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 ofz_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) withinR_vir. We used aβmodel 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) andβ = 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 > 2.8σand theS_X, CGMcan be described by aβmodel withβ ≈ 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 withinR_virto 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 toR_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.

EROSITA (extended ROentgen Survey with an Imaging Telescope Array) will be one out of three main instruments on the Russian new Spectrum-RG mission which will be launched in the timeframe 2010-2011 into an equatorial Low Earth Orbit. The other two instruments are the wide field X-ray monitor Lobster (Leicester University, UK) and ART (IKI, Russia), an X-ray concentrator based on a Kumakhov optics. eROSITA consists of seven Wolter-I telescope modules similar to the German mission ABRIXAS which failed in 1999 and ROSITA, a telescope which was planned to be installed on the International Space Station ISS. Unlike these, the eROSITA telescope modules will be extended by adding another 27 mirror shells to the already existing ABRIXAS design. This will increase the effective area by a factor of ~5 at low energies. The additional shells do not contribute to the area at higher energies ( > 5 keV) due to the relative large grazing angles. Here we stay with the old ABRIXAS/ROSITA effective area. However, the primary scientific goal has changed since ABRIXAS: we are now aiming primarily for the detection of 50-100 thousands Clusters of Galaxies up to redshifts z > 1 in order to study the large scale structure in the Universe and test cosmological models including the Dark Energy, which was not yet known at ABRIXAS times. For the detection of clusters, a large effective area is needed at low (< 2 kev) energies. The mission scenario comprises a wide survey of the complete extragalactic area and a deep survey in the neighborhood of the Galactic Poles. Both are accomplished by an all-sky survey with a tilt of the rotation axis in order to shift the deepest exposures away from the ecliptic poles towards the galactic poles.

The evolution of the cluster mass function traces the growth of the linear density perturbations and can be utilized to constrain the parameters of cosmological and alternative gravity models. In this context, we present new constraints on potential deviations from general relativity by investigating the Hu-Sawicki parametrization of the f (R) gravity with the first Spectrum Roentgen Gamma (SRG)/eROSITA All-Sky Survey (eRASS1) cluster catalog in the western Galactic hemisphere in combination with the overlapping Dark Energy Survey Year-3, KiloDegree Survey, and Hyper Suprime-Cam data for weak lensing mass calibration. For the first time, we present constraints obtained from cluster abundances only. When we consider massless neutrinos, we find a strict upper limit of log |f_R0| < −4.31 at a 95% confidence level. Massive neutrinos suppress structure growth at small scales, and thus have the opposite effect of f (R) gravity. We consequently investigate the joint fit of the mass of the neutrinos with the modified gravity parameter. We obtain log |f_R0| < −4.08 jointly with ∑ m_ν < 0.49 eV at a 95% confidence level, which is tighter than the limits in the literature utilizing cluster counts only. At log |f_R0| = −6, the number of clusters is not significantly changed by the theory. Consequently, we do not find any statistical deviation from general relativity in the study of eRASS1 cluster abundance. Deeper surveys with eROSITA, increasing the number of detected clusters, will further improve constraints on log |f_R0| and investigate alternative gravity theories.