Abstract

This thesis has focused on observational studies of galaxy evolution. We combine multiwavelength data to derive various physical properties of nearby galaxies. In particular, we study the recent star formation histories (SFHs) of galaxies from their optical spectra, and the relations between molecular gas and star formation of galaxies from their radio, ultraviolet, and infrared observations. First, we constrain the radial dependence of the recent SFHs of about 200 local galaxies with the long-slit spectroscopy data by fitting stellar population models to the combination of specific star formation rate (sSFR), 4000 Angstrom break strength and Balmer absorption lines. The late-type and early-type galaxies show distinct behaviors in their recent star formation histories. In late-type systems, bursts occur both in the inner and in the outer regions of the galaxy. The fraction of stars formed in a single burst episode is typically around 15% of the total stellar mass in the inner regions of the galaxy and around 5% of the mass in the outer regions. On the other hand, bursts occur predominantly in the outer disk in massive and bulge-dominated galaxies, and the fraction of stars formed in a single episode is only 2 - 3% of the underlying stellar mass. One of the most fundamental questions in modern astrophysics is how galaxies convert their gas into stars, and how this process may change with the galaxy internal properties and/or across cosmic time. We study the variations in molecular gas depletion time (tdep), defined as the molecular gas mass divided by the star formation rate (SFR), and which tells us how fast the gas will be consumed under the current SFR. We establish that the main parameter dependence of tdep is upon sSFR on both local and global scales. The strong correlation between tdep and sSFR extends continuously over a factor of 10 in tdep and from log sSFR = -11.5 to -9, i.e., from nearly quiescent patches of the disc to disc regions with very strong star formation. This leads to the conclusion that the local molecular gas depletion time in galactic disks is dependent on the local fraction of young-to-old stars and that galaxies with high current-to-past-averaged star formation activity, will drain their molecular gas reservoir sooner. We further study the impact of galaxy internal structures such as the bulge, arm, bar and ring on the variation of tdep on kiloparsec and global scales. The displacements in the main tdep-sSFR plane for different structures is linked to the variations in stellar, rather than gas surface densities: regions with high stellar surface densities such as the central bulges of galaxies have a reduced tdep at a given sSFR, while regions with low stellar surface densities such as the disk of galaxies have a longer tdep at a given sSFR. We provide our best current predictor for tdep, both globally and for 1kpc grids.