FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45-m Telescope (FUGIN) V: Dense gas mass fraction of molecular gas in the Galactic plane

Recent observations of the nearby Galactic molecular cloudsindicate that the dense gas in molecular cloudshave quasi- universal propertieson star formation, and observational studies of extra galaxies have shown a galactic- scale correlationbetween the star formationrate (SFR) and surface density of molecular gas. To reach a comprehensive understanding of both properties, it is important to quantify the fractional mass of the dense gas in molecular cloudsf_DG. In particular, for the Milky Way (MW), there are no previous studies resolving the f_DG disk over a scale of several kpc. In this study, the f_DG was measured over 5kpc in the first quadrant of the MW, based on the CO J=1-0 data in l=10-50 deg obtained as part of the FOREST Unbiased Galactic PlaneImaging Survey with the Nobeyama 45-m Telescope (FUGIN) project. The total molecular mass was measured using 12CO, and the dense gas mass was estimated using C18O. The fractional masses including f_DG in the region within ~30% of the distances to the tangential points of the Galactic rotation (e.g., the Galactic Bar, Far-3kpc Arm, Norma Arm, Scutum Arm, SagittariusArm, and inter-arm regions) were measured. As a result, an averaged f_DG of 2.9^{+2.6}_{-2.6} % was obtained for the entirety of the target region. This low value suggests that dense gas formation is the primary factor of inefficient star formationin galaxies. It was also found that the f_DG shows large variations depending on the structures in the MW disk. The f_DG in the Galactic arms were estimated to be ~4-5%, while those in the bar and inter-arm regions were as small as ~0.1-0.4%. These results indicate that the formation/destruction processes of the dense gas and their timescales are different for different regions in the MW, leading to the differences in SFRs.