Metallicity gradients in small and nearby spiral galaxies

Abstract: Spectra of HII regions obtained with Gemini/GMOS are used to derive the radial metallicity gradients of four small, low-mass spiral galaxies. The analysis of the outer disk of one of them, NGC 1058, uncovers the characteristic flattening found in similar extended disk galaxies. After combining these data with published long-slit observations of nearby spiral galaxies, no evidence for a dependence of the disk scale length-normalized metallicity gradients with stellar mass is found, down to log(M*/Msun) ~ 8.5. The abundance gradients derived from these observations are compared to predictions from recent cosmological simulations of galaxy evolution, finding that in several cases the simulations fail to reproduce the mean steepening of the gradients, expressed in dex/kpc, with decreasing stellar mass for present-day galaxies, or do not extend to sufficiently small stellar masses for a meaningful comparison. The mean steepening of the abundance gradients (in dex/kpc) with decreasing disk scale length is in qualitative agreement with predictions from the inside-out model of Boissier & Prantzos, although the predicted slopes are systematically steeper than observed. This indicates the necessity of including processes such as outflows and radial mixing in similar models of galactic chemical evolution. Published spatially resolved metallicity and photometric data of dwarf irregular galaxies suggest that significant, but transitory, metallicity gradients can develop for systems that have experienced recent (t < 100 Myr) enhanced star formation in their inner disks.