Quantifying Dark Gas

TL;DR

This paper investigates the elusive dark molecular gas (DMG) in the interstellar medium using absorption techniques, revealing that OH is a promising tracer for DMG regions lacking CO, with implications for galaxy evolution.

Contribution

It presents preliminary observational results on DMG using CO and OH absorption surveys, highlighting OH as an effective DMG tracer in CO-free regions.

Findings

OH excitation temperature is close to the Galactic background.

OH effectively traces DMG co-existing with H$_2$ in CO-free regions.

Absorption mapping with SKA and ALMA will enhance understanding of ISM components.

Abstract

A growing body of evidence has been supporting the existence of so-called "dark molecular gas" (DMG), which is invisible in the most common tracer of molecular gas, i.e., CO rotational emission. DMG is believed to be the main gas component of the intermediate extinction region between A$\rm_v$$\sim$0.05-2, roughly corresponding to the self-shielding threshold of H$_2$ and $^{13}$CO. To quantify DMG relative to HI and CO, we are pursuing three observational techniques, namely, HI self-absorption, OH absorption, and TeraHz C$^+$ emission. In this paper, we focus on preliminary results from a CO and OH absorption survey of DMG candidates. Our analysis show that the OH excitation temperature is close to that of the Galactic continuum background and that OH is a good DMG tracer co-existing with molecular hydrogen in regions without CO. Through systematic "absorption mapping" by Square Kilometer Array (SKA) and ALMA, we will have unprecedented, comprehensive knowledge of the ISM components including DMG in terms of their temperature and density, which will impact our understanding of galaxy evolution and star formation profoundly.