Thermal Pressures in the Interstellar Medium away from Stellar Environments

TL;DR

This study measures interstellar thermal pressures using C I absorption lines in extragalactic sources, confirming the presence of high-pressure gas in the general interstellar medium independent of stellar influences.

Contribution

It extends previous Galactic star-based measurements to extragalactic sources, demonstrating that high-pressure gas exists broadly in the interstellar medium without stellar environment bias.

Findings

Low-pressure distribution favors slightly higher pressures than earlier

Fraction of high-pressure gas remains consistent with previous results

High-pressure gas exists in the general interstellar medium, not just stellar regions

Abstract

Interstellar thermal pressures can be measured using C I absorption lines that probe the pressure-sensitive populations of the fine-structure levels of its ground state. In a survey of C I absorption toward Galactic hot stars, Jenkins & Tripp (2011) found evidence of small amounts ($\sim 0.05\%$) of gas at high pressures ($p/k \gg 10^4{\rm cm^{-3}K}$) mixed with a more general presence of lower pressure material exhibiting a log normal distribution that spanned the range $10^3 \lesssim p/k \lesssim 10^4{\rm cm^{-3}K}$. In this paper, we study Milky Way C I lines in the spectra of extragalactic sources instead of Galactic stars and thus measure the pressures without being influenced by regions where stellar mass loss and H II region expansions could create localized pressure elevations. We find that the distribution of low pressures in the current sample favors slightly higher pressures than the earlier survey, and the fraction of gaseous material at extremely high pressures is about the same as that found earlier. Thus we conclude that the earlier survey was not appreciably influenced by the stellar environments, and the small amounts of high pressure gas indeed exist within the general interstellar medium.