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

TL;DR

This study provides evidence of in situ star formation in the thick disk of NGC 4013, revealing extraplanar H II regions powered by young star clusters, and estimates the galaxy's star formation rate in this region.

Contribution

It presents the first detailed observations confirming in situ star formation in the thick disk of a spiral galaxy, with spectroscopic and imaging data supporting the existence of extraplanar H II regions.

Findings

Extraplanar H II region powered by a young star cluster.

Star formation rate in the thick disk estimated at ~5 x 10^-4 M_sun/yr.

Star formation likely occurs in dense clouds within the thick disk.

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 $M_{V} = -8.21\pm0.24$. 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.