Spatially-resolved star formation histories and molecular gas depletion time of nearby galaxies

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.