITA and three hydrodynamic simulations. Left shows values in the cores of the groups, center at the mid-regions of groups, and right at the outskirts of groups. The grey data represents values for each observational bin of galaxy groups, while the blue and yellow points represent the averaged values at a given temperature.
Adapted from Figure 16 in Bahar et al. 2024.

Figure 3 showcases these results, comparing temperatures and entropies at three different radii: the cores, mid-regions, and outskirts of groups. The entropies at each radius vary, as does the alignment between simulation and observation. The eROSITA measurements presented in this paper demonstrate a strong ability to constrain feedback treatment in simulations, as there are clear differences between observation and simulation in several cases. Overall, the Magneticum simulation aligns best, although OWLS does a better job in group cores. Due to these mixed results, the authors suggest that the feedback models in all three simulations are not quite adequate when it comes to studies at group scales, although the strong alignment of Magneticum at group outskirts shows promise. MillenniumTNG has the weakest feedback prescriptions of the three, so in general, these results favor strong AGN feedback models. As the authors acknowledge, pushing measurements to lower temperatures could be quite useful for further constraints on AGN feedback models. 

Overall, this work provides an exciting glimpse into the powers of new X-ray telescopes when it comes to studying gas conditions. Constraining AGN feedback is a big open problem in astrophysics today, and these results demonstrate the usefulness of X-ray observations in galaxy groups.

Astrobite edited by Sowkhya Shanbhog and Lynnie Saade

Featured image credit: MPE, J. Sanders for the eROSITA consortium

About Skylar Grayson

Skylar Grayson is an Astrophysics PhD Candidate and NSF Graduate Research Fellow at Arizona State University. Her primary research focuses on AGN feedback processes in cosmological simulations. She also works in astronomy education research, studying online learners in both undergraduate and free-choice environments. In her free time, Skylar keeps herself busy doing science communication on social media, playing drums and guitar, and crocheting!

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