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تسجيل مستخدم جديدExtraplanar H II Regions in Spiral Galaxies. I. Low-Metallicity Gas Accreting through the Disk-Halo Interface of NGC 4013
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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 $-0.32pm 0.09$ 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.
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We present observations of an H$alpha$ emitting knot in the thick disk of NGC 4013, demonstrating it is an H II region surrounding a cluster of young hot stars $z = 860$ pc above the plane of this edge-on spiral galaxy. With LBT/MODS spectroscopy we
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