Revealing The CO X-factor In Dark Molecular Gas through Sensitive ALMA Absorption Observations

TL;DR

This study uses sensitive ALMA absorption observations to investigate the properties of CO and HCO+ in diffuse molecular gas, revealing that HCO+ is a more reliable tracer of H2 than CO in such environments.

Contribution

The paper provides the first direct measurements of excitation temperatures and the CO X-factor in diffuse molecular gas using sensitive absorption lines, highlighting HCO+ as a superior tracer of H2.

Findings

CO detected in very low extinction regions (A_v<0.32 mag)

HCO+ shows a strong correlation with H2 (P ~ 0.93)

Derived CO X-factor is about 6 times higher than the Milky Way average

Abstract

Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be under-abundant and/or sub-thermally excited. We performed one of the most sensitive (down to $\mathrm{\tau_{rms}^{CO} \sim 0.002}$ and $\mathrm{\tau_{rms}^{HCO^+} \sim 0.0008}$) sub-millimeter molecular absorption line observations towards 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to $\mathrm{A_v< 0.32\,mag}$ and \hcop was down to $\mathrm{A_v < 0.2\,mag}$, where atomic gas and dark molecular gas (DMG) starts to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures $\mathrm{T_{ex}}$ of 4.1\,K and 2.7\,K, for CO and for \hcop, respectively, which are close to the cosmic microwave background (CMB) and provide explanation for their being undercounted in emission surveys. A stronger linear correlation was found between $\mathrm{N_{HCO^+}}$ and $\mathrm{N_{H_2}}$ (Pearson correlation coefficient P $\sim$ 0.93) than that of $\mathrm{N_{CO}}$ and $\mathrm{N_{H_2}}$ (P $\sim$ 0.33), suggesting \hcop\ being a better tracer of H$_2$ than CO in diffuse gas. The derived CO-to-\h2 conversion factor (the CO X-factor) of (14 $\pm$ 3) $\times$ 10$^{20}$ cm$^{-2}$ (K \kms)$^{-1}$ is approximately 6 times larger than the average value found in the Milky Way.