Extraplanar H II Regions in Spiral Galaxies. II. In Situ Star Formation in the Interstellar Thick Disk of NGC 4013

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 show this H II region has an H\(\alpha\) lumi...

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Bibliographic Details
Published in:arXiv.org 2018-02
Main Authors: Howk, J Christopher, Rueff, Katherine M, Lehner, Nicolas, Wotta, Christopher B, Croxall, Kevin, Savage, Blair D
Format: Article
Language:English
Subjects:
H II regions
Hot stars
Interstellar
Knots
Luminosity
Orion nebula
Photons
Spiral galaxies
Star & galaxy formation
Star clusters
Star formation rate
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Summary: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 show this H II region has an H\(\alpha\) luminosity \(\sim 4\) - 7 times that of the Orion nebula, with an implied ionizing photon production rate \(\log Q_0 \gtrsim 49.4\) (photons s\(^{-1}\)). HST/WFPC2 imaging reveals an associated blue continuum source with \(M_{V} = -8.21\pm0.24\). Together these properties demonstrate the H II region is powered by a young cluster of stars formed {\em in situ} in the thick disk with an ionizing photon flux equivalent to \(\sim\)6 O7 V stars. If we assume \(\approx6\) other extraplanar \halpha -emitting knots are H II regions, the total thick disk star formation rate of \ngc 4013 is \(\sim 5 \times 10^{-4}\) M\(_\odot\) yr\(^{-1}\). The star formation likely occurs in the dense clouds of the interstellar thick disk seen in optical images of dust extinction and CO emission.
ISSN:2331-8422
DOI:10.48550/arxiv.1802.10166