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 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 $M_V=-8.21\pm 0.24$. 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 $\approx 6$ 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.

Astrophysical Journal, in press. 9 pages