The hot circumgalactic medium in the eROSITA All-Sky Survey
Authors: Zhang Yi , Comparat Johan , Ponti Gabriele , Merloni Andrea , Nandra Kirpal , Haberl Frank , Truong Nhut , Pillepich Annalisa , Popesso Paola , Locatelli Nicola , Zhang Xiaoyuan , Sanders Jeremy , Zheng Xueying , Liu Ang , Liu Teng , Predehl Peter , Salvato Mara , Bruggen Marcus , Shreeram Soumya , Yeung Michael C. H. 
Published: December 2024

Aims The galaxy population shows a characteristic bimodal distribution based on the star formation activity and is sorted into star-forming or quiescent. These two subpopulations have a tendency to be located in different mass halos. The circumgalactic medium (CGM), as the gas repository for star formation, might contain the answer to the mystery of the formation of such bimodality. Here we consider the bimodality of the galaxy population and study the difference between the properties of the hot CGM around star-forming and quiescent galaxies.
Methods. We used the X-ray data from the first four SRG/eROSITA all-sky surveys (eRASS:4). We selected central star-forming and quiescent galaxies from the Sloan Digital Sky Survey DR7 with stellar mass 10.0 < log(M*/M⊙) < 11.5 or halo mass 11.5 < log(M200 m/M⊙) < 14.0 within spectroscopic redshift zspec < 0.2, and we built approximately volume-limited galaxy samples. We stacked the X-ray emission around star-forming and quiescent galaxies, respectively. We masked detected point sources and carefully modeled the X-ray emission from unresolved active galaxy nuclei (AGN) and X-ray binaries (XRB) to detect the X-ray emission from the hot CGM. We measured the X-ray surface brightness (SX, CGM) profiles and integrated the X-ray emission from hot CGM within R500c (LX, CGM) to provide the scaling relations between LX, CGM and galaxies’ stellar or halo mass.
Results. We detect extended X-ray emission from the hot CGM around star-forming galaxies with log(M*/M⊙) > 11.0 and quiescent galaxies with log(M*/M⊙) > 10.5, extending out to R500c. The SX, CGM profile of quiescent galaxies follows a β model with β ≈ 0.4, where β quantifies the slope of the profile. Star-forming galaxies with median stellar masses log(M*, med/M⊙) = 10.7, 11.1, 11.3 have LX, CGM ≈ 0.8,  2.3,  4.0 × 1040 erg/s, while for quiescent galaxies with log(M*, med/M⊙) = 10.8, 11.1, 11.4, LX, CGM ≈ 1.1,  6.2,  30 × 1040 erg/s. Notably, quiescent galaxies with log(M*, med/M⊙) > 11.0 exhibit brighter hot CGM than their star-forming counterparts. In halo mass bins, we detect similar X-ray emission around star-forming and quiescent galaxies with log(M200 m/M⊙) > 12.5, suggesting that galaxies in the same mass dark matter halos host equally bright hot CGM. We emphasize that the observed LX, CGM − M500c relations of star-forming and quiescent galaxies are sensitive to the stellar-to-halo mass relation (SHMR). A comparison with cosmological hydrodynamical simulations (EAGLE, TNG100, and SIMBA) reveals varying degrees of agreement, contingent on the simulation and the specific stellar or halo mass ranges considered.
Conclusions. Either selected in stellar mass or halo mass, the star-forming galaxies do not host brighter stacked X-ray emission from the hot CGM than their quiescent counterparts at the same mass range. The result provides useful constraints on the extent of feedback’s impacts as a mechanism for quenching star formation as implemented in current cosmological simulations.