The dark neutral medium is (mostly) molecular hydrogen^★

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
e-mail: hliszt@nrao.edu
² LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Paris, France
e-mail: maryvonne.gerin@obspm.fr

Received: 27 February 2023
Accepted: 12 May 2023

Abstract

Context. More gas is sometimes inferred in molecular cloud complexes than is represented in HI or CO emission, and this is called dark neutral medium (DNM).

Aims. Our aim is to extend a study of DNM along 13 directions in the outskirts of Chamaeleon by determining the atomic or molecular character of the DNM along a larger sample of sightlines.

Methods. We acquired ALMA ground rotational state absorption profiles of HCO⁺ and other molecules toward 33 compact extragalactic continuum background sources seen toward the Galactic anticenter, deriving N(H₂)=N(HCO⁺)/3 × 10⁻⁹ as before. We observed J = 1−0 CO emission with the IRAM 30m telescope in directions where HCO⁺ was newly detected.

Results. HCO⁺ absorption was detected in 28 of 33 new directions and CO emission along 19 of those 28. The five sightlines lacking detectable HCO⁺ have three times lower <E_B–V> and <N(DNM)>. Binned in E_B–V, N(H₂) and N(DNM) are strongly correlated and vary by factors of 50–100 over the observed range E_B–V ≈0.05–1 mag, while N(HI) varies by factors of only 2–3. On average N(DNM) and N(H₂) are well matched, and detecting HCO⁻ absorption adds little to no H₂ in excess of the previously inferred DNM. There are five cases where 2N(H₂) < N(DNM)/2 indicates saturation of the HI emission profile. For sightlines with W_CO ≥ 1 K-km s^‒1, the CO-H₂ conversion factor N(H₂)/W_CO = 2–3 × 10²⁰ cm^‒2 /(1 K-km s^‒1) is higher than is derived from studies of resolved clouds in γ-rays.

Conclusions. Our work sampled primarily atomic gas with a mean H₂ fraction ≈1/3, but the DNM is almost entirely molecular. CO fulfills its role as an H₂ tracer when its emission is strong, but large-scale CO surveys are not sensitive to H₂ columns associated with typical values N(DNM) = 2–6 × 10²⁰ cm^‒2. Lower X_CO values from γ-ray studies arise in part from different definitions and usage. Sightlines with W_CO ≥ 1 K-km s^‒1 represent 2/3 of the H₂ detected in HCO+ and detecting 90% of the H₂ would require detecting CO at levels W_co ≈ 0.2–0.3 K-km s^‒1.