Unveiling the hot interstellar medium in nearby galaxies with eROSITA

Abstract

Diffuse X-ray emission in galaxies arises from both stellar-feedback-heated gas and unresolved compact sources. Previous studies mainly focused on the integrated X-ray luminosity, leaving the thermal component, especially in low star formation rate (SFR) galaxies, largely unexplored. We aim to characterize the diffuse X-ray emission of nearby galaxies over a wide range of SFRs and to test how the X-ray luminosities L_ scale with star formation in a blind volume-limited sample. We analyzed data of the four eROSITA/ all-sky surveys (eRASS:4) for 140 galaxies within 25,Mpc drawn from the Updated Nearby Galaxy Catalog and cross-matched with HECATEv2. Early-type galaxies were treated separately, while the star-forming galaxies were divided by specific SFR (sSFR) into low-, medium-, and high-sSFR subsamples. We extracted spectra of the integrated emission and modeled them with thermal plasma and power law components. For each subsample, we stacked the spectra and derived total and thermal-component X-ray luminosities. We then fitted relations for the star-forming galaxies. Spectrum-Roentgen-Gamma The stacked spectra show a systematic change with sSFR: High-sSFR galaxies exhibit a pronounced soft thermal component, whereas low-sSFR and early-type galaxies are increasingly dominated by the harder nonthermal continuum. For the total 0.5--8,keV luminosity, we find łog L_ ∼ 39.5 + 0.9,łog X SFR , with an intrinsic scatter of ∼0.9, , with a smaller intrinsic scatter of ∼0.7, For the 0.5--2,keV luminosity of the thermal component, we find łog L_ ∼ 38.7 + 1.1,łog X SFR The X-ray luminosity of the thermal component remains well correlated with the SFR down to ∼10^ -5 , and is consistent with previous relations for star-forming galaxies, while the total L_X- -1 relation is flatter and more scattered. This result suggests that at low SFR, the total X-ray output is likely dominated by a small number of unresolved X-ray binaries, whereas the thermal X-ray luminosity remains a robust tracer of stellar-feedback-related X-ray emission.