X-ray emission from star-forming galaxies - II. Hot interstellar medium

TL;DR

This study analyzes the hot interstellar medium in nearby star-forming galaxies, revealing thermal components, their temperatures, and correlations with star formation rates, providing insights into the energy conversion efficiency of supernovae.

Contribution

It presents the first comprehensive analysis of the hot interstellar medium's thermal properties across a broad galaxy sample, including temperature components and energy conversion estimates.

Findings

Presence of at least one thermal emission component in all galaxies.

Average temperature of hot gas is 0.24 keV; a second component at 0.71 keV in some galaxies.

Diffuse X-ray luminosity correlates linearly with star formation rate.

Abstract

We study the emission from the hot interstellar medium in a sample of nearby late type galaxies defined in Paper I. Our sample covers a broad range of star formation rates, from ~0.1 Msun/yr to ~17 Msun/yr and stellar masses, from ~3x10^8 Msun to ~6x10^10 Msun. We take special care of systematic effects and contamination from bright and faint compact sources. We find that in all galaxies at least one optically thin thermal emission component is present in the unresolved emission, with the average temperature of <kT>= 0.24 keV. In about ~1/3 of galaxies, a second, higher temperature component is required, with the <kT>= 0.71 keV. Although statistically significant variations in temperature between galaxies are present, we did not find any meaningful trends with the stellar mass or star formation rate of the host galaxy. The apparent luminosity of the diffuse emission in the 0.5-2 keV band linearly correlates with the star formation rate with the scale factor of Lx/SFR\approx 8.3x10^38 erg/s per Msun/yr, of which in average ~30-40% is likely produced by faint compact sources of various types. We attempt to estimate the bolometric luminosity of the gas and and obtained results differing by an order of magnitude, log(Lbol/SFR)\sim39-40, depending on whether intrinsic absorption in star-forming galaxies was allowed or not. Our theoretically most accurate, but in practice the most model dependent result for the intrinsic bolometric luminosity of ISM is Lbol/SFR\sim 1.5x10^40 erg/s per Msun/yr. Assuming that core collapse supernovae are the main source of energy, it implies that \epsilon_SN\sim5x10^-2 (E_SN/10^51)^-1 of mechanical energy of supernovae is converted into thermal energy of ISM.