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...

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Bibliographic Details
Published in:arXiv.org 2018-04
Main Authors: Howk, J Christopher, Rueff, Katherine M, Lehner, Nicolas, Wotta, Christopher B, Croxall, Kevin, Savage, Blair D
Format: Article
Language:English
Subjects:
Accretion disks
Emission spectroscopy
H II regions
Hot stars
Interstellar gas
Luminosity
Measurement methods
Metallicity
Orion nebula
Outflow
Spectrum analysis
Spiral galaxies
Spirals
Star & galaxy formation
Star formation
Stars & galaxies
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Summary: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.
ISSN:2331-8422
DOI:10.48550/arxiv.1802.10165