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.