Extraplanar H II Regions in Spiral Galaxies. I. Low-Metallicity Gas
  Accreting through the Disk-Halo Interface of NGC 4013
  The interstellar thick disks of galaxies serve as the interface between the
thin star-forming disk, where feedback-driven outflows originate, and the
distant halo, the repository for accreted gas. We present optical emission line
spectroscopy of a luminous thick disk H II region located at $z = 860$ pc above
the plane of the spiral galaxy NGC 4013 taken with the Multi-Object Double
Spectrograph on the Large Binocular Telescope. This nebula, with an H$\alpha$
luminosity $\sim4-7$ times that of the Orion nebula, surrounds a luminous
cluster of young, hot stars that ionize the surrounding interstellar gas of the
thick disk, providing a measure of the properties of that gas. We demonstrate
that strong emission line methods can provide accurate measures of relative
abundances between pairs of H II regions. From our emission line spectroscopy,
we show that the metal content of the thick disk H II region is a factor of
$\approx2$ lower than gas in H II regions at the midplane of this galaxy (with
the relative abundance of O in the thick disk lower by $-0.32\pm 0.09$ dex).
This implies incomplete mixing of material in the thick disk on small scales
(100s of parsecs) and that there is accretion of low-metallicity gas through
the thick disks of spirals. The inclusion of low-metallicity gas this close to
the plane of NGC 4013 is reminiscent of the recently-proposed "fountain-driven"
accretion models.
