Molecular Gas and Dark Neutral Medium in the Outskirts of Chamaeleon

TL;DR

This study used molecular absorption lines to detect and quantify molecular gas in the outskirts of Chamaeleon, revealing that the dark neutral medium is predominantly molecular rather than atomic, and accounting for previously inferred unseen gas.

Contribution

It demonstrates that H2 inferred from HCO+ absorption explains the dark neutral medium in Chamaeleon, showing the DNM is mainly molecular and clarifying the gas composition in the region.

Findings

HCO+ absorption detected in nearly all directions, indicating widespread molecular gas.

The DNM is mostly molecular, with a molecular fraction of 46% in high-DNM regions.

No substantial reservoir of undetected atomic or molecular dark gas remains.

Abstract

%context More gas is inferred to be present in molecular cloud complexes than can be accounted for by HI and CO emission, a phenomenon known as dark neutral medium (DNM) or CO-dark gas for the molecules. %aims To see if molecular gas can be detected in Chamaeleon when gas column densities in the DNM were inferred and CO emission was not detected. % methods We took 3mm absorption profiles of HCO+ and other molecules toward quasars across Chamaeleon, 1 of which had detectable CO emission. We derived N(H2) assuming N(HC+)/N(H2) = 3x10^{-9}. %results With the possible exception of 1 weak continuum target HCO+ absorption was detected in all directions, \cch\ in 8 and HCN in 4 directions. The sightlines divide in 2 groups according to their DNM content with 1 group of 8 directions having N(DNM) \ga 2x10^{20} \pcc and another group of 5 directions having N(DNM) < .5x10^{20}\pcc. The groups have comparable <N(HI)> in Chamaeleon 6-7 x 10^{20}\pcc and <N(H)/E(B-V) ~ 6-7x10^{21}\pcc/mag. They differ in having quite different <E(B-V)> 0.33 vs 0.18 mag, <N(DNM)> 3.3 vs .14 x 10^{20}\pcc and <2N(H2)> = 5.6 vs 0.8 x 10^{20} \pcc. Gas at more positive velocities is enriched in molecules and DNM. %conclusion Overall the quantity of H2 inferred from HCO+ fully accounts for the previously-inferred DNM along the studied sightlines. H2 is concentrated in the high-DNM group, where the molecular fraction is 46% vs. 13% otherwise and 38% overall. Thus, neutral gas in the outskirts of the complex is mostly atomic but the DNM is mostly molecular. Saturation of the HI emission may occur along 3 of the 4 sightlines having the largest DNM column densities but there is no substantial reservoir of 'dark' atomic or molecular gas that remains undetected as part of the inventory of dark neutral medium.