Yasuo Fukui (Nagoya)

Yasuo Fukui Colloquium: A new tool to measure dense HI gas based on the Planck dust properties

Marsfield Lecture Theatre

Abstract

Title

A new tool to measure dense HI gas based on the Planck dust properties:
Evidence for massive HI envelopes surrounding CO clouds

Abstract

The 21 m HI line emission has been useful in probing the major part of the interstellar gas over more than a half century. A difficulty has been due to the uncertainty in the HI physical conditions, i.e., the spin temperature Ts and line optical depth which cannot be directly derived from single observed parameter, the HI brightness. It is discussed that there is “dark gas” which is not detectable either in the HI or CO emission in the Galaxy, and that H2 without CO may offer a possible explanation for the dark gas whose density may lie around several 100 cm-3.

The dust properties derived by the Planck satellite offer a new accurate measure of the interstellar dust column density, and accordingly the gas column density. The new method, which I will present, allows us to estimate both Ts and HI column density. In this talk, I will present a comparison among CO, HI and Planck dust properties toward high-latitude CO clouds (MBM53-55), and show that the HI envelope around the CO clouds is very massive, in the order of 20,000Mo, as compared with the CO cloud whose mass is 2,000 Mo. Ts of the HI envelope is low in a range 20-80 K, where 70 % of the HI has Ts lower than 50 K and density higher than 100 cm-3. I will show that a similar trend is observed for the whole sky, and suggest that the cold and dense HI gas is common in the interstellar medium. I finally discuss such cold HI as the “dark gas” candidate.