Diffuse X-Ray-emitting Gas in the Central Region of Star-Forming Galaxies

Chunyi Zhang, Junfeng Wang

Published 2025-06-21Version 1

The interstellar medium of galaxies, with temperatures reaching several million degrees, provides a pivotal perspective for understanding the physical and chemical properties of star formation, galactic evolution, and their associated feedback mechanisms. We use archival data from $Chandra$ observations to extract the diffuse X-ray emission from 23 nearby star-forming galaxies and study its correlation with star formation activity in the central region of these galaxies. The surface brightness profile of each galaxy presents a sharp decrease in the central region of $\thicksim$0.3$-$2 kpc and then varies slowly outside this range. Compared to the global relation between the diffuse thermal X-ray luminosity from hot gas ($L_{\rm 0.5 - 2\,keV}^{\rm gas}$) and the star formation rate (SFR), we found a super-linear relation of ${\rm log}(L_{\rm 0.5-2\,keV}^{\rm gas} /{\rm erg\,s^{-1}})=1.34\,{\rm log}({\rm SFR}/{M_{\rm \odot}}\,{\rm yr^{-1}})+40.15$ for the center of these sample galaxies. This result suggests that more intense stellar feedback is associated with stronger star formation activity in the central region of star-forming galaxies, where more energy output from supernovae (SNe) and stellar winds is converted into X-ray flux. Furthermore, the slope of the $L_{\rm 0.5 - 2\,keV}^{\rm gas}$$-$SFR relation anticorrelates with spatial scale in the galactic central region. This indicates that the characteristics of central hot gas emission are gradually averaged over larger areas. The diffuse X-ray luminosity also shows a good correlation with molecular gas, stellar mass, and mid-plane pressure traced by the baryonic mass, although these relations show relatively large scatter.