Variations between Dust and Gas in the Diffuse Interstellar Medium. 2. Search for Cold Gas

TL;DR

This study investigates the discrepancy between dust-inferred gas content and 21-cm emission measurements in diffuse interstellar clouds, testing if cold atomic hydrogen accounts for the excess 'dark' gas, and finds it does not.

Contribution

It provides evidence that cold atomic hydrogen is not the primary source of the 'dark' gas in diffuse interstellar clouds, refining our understanding of interstellar medium composition.

Findings

21-cm absorption is insufficient to explain the excess gas

Cold atomic hydrogen does not account for the 'dark' gas

Dust-based estimates suggest more gas than 21-cm emission indicates

Abstract

The content of interstellar clouds, in particular the inventory of diffuse molecular gas, remains uncertain. We identified a sample of isolated clouds, approximately 100 solar masses in size, and used the dust content to estimate the total amount of gas. In Paper 1, the total inferred gas content was found significantly larger than that seen in 21-cm emission measurements of atomic hydrogen. In this paper we test the hypothesis that the apparent excess 'dark' gas is cold atomic hydrogen, which would be evident in absorption but not in emission due to line saturation. The results show there is not enough 21-cm absorption toward the clouds to explain the total amount of 'dark' gas.