Cross-correlation between soft X-rays and galaxies

This paper presents the construction and validation of complete stellar mass-selected, volume-limited galaxy samples using the Legacy Survey (data release 10) galaxy catalogs, covering ∼16 800 deg2 of extra-galactic sky and extending to redshifts of z < 0.35. We used companion mock catalogs to ensure a controlled galaxy selection. We measured the two-point correlation function of these galaxies with tiny statistical uncertainties at the percent level and systematic uncertainties up to 5%. We fitted a four-parameter halo occupation distribution (HOD) model to retrieve the population of host halos, yielding results on the stellar to halo mass relation that are consistent with the current models of galaxy formation and evolution. Using these complete galaxy samples, we measured and analyzed the cross-correlation between galaxies and all soft X-ray photons observed by SRG/eROSITA in the 0.5–2 keV band over ∼13 000 deg2. The cross-correlation measurements have an unprecedented sub-percent statistical uncertainty and 5–10% systematic uncertainty. We introduced a novel extension to the halo model to interpret the cross-correlation, decomposing contributions from X-ray point sources, hot gas, satellites, and the two-halo term. The model offers a new comprehensive view of the relation between the complete 0.5–2 keV X-ray photon field and complete sets of galaxies at low redshift and their host halos. For low stellar mass thresholds (log M*/M⊙ > 10, 10.25, 10.5), we find that the point source emission dominates the cross-correlation at small separation (r < 80 kpc). Then, in the range of 80 < r < 2 Mpc, the emission from large halos hosting satellite galaxies dominates. Finally, on scales beyond those considered here (r > 2 Mpc), the two-halo term becomes dominant. Interestingly, there is no scale at which the hot gas dominates. In the range (20 < r < 200 kpc), the hot gas contributes to more than 10% of the signal. Progressively, with the minimum stellar mass increasing, the hot gas emission increases. For the log M*/M⊙ > 10.75 sample, in the range 50–60 kpc, the three components contribute each the same surface brightness. For the log M*/M⊙ > 11 sample, the hot gas is the dominating emission source over the range of 30–200 kpc. Finally, for the log M*/M⊙ > 11.25 and (11.5) samples, the hot gas emission dominates over other components until 400 (700) kpc. We constrained the slope of the scaling relation between halo mass and X-ray luminosity (over three orders of magnitude in mass) at the 5% level, using the samples with the lowest mass threshold. We find a slope of $ 1.629^{+0.091}_{-0.089} $. Additional analyses explore the energy dependence of the cross-correlation and differences between red sequence and blue cloud galaxies, revealing sensitivity to galaxy quiescent fractions and opening avenues for a more complex, unified modeling of galaxies, active galactic nuclei (AGNs), and hot gas in the optical and X-rays.