Poster Presentation

Study of physical states of molecular gas in NGC 3627 by CO multi-line observations with ALMA

Author(s): Shibata Kazuki(the University of Tsukuba), Watanabe Yoshimasa (Nihon University), Kuno Nario(the University of Tsukuba), Sorai Kazuo(Hokkaido University)

Presenter: Kazuki Shibata (the University of Tsukuba)

In nearby galaxies, it is known that star formation efficiencies are different from region to region within a disk. Elucidating the origin of the diversity of star formation is one of the challenges in the star formation studies of nearby galaxy. Therefore, it is important to investigate the physical state of the molecular gas that is the material for star formation. For this purpose, we evaluate the physical state of molecular gas in the nearby galaxy NGC 3627, because NGC 3627 is known to have more active star formation at the bar-ends (both ends of the bar structure) than the spiral arm. We estimated the physical state of molecular gas at the bar-end and the spiral arm on the south side of NGC 3627 and compared it with star-forming activity. In this study, we use molecular emission line data obtained with the ALMA. By comparing three spectral lines of 12CO (J = 1 − 0), 12CO (J = 2 − 1) and 13CO (J = 1 − 0) with a model based on statistical equilibrium using RADEX(Van der Tak et al. 2007), we can estimate the physical state of molecular clouds such as density and temperature. Since the molecular emission line of 12CO is optically thick, we assume that there is a large velocity gradient in the molecular cloud and applied the LVG approximation method to estimate the physical state of the molecular cloud with local statistical equilibrium. From this analysis, the kinematic temperature and the number density of molecular hydrogen are estimated to be 30 K and 8.5 x 10^4 cm^-3, respectively, at the southern bar-end which has the most active star forming region. These temperature and density are found to be higher than those in the spiral arm by factors of three and six, respectively. Beuther et al. (2017) has indicated that molecular clouds with different velocities are colliding at the boundary of the spiral arm and the bar-end. Therefore, our result suggests that the active star formation at the bar-end is originated by dense molecular gas which is efficiently produced by gravitational contraction after the collisions of molecular clouds which occurred at the upstream of the bar-end.