The dark neutral medium is (mostly) molecular hydrogen

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 (H 2 )= 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 and . Binned in E B–V , N (H 2 ) 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 (H 2 ) are well matched, and detecting HCO − absorption adds little to no H 2 in excess of the previously inferred DNM. There are five cases where 2 N (H 2 ) < N (DNM)/2 indicates saturation of the HI emission profile. For sightlines with W CO ≥ 1 K-km s ‒1 , the CO-H 2 conversion factor N (H 2 )/ W CO = 2–3 × 10 20 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 H 2 fraction ≈1/3, but the DNM is almost entirely molecular. CO fulfills its role as an H 2 tracer when its emission is strong, but large-scale CO surveys are not sensitive to H 2 columns associated with typical values N (DNM) = 2–6 × 10 20 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 H 2 detected in HCO+ and detecting 90% of the H 2 would require detecting CO at levels W co ≈ 0.2–0.3 K-km s ‒1