Optically Thick H I Dominant in the Local Interstellar Medium: An Alternative Interpretation to “Dark Gas”

Abstract

Dark gas in the interstellar medium (ISM) is believed to not be detectable either in CO or H i radio emission, but it is detectable by other means including γ rays, dust emission, and extinction traced outside the Galactic plane at |b| > 5• . In these analyses, the 21 cm H i emission is usually assumed to be completely optically thin. We have reanalyzed the H i emission from the whole sky at |b| > 15• by considering temperature stratification in the ISM inferred from the Planck/IRAS analysis of the dust properties. The results indicate that the H i emission is saturated with an optical depth ranging from 0.5 to 3 for 85% of the local H i gas. This optically thick H i is characterized by spin temperature in the range 10 K-60 K, significantly lower than previously postulated in the literature, whereas such low temperature is consistent with emission/absorption measurements of the cool H i toward radio continuum sources. The distribution and the column density of the H i are consistent with those of the dark gas suggested by γ rays, and it is possible that the dark gas in the Galaxy is dominated by optically thick cold H i gas. This result implies that the average density of H i is 2-2.5 times higher than that derived on the optically thin assumption in the local ISM.

Quantifying the Interstellar Medium and Cosmic Rays in the MBM 53, 54, and 55 Molecular Clouds and the Pegasus Loop Using Fermi-Lat Gamma-Ray Observations

“…The applicability of their results to more translucent clouds of complicated geometries is not clear and theoretical investigations are needed. Another possibility to explain the large dark-gas fraction (compared with the H 2 traced by CO) and the scatter in the W H I -N (H tot ) relation, both seen in Figure 9, is the optical thickness of the H I 21-cm line (e.g., Fukui et al 2014Fukui et al , 2015. Then, W H I can be correlated with N (H tot ) as a function of the spin temperature T s as…”

“…By comparing the Planck dust emission model, and the H I and CO data, the Planck Collaboration (2011) estimated the mass of dark gas to be ∼30% of the atomic gas and ∼120% of the CO-bright molecular gas in the solar neighborhood. By comparing the Planck dust optical depth map at 353 GHz (τ 353 ), and the H I/CO data and assuming that the total gas column density was proportional to τ 353 , Fukui et al (2014Fukui et al ( , 2015 proposed that a significant amount of the atomic hydrogen was optically thick in areas with low dust temperature (T d ), resulting in an excess mass comparable to the mass of H I in the optically thin case. The Planck Collaboration (2014), on the other hand, found that the dust radiance R (bolometric luminosity) was well correlated with the integrated H I 21-cm line intensity, W H I , in wide range of T d in the diffuse ISM, and proposed that it would be a better tracer of the dust (and the total gas) column density.…”

Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks

“…The estimated face values themselves are important, but moreover, our results strongly suggest that understanding the fate of dispersed gas is inevitable to study the gas resurrecting processes in the ISM. The gas phases that are not well observed yet (e.g., CO-dark H 2 gas (Hosokawa & Inutsuka 2006;Tang et al 2016;Xu et al 2016) and optically thick HI gas (Fukui et al 2015a)) also come into play as well as usual CO-bright molecular gas. Three dimensional detailed magnetohydrodynamics simulation, for example, is required to understand the evolution of those unseen gas phases and to test whether it reproduces ε res that we predict here.…”

Formation and Fractionation of CO (Carbon Monoxide) in Diffuse Clouds Observed at Optical and Radio Wavelengths

“…It also bears on the question of "dark" gas generally, where the presence of more gas is indicated by gamma-rays or dust emission/extinction than would ordinarily be inferred from 21cm H I and mm-wave CO emission (Grenier et al 2005;Planck Collaboration et al 2011, 2015. The gas shortfall has been variously attributed to optically thick H I that is underrepresented in H I emission (Fukui et al 2015, but see Stanimirović et al (2014)) or to H 2 that is missed in CO emission (Wolfire et al 2010). …”