Gas infall and stochastic star formation in galaxies in the local universe

We study the recent star formation histories of local galaxies by analysing the scatter in their colours and spectral properties. We present evidence that the distribution of star formation histories changes qualitatively above a characteristic stellar surface mass density of 3 x 10(8)M circle dot kpc(-2), corresponding to the transition between disc-dominated (late-type) galaxies and bulge-dominated (early-type) systems. When we average over subpopulations of galaxies with densities below this value, we find that subpopulations of all masses and densities form their stars at the same average rate per unit stellar mass. However, the scatter in galaxy colours, stellar absorption-line indices and emission-line strengths is larger for more compact galaxies of a given mass. This suggests that star formation occurs in shorter, higher amplitude events in galaxies with smaller sizes. Above the characteristic density, galaxy growth through star formation shuts down and the scatter in galaxy colours and spectral properties decreases. We propose that in low-density galaxies, star formation events are triggered when cold gas is accreted on to a galaxy. We have used a new high-resolution numerical simulation of structure formation in a 'concordance' Lambda cold dark matter (Lambda CDM) universe to quantify the incidence of these accretion events, and we show that the observational data are well fitted by a model in which the consumption time of accreted gas decreases with the surface density of the galaxy as t(cons) proportional to mu(-1)(double dagger). The dark matter haloes that host massive galaxies with high stellar surface mass densities are also expected to grow through accretion, but the observations indicate that in bulge-dominated galaxies, star formation is no longer coupled to the hierarchical build-up of these systems.