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

摘要

We present new H I 21 cm absorption data and ultraviolet spectroscopy fromHST/STIS of the QSO/galaxy pair 3C 232/NGC 3067. The QSO sightline lies nearthe minor axis and 1.8 arcmin (11 kpc) above the plane of NGC 3067, a nearbyluminous (cz = 1465 km/s, L = 0.5L*) starburst galaxy with a moderate starformation rate of 1.4 Solar masses per year. The UV spectra show that the Si IVand 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, implyingthat the low and high ionization gas are both found in three distinct absorbingclouds (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 andkinetic 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 andSi IV are explained more easily as collisionally-ionized boundary layers of thephotoionized clouds. Due to their small inferred space velocity offsets (-260,-130, and +170 km/s) relative to the nucleus of NGC 3067 and the spatialcoincidence of low and high ionization gas, we propose that these absorbers areanalogous to Galactic high velocity clouds (HVCs). A comparison of the NGC 3067clouds and Galactic HVCs finds similar H I column densities, kinematics,metallicities, spin temperatures, and inferred sizes. We find no compellingevidence that any halo gas along this sightline is escaping the gravitationalpotential of NGC 3067, despite its modest starburst.