H i, CO, and Dust in the Perseus Cloud

Comparison analyses between the gas emission data ( line and line) and the Planck/IRAS dust emission data (optical depth at and dust temperature ) allow us to estimate the amount and distribution of the hydrogen gas more accurately, and our previous studies revealed the existence of a large amount of optically thick gas in the solar neighborhood. Referring to this, we discuss the neutral hydrogen gas around the Perseus cloud in the present paper. By using the J-band extinction data, we found that increases as a function of the 1.3th power of column number density of the total hydrogen ( ), and this implies dust evolution in high density regions. This calibrated relationship shows that the amount of the gas can be underestimated to be if the optically thin method is used. Based on this relationship, we calculated the optical depth of the line ( ) and found that around the molecular cloud. The effect of is still significant, even if we take into account the dust evolution. We also estimated a spatial distribution of the -to- conversion factor ( ), and we found its average value is . Although these results are inconsistent with some previous studies, these discrepancies can be well explained by the difference of the data and analyses methods.