Molecular Gas And Star Formation In Nearby Galaxies

In the local Universe, stars form within molecular clouds. Therefore, the properties of molecular clouds may determine the star formation rate. Conversely, star formation also gives feedback to the clouds where the stars reside. In this dissertation, I present the interplay between the molecular gas and star formation, through three parts below. First, I identify and characterize the properties of molecular clouds in NGC4526, resulting in the first catalog of molecular clouds in an early-type galaxy. As a population, the molecular clouds in NGC4526 are gravitationally bound and have a steeper mass distribution than that in the Milky Way. These molecular clouds are also more luminous, denser, and have a higher velocity dispersion than their counterparts in the Milky Way. These different properties may be due to a more intense interstellar radiation field than in the Galactic disk and a weaker external pressure than in the Galactic center. Second, I combine the mm-wave interferometric data from CARMA and the optical Integral Field Unit data from CALIFA to study the molecular depletion time on kilo-parsec scales of nearby galaxies. In particular, the molecular depletion time between the galactic centers and disks is compared. I find that some galactic centers have shorter depletion time than that in the disks, which means that those centers form stars more efficiently per unit molecular gas mass. This places the galactic centers as an intermediate regime between galactic disks and starburst galaxies. The central drop of depletion time is also correlated with a central increase in the stellar mass surface density, suggesting that a shorter depletion time is associated with the molecular gas compression by the stellar gravitational potential. Third, the feedback from star formation to maintain turbulence in the interstellar matter of M33 is investigated. I show that supernovae have enough energy to maintain atomic gas turbulence inside 4 kpc radius and within molecular clouds, assuming a constant value of turbulent dissipation time of 9.8 Myrs. In the outer parts, the energy from the differential rotation of galaxy is large enough to maintain atomic gas turbulence through the magneto-rotational instability (MRI). I conclude that the sum of supernovae and MRI energy maintains turbulence at all radii where atomic hydrogen is detected in M33.