Gas-Phase Oxygen Gradients in Strongly Interacting Galaxies: I.
  Early-Stage Interactions
  A consensus is emerging that interacting galaxies show depressed nuclear gas
metallicities compared to isolated star-forming galaxies. Simulations suggest
that this nuclear underabundance is caused by interaction-induced inflow of
metal-poor gas, and that this inflow concurrently flattens the radial
metallicity gradients in strongly interacting galaxies. We present
metallicities of over 300 HII regions in a sample of 16 spirals that are
members of strongly interacting galaxy pairs with mass ratio near unity. The
deprojected radial gradients in these galaxies are about half of those in a
control sample of isolated, late-type spirals. Detailed comparison of the
gradients with simulations show remarkable agreement in gradient distributions,
the relationship between gradients and nuclear underabundances, and the shape
of profile deviations from a straight line. Taken together, this evidence
conclusively demonstrates that strongly interacting galaxies at the present day
undergo nuclear metal dilution due to gas inflow, as well as significant
flattening of their gas-phase metallicity gradients, and that current
simulations can robustly reproduce this behavior at a statistical level.
