Absorption‐Line Study of Halo Gas in NGC 3067 toward the Background Quasar 3C 232

Abstract

We present new H I 21 cm absorption data and ultraviolet spectroscopy from HST/STIS of the QSO/galaxy pair 3C 232/NGC 3067. The QSO sightline lies near the minor axis and 1.8 arcmin (11 kpc) above the plane of NGC 3067, a nearby luminous (cz = 1465 km/s, L = 0.5L*) starburst galaxy with a moderate star formation rate of 1.4 Solar masses per year. The UV spectra show that the Si IV and C IV doublets have the same three velocity components at cz = 1369, 1417, and 1530 km/s found in Ca II H & K, Na I D, Mg I, Mg II, and Fe II, implying that the low and high ionization gas are both found in three distinct absorbing clouds (only the strongest component at 1420 km/s is detected in H I 21 cm). The new Lyman alpha observation allows the first measurements of the spin and kinetic temperatures of halo gas: T_s = 435 +/- 140 K and T_k/T_s ~ 1. However, while a standard photoionization model can explain the low ions, the C IV and Si IV are explained more easily as collisionally-ionized boundary layers of the photoionized clouds. Due to their small inferred space velocity offsets (-260, -130, and +170 km/s) relative to the nucleus of NGC 3067 and the spatial coincidence of low and high ionization gas, we propose that these absorbers are analogous to Galactic high velocity clouds (HVCs). A comparison of the NGC 3067 clouds and Galactic HVCs finds similar H I column densities, kinematics, metallicities, spin temperatures, and inferred sizes. We find no compelling evidence that any halo gas along this sightline is escaping the gravitational potential of NGC 3067, despite its modest starburst.