Global Hot Gas Excess in (U)LIRGs: Replicating Galactic Nuclei Scaling Relations between Diffuse X-ray Emission and Star Formation on Galaxy-Wide Scales

Abstract

Hot ionized interstellar medium interlinks star formation and stellar feedback processes, redistributing energy, momentum, and material throughout galaxies. We use X-ray data from $Chandra$ to extract the hot gas emission from 78 of the most luminous infrared-selected galaxies in the local Universe. In the extreme star-forming environments, the intrinsic thermal X-ray luminosity of hot gas ($L_{\rm 0.5 - 2\,keV}^{\rm gas}$) shows a significant excess over the predictions of the standard linear $L_{\rm X}$$-$SFR relation for most objects with very high star formation rates (SFRs). The contribution of active galactic nuclei (AGNs) appears to have little impact on the global hot gas luminosity. For galaxies with SFR $\gt$ 50 ${M_{\rm \odot}}\,\,{\rm yr^{-1}}$, the Bayesian analysis gives a super-linear relation of ${\rm log}(L_{\rm 0.5-2\,keV}^{\rm gas} /{\rm erg\,s^{-1}})=1.34\,{\rm log}({\rm SFR}/{M_{\rm \odot}}\,{\rm yr^{-1}})+39.82$, similar to that found in the central regions of normal spiral galaxies. These results suggest a scenario in which the merger of galaxies delivers substantial amounts of gas, triggering intense star formation in both the nuclear region and the galactic disk, and ultimately enhancing the global thermal X-ray emission. The ratio of the apparent thermal luminosity in the 0.5$-$2 keV band ($L_{\rm 0.5 - 2\,keV}^{\rm appar}$) to $L_{\rm 0.5 - 2\,keV}^{\rm gas}$ shows statistically significant negative correlations with the intrinsic column density ($N_{\rm H}$) and SFR. Moreover, in contrast to the luminosity ratio, SFR shows a moderate positive correlation with intrinsic $N_{\rm H}$. This suggests that the correlation between $L_{\rm 0.5 - 2\,keV}^{\rm appar}$/$L_{\rm 0.5 - 2\,keV}^{\rm gas}$ and SFR may be driven by the underlying $L_{\rm 0.5 - 2\,keV}^{\rm appar}$/$L_{\rm 0.5 - 2\,keV}^{\rm gas}$$-$$N_{\rm H}$ and SFR$-$$N_{\rm H}$ relations.