Evidence for the Accretion of Gas in Star-Forming Galaxies: High N/O
  Abundances in Regions of Anomalously-Low Metallicity
  While all models for the evolution of galaxies require the accretion of gas
to sustain their growth via on-going star formation, it has proven difficult to
directly detect this inflowing material. In this paper we use data of nearby
star-forming galaxies in the SDSS IV Mapping Nearby Galaxies at Apache Point
Observatory (MaNGA) survey to search for evidence of accretion imprinted in the
chemical composition of the interstellar medium. We measure both the O/H and
N/O abundance ratios in regions previously identified as having anomalously low
values of O/H. We show that the unusual locations of these regions in the N/O
vs. O/H plane indicate that they have been created through the mixing of disk
gas having higher metallicity with accreted gas having lower metallicity. Taken
together with previous analysis on these anomalously low-metallicity regions,
these results imply that accretion of metal-poor gas can probably sustain star
formation in present-day late-type galaxies.
