Spatially resolved star formation histories of nearby galaxies: evidence
  for episodic star formation in discs
  We use long-slit spectroscopy from Moran et al. to study the radial
dependence of the recent star formation histories of nearby galaxies with
stellar masses greater than 10^10M_sun. We fit stellar population models to the
combination of SSFR, D4000 and Hdelta_A and show that many galaxies have Balmer
absorption line equivalent widths that require recent short-lived episodes or
bursts of star formation. The fraction of galaxies that have experienced
episodic rather than continuous star formation is highest for late-type
galaxies with low stellar masses. In these systems, bursts occur both in the
inner and 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. When we
average over the population, we find that such bursts contribute around a half
of the total mass in stars formed in the last 2 Gyr. In massive galaxies,
bursts occur predominantly in the outer disk. Around a third of all massive,
bulge-dominated galaxies have experienced recent star formation episodes that
are fully confined to their outer (R > 0.7R_90) regions. The fraction of stars
formed in a single episode is only 2 - 3 % of the underlying stellar mass, but
such bursts contribute nearly all the stellar mass formed in the last 2 Gyr.
Recent star formation in outer disks is strongly correlated with the global
atomic gas fraction of the galaxy, but not its global molecular gas fraction.
We suggest that outer episodic star formation is triggered by gas accretion
events.
