H ɪ, CO, and Planck/IRAS dust properties in the high latitude cloud complex, MBM 53, 54, 55 and HLCG 92-35. Possible evidence for an optically thick HI envelope around the CO clouds

We present an analysis of the Hɪ and CO gas in conjunction with the Planck/IRAS submillimeter/far-infrared dust properties toward the most outstanding high latitude clouds MBM53, 54, 55 and HLCG 92 − 35 at b = −30° to−45°. The CO emission, dust opacity at 353 GHz (τ₃₅₃), and dust temperature (T_d) show generally good spatial correspondence. On the other hand, the correspondence between the Hɪ emission and the dust properties is less clear than in CO. The integrated Hɪ intensity W_Hɪ and τ₃₅₃ show a large scatter with a correlation coefficient of ∼0.6 for a T_d range from 16K to 22K. We find, however, that W_Hɪ and τ₃₅₃ show better correlation for smaller ranges of T_d every 0.5K, generally with a correlation coefficient of 0.7–0.9.We set up a hypothesis that the Hɪ gas associated with the highest T_d ≥ 21.5K is optically thin, whereas the Hɪ emission is generally optically thick for T_d lower than 21.5K. We have determined a relationship for the optically thin Hɪ gas between atomic hydrogen column density and τ₃₅₃, N_Hɪ (cm⁻²) = (1.5 × 10²⁶)· τ₃₅₃, under the assumption that the dust properties are uniform and we have applied this to estimate N_Hɪ from τ₃₅₃ for the whole cloud. N_Hɪ was then used to solve for τ_s and N_Hɪ over the region. The result shows that the Hɪ is dominated by optically thick gas having a low spin temperature of 20–40K and a density of 40–160 cm⁻³. The Hɪ envelope has a total mass of ∼1.2 × 10⁴ M_☉, an order of magnitude larger than that of the CO clouds. The Hɪ envelope properties derived by this method do not rule out a mixture of Hɪ and H₂ in the dark gas, but we present indirect evidence that most of the gas mass is in the atomic state.