H I, 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 H I Envelope around the CO Clouds

Abstract

We present an analysis of the H I and CO gas in conjunction with the Planck/IRAS submillimeter/far-infrared dust properties toward the most outstanding high latitude clouds MBM 53, 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 H I emission and the dust properties is less clear than in CO. The integrated H I intensity W_{{H {I}}} and τ₃₅₃ show a large scatter with a correlation coefficient of ~0.6 for a T _d range from 16 K to 22 K. We find, however, that W_{{H {I}}} and τ₃₅₃ show better correlation for smaller ranges of T _d every 0.5 K, generally with a correlation coefficient of 0.7-0.9. We set up a hypothesis that the H I gas associated with the highest T _d >= 21.5 K is optically thin, whereas the H I emission is generally optically thick for T _d lower than 21.5 K. We have determined a relationship for the optically thin H I gas between atomic hydrogen column density and τ₃₅₃, N_{{H {I}}} (cm^-2)=(1.5× 10²⁶)\cdot τ ₃₅₃, under the assumption that the dust properties are uniform and we have applied this to estimate N_{{H {I}}} from τ₃₅₃ for the whole cloud. N_{{H {I}}} was then used to solve for T _s and τ _{{H {I}}} over the region. The result shows that the H I is dominated by optically thick gas having a low spin temperature of 20-40 K and a density of 40-160 cm^-3. The H I envelope has a total mass of ~1.2 × 10⁴ M _⊙, an order of magnitude larger than that of the CO clouds. The H I envelope properties derived by this method do not rule out a mixture of H I and H₂ in the dark gas, but we present indirect evidence that most of the gas mass is in the atomic state.