Abstract

Contributed Talk - Splinter StarFormation   (MW-1250)

Unveiling the Hot Interstellar Medium in Nearby Galaxies with eROSITA

Roman Laktionov, Manami Sasaki, Elias Kyritsis, Andreas Zezas, Steven Hämmerich, Jonathan R. Knies, Jörn Wilms, Frank Haberl, Martin G. H. Krause, Ashley Coombs, Philipp Weber, and Aafia Zainab
Dr. Karl Remeis Observatory Bamberg, ECAP, FAU; Max-Planck-Institut für extraterrestrische Physik, Garching; Physics Department & Institute of Theoretical & Computational Physics, University of Crete; Institute of Astrophysics, Foundation for Research and Technology-Hellas, Heraklion; Centre for Astrophysics Research, University of Hertfordshire

Context. 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. Aims. 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 LX scale with star formation in a blind, volume-limited sample. Methods. We analyzed data of the four eROSITA/SRG (Spectrum Roentgen Gamma) all-sky surveys (eRASS:4) for 149 galaxies within 25 Mpc, drawn from the Updated Nearby Galaxy Catalog (Karachentsev et al. 2013), and cross-matched with HECATEv2 (Kyritsis et al. 2026). 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 diffuse 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. Results. 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, non-thermal continuum. For the total 0.5–8 keV luminosity, we obtain log LX ∼ 38.9 + 0.7 log SFR, with an intrinsic scatter of ∼1.0 dex. For the 0.5–2 keV luminosity of the thermal component, we find log LX ∼ 38.6 + 1.1 log SFR, with a smaller intrinsic scatter of ∼0.50 dex. Conclusions. The X-ray luminosity of the thermal component remains well correlated with the SFR down to ∼10−4 M⊙ yr−1 and is consistent with previous relations for star-forming galaxies, while the total LX − SFR relation is significantly 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.