Spatially resolved star formation histories of nearby galaxies: evidence for episodic star formation in discs

Using long-slit spectroscopy from Moran et al. (2012), we constrain the radial dependence of the recent star formation histories of nearby galaxies with stellar masses greater than 1010 M⊙. By fitting stellar population models to the combination of specific star formation rate, Dn4000 and HγA, we show that the star formation histories of many disc galaxies cannot be accurately represented if their star formation rates declined exponentially with time. 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, low surface densities and low concentrations. In these 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 per cent of the total stellar mass in the inner regions of the galaxy and around 5 per cent 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 disc. Around a third of all massive, bulge-dominated galaxies have experienced recent star formation episodes that are fully confined to their outer (R > 0.7R90) regions. The fraction of stars formed in bursts is only ˜2-3 per cent of the underlying stellar mass, but when we average over the population, we find that such bursts contribute nearly all the stellar mass formed in the last 2 Gyr. Recent star formation in outer discs 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'. Episodic star formation in the inner regions is suppressed in galaxies with large bulge-to-disc ratio. This supports the idea that inner bursts are linked to instability-driven gas inflows.