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The interstellar thick disks of galaxies serve as the interface between thethin star-forming disk, where feedback-driven outflows originate, and thedistant halo, the repository for accreted gas. We present optical emission linespectroscopy of a luminous thick disk H II region located at $z = 860$ pc abovethe plane of the spiral galaxy NGC 4013 taken with the Multi-Object DoubleSpectrograph on the Large Binocular Telescope. This nebula, with an H$\alpha$luminosity $\sim4-7$ times that of the Orion nebula, surrounds a luminouscluster of young, hot stars that ionize the surrounding interstellar gas of thethick disk, providing a measure of the properties of that gas. We demonstratethat strong emission line methods can provide accurate measures of relativeabundances 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 (withthe 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 throughthe thick disks of spirals. The inclusion of low-metallicity gas this close tothe plane of NGC 4013 is reminiscent of the recently-proposed "fountain-driven"accretion models.

Extraplanar H II Regions in Spiral Galaxies. I. Low-Metallicity Gas Accreting through the Disk-Halo Interface of NGC 4013