Evidence for recent accretion in nearby galaxies

I discuss observations of magnitude residuals from the B-band Tully-Fisher relationship, B — Vcolor, chemical abundance gradients, and asymmetries in the H I and stellar disks of nearby spiral galaxies within the context of a model in which small satellites or H I clouds are accreted onto the outer disks of spiral galaxies. Correlations between the various observables support the hypothesis that accretion dilutes the gas-phase abundances in the outer disk, steepens the abundance gradient across the disk, increases the star formation rate, and creates asymmetries in the outer disk. By estimating the duration of steep abundance gradients, elevated rates of star formation, or outer disk asymmetries, we can place constraints on the rate of accretion events. The data suggest that accretion events at the current time are common.