Gas Metallicities in the Extended Disks of NGC 1512 and NGC 3621.
  Chemical Signatures of Metal Mixing or Enriched Gas Accretion?
  (Abridged) 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 R25 (from 2-3 kpc to 18-25 kpc). We
find that the excitation properties of nebulae in the outer (R>R25) 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 ~0.06 dex. Based on the excitation and
chemical (N/O ratio) analysis we find no compelling evidence for variations in
the upper initial mass function of the ionizing clusters of extended disks. The
O/H abundance in the outer disks of the target galaxies corresponds to 35% of
the solar value (or higher, depending on the metallicity diagnostic). This
conflicts with the notion that metallicities in extended disks of spiral
galaxies are necessarily low. The observed metal enrichment cannot be produced
with the current level of star formation. 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.
