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Description
We analyze molecular-gas formation in neutral atomic hydrogen (HI) clouds using the latest CO data, obtained from the FOREST (four-beam receiver system on the 45m telescope) unbiased Galactic plane imaging survey with the Nobeyama 45m telescope, and using HI data taken from the Very Large Array Galactic plane survey. We applied a dendrogram algorithm to the HI data cube to identify HI clouds, and we calculated the HI mass and molecular-gas mass by summing the CO line intensity within each HI cloud. On the basis of the results, we created a catalog of 5737 identified HI clouds with local standard of rest (LSR) velocity of VLSR<=-20km/s in galactic longitude and latitude ranges of 20{deg}<=l<=50{deg} and -1{deg}<=b<=1{deg}, respectively. We found that most of the HI clouds are distributed within a Galactocentric distance of 16kpc, and most of them are in the cold neutral medium phase. In addition, we determined that the high-mass end of the HI mass function is fitted well with a power-law function with an index of 2.3. Although two sequences of self-gravitating and diffuse clouds are expected to appear in the M_tot_-M_H2_ diagram according to previous works based on a plane-parallel model, the observational data show only a single sequence with large scattering within these two sequences. This implies that most of the clouds are mixtures of these two types of clouds. Moreover, we suggest the following scenario of molecular-gas formation: an HI-dominant cloud evolved with increasing H2 mass along a path of M_H2_{prop.to}M^2^_tot_ by collecting diffuse gas before reaching and moving along the curves of the two sequences.
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