Extraplanar H II Regions in Spiral Galaxies. II. In Situ Star Formation in the Interstellar Thick Disk of NGC 4013
Abstract
We present observations of an Hα-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 that this H II region has an Hα luminosity ∼4-7 times that of the Orion nebula, with an implied ionizing photon production rate log Q 0 ≈ 49.4 (photons s-1). HST/WFPC2 imaging reveals an associated blue continuum source with M V = -8.21 ± 0.24. Together, these properties demonstrate that the H II region is powered by a young cluster of stars formed in situ in the thick disk, with an ionizing photon flux equivalent to ∼6 O7 V stars. If we assume ≈6 other extraplanar Hα-emitting knots are H II regions, the total thick disk star formation rate of NGC 4013 is ∼5 × 10-4 M ⊙ 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2018
- DOI:
- 10.3847/1538-4357/aab105
- arXiv:
- arXiv:1802.10166
- Bibcode:
- 2018ApJ...856..167H
- Keywords:
-
- galaxies: individual: NGC 4013;
- galaxies: ISM;
- galaxies: spiral;
- H ii regions;
- ISM: abundances;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Astrophysical Journal, in press. 9 pages