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Context. The circumgalactic medium (CGM) provides the material needed for galaxy formation and influences galaxy evolution. The hot ( T > 10 6 K ) CGM is poorly detected around galaxies with stellar masses ( M * ) lower than 3 × 10 11 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 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 3.1 σ (4.7 σ ) 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 > 2.8 σ 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 ≈3 R 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.
Zhang et al. (2024) studied this question.