Fire and Ice in the Whirlpool: Spatially Resolved Scaling Relations between X-ray Emitting Hot Gas and Cold Molecular Gas in M51

TL;DR

This study investigates the spatial relationships between hot X-ray emitting gas and cold molecular gas in M51, revealing correlations influenced by supernova heating and star formation activity on kiloparsec scales.

Contribution

It provides the first detailed spatially resolved analysis of the connection between hot and cold gas phases and their relation to star formation in M51.

Findings

Hot gas surface brightness decreases within 2 kpc of M51's center.

Positive correlation between infrared and hot gas luminosities varies radially.

Supernovae are the main heating source for hot gas in the galactic center.

Abstract

The cold and hot interstellar medium (ISM) in star forming galaxies resembles the reservoir for star formation and associated heating by stellar winds and explosions during stellar evolution, respectively. We utilize data from deep $Chandra$ observations and archival millimeter surveys to study the interconnection between these two phases and the relation to star formation activities in M51 on kiloparsec scales. A sharp radial decrease is present in the hot gas surface brightness profile within the inner 2 kpc of M51. The ratio between the total infrared luminosity ($L_{\rm IR}$) and the hot gas luminosity ($L_{\rm 0.5 - 2\,keV}^{\rm gas}$) shows a positive correlation with the galactic radius in the central region. For the entire galaxy, a twofold correlation is revealed in the $L_{\rm 0.5 - 2\,keV}^{\rm gas}$${-}$$L_{\rm IR}$ diagram, where $L_{\rm 0.5 - 2\,keV}^{\rm gas}$ sharply increases with $L_{\rm IR}$ in the center but varies more slowly in the disk. The best fit gives a steep relation of ${\rm log}(L_{\rm 0.5-2\,keV}^{\rm gas} /{\rm erg\,s^{-1}})=1.82\,{\rm log}(L_{\rm IR} /{L_{\rm \odot}})+22.26$ for the center of M51. The similar twofold correlations are also found in the $L_{\rm 0.5 - 2\,keV}^{\rm gas}$${-}$molecular line luminosity ($L^\prime_{\rm gas}$) relations for the four molecular emission lines CO(1-0), CO(2-1), HCN(1-0), and HCO$^+$(1-0). We demonstrate that the core-collapse supernovae (SNe) are the primary source of energy for heating gas in the galactic center of M51, leading to the observed steep $L_{\rm 0.5 - 2\,keV}^{\rm gas}$${-}$$L_{\rm IR}$ and $L_{\rm 0.5 - 2\,keV}^{\rm gas}$${-}$$L^\prime_{\rm gas}$ relations, as their X-ray radiation efficiencies ($\eta$ $\equiv$ $L_{\rm 0.5 - 2\,keV}^{\rm gas}$/$\dot{E}_\mathrm{SN}$) increase with the star formation rate surface densities, where $\dot{E}_\mathrm{SN}$ is the SN mechanical energy input rate.