GAS METALLICITIES IN THE EXTENDED DISKS OF NGC 1512 AND NGC 3621. CHEMICAL SIGNATURES OF METAL MIXING OR ENRICHED GAS ACCRETION?

We have obtained spectra of 135 HII regions located in the inner and extended disks of the spiral galaxies NGC 1512 and NGC 3621, spanning the range of galactocentric distances 0.2-2 x R-25 (from similar to 2-3 kpc to similar to 18-25 kpc). We find that the excitation properties of nebulae in the outer (R > R-25) disks are similar to those of the inner disks, but on average younger HII regions tend to be selected in the bright inner disks. Reddening by dust is not negligible in the outer disks and subject to significant large-scale spatial variations. For both galaxies, the radial abundance gradient flattens to a constant value outside of the isophotal radius. The outer disk O/H abundance ratio is highly homogeneous, with a scatter of only similar to 0.06 dex. In the case of the interacting galaxy NGC 1512 we find a number of HII regions with peculiar metallicity for their radius, a result which can be interpreted by gas flows activated by the gravitational encounter with NGC 1510. Based on the excitation and chemical (N/O ratio) analysis, we find no compelling evidence for variations in the upper initialmass function of ionizing clusters of extended disks. The O/H abundance in the outer disks of the target galaxies corresponds to similar to 35% of the solar value (or higher, depending on the metallicity diagnostic). This agrees with our earlier measurements in M83 and NGC 4625, and conflicts with the notion that metallicities in extended disks of spiral galaxies are low and on the order of similar to 0.1 x Z(circle dot). We show that, in general, the observed metal enrichment cannot be produced with the current level of star formation, even if the latter extends over a Hubble time. We discuss the possibility that metal transport mechanisms from the inner disks lead to metal pollution of the outer disks. Gas accretion from the intergalactic medium, enriched by outflows, offers an alternative solution, justified within the framework of hydrodynamic simulations of galaxy evolution. Specific model predictions of the chemical enrichment and the flat gradients in extended disks of nearby galaxies will be valuable to discriminate between these different scenarios.