The hot circumgalactic medium in the eROSITA All-Sky Survey

I. X-ray surface brightness profiles

¹ Max-Planck-Institut für extraterrestrische Physik (MPE), Gießenbachstraße 1, 85748 Garching bei München, Germany
² INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
³ European Southern Observatory, Karl Schwarzschildstrasse 2, 85748 Garching bei München, Germany
⁴ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
⁵ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
⁶ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ Center for Space Sciences and Technology, University of Maryland, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁸ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 30 January 2024
Accepted: 26 July 2024

Abstract

Context. The circumgalactic medium (CGM) provides the material needed for galaxy formation and influences galaxy evolution. The hot (T > 10⁶K) CGM is poorly detected around galaxies with stellar masses (M_*) lower than 3 × 10¹¹ M_⊙ due to the low surface brightness.

Aims. We aim to detect the X-ray emission from the hot CGM around Milky Way-mass (MW-mass, log(M_*/M_⊙) = 10.5 − 11.0) and M31-mass (log(M_*/M_⊙) = 11.0 − 11.25) galaxies, in addition to measuring the X-ray surface brightness profile of the hot CGM.

Methods. We applied a stacking technique to gain enough statistics to detect the hot CGM. We used the X-ray data from the first four SRG/eROSITA All-Sky Surveys (eRASS:4). We discussed how the satellite galaxies could bias the stacking and the method we used to carefully build the central galaxy samples. Based on the SDSS spectroscopic survey and halo-based group finder algorithm, we selected central galaxies with spectroscopic redshifts of z_spec < 0.2 and stellar masses of 10.0 < log(M_*/M_⊙) < 11.5 (85 222 galaxies) – or halo masses of 11.5 < log(M_200m/M_⊙) < 14.0 (125,512 galaxies). By stacking the X-ray emission around galaxies, we obtained the mean X-ray surface brightness profiles. We masked the detected X-ray point sources and carefully modeled the X-ray emission from the unresolved active galactic nuclei (AGN) and X-ray binaries (XRB) to obtain the X-ray emission from the hot CGM.

Results. We measured the X-ray surface brightness profiles for central galaxies of log(M_*/M_⊙) > 10.0 or log(M_200m/M_⊙) > 11.5. We detected the X-ray emission around MW-mass and more massive central galaxies extending up to the virial radius (R_vir). The signal-to-noise ratio (S/N) of the extended emission around MW-mass (M31-mass) galaxy is about 3.1σ (4.7σ) within R_vir. We used a β model to describe the X-ray surface brightness profile of the hot CGM (S_X, CGM). We obtained a central surface brightness of log(S_X,0[erg s⁻¹ kpc⁻²]) = 36.7_−0.4^+1.4 (37.1_−0.4^+1.5) and β = 0.43_−0.06^+0.10 (0.37_−0.02^+0.04) for MW-mass (M31-mass) galaxies. For galaxies with log(M_200m/M_⊙) > 12.5, the extended X-ray emission is detected with S/N > 2.8σ and the S_X, CGM can be described by a β model with β ≈ 0.4 and log(S_X,0[erg s⁻¹ kpc⁻²]) > 37.2. We estimated the baryon budget of the hot CGM and obtained a value that is lower than the prediction of ΛCDM cosmology, indicating significant gas depletion in these halos. We extrapolated the hot CGM profile measured within R_vir to larger radii and found that within ≈3R_vir, the baryon budget is close to the ΛCDM cosmology prediction.

Conclusions. We measured the extended X-ray emission from representative populations of central galaxies around and above MW-mass out to R_vir. Our results set a firm footing for the presence of the hot CGM around such galaxies. These measurements constitute a new benchmark for galaxy evolution models and possible implementations of feedback processes therein.