COMPLEX Mg Ⅱ ABSORPTION IN THE OUTER DISK OF M61

摘要

The increasing availability of high-quality spectra of QSO absorption-line systems at resolutions of only a few kilometers per second is expected to facilitate the translation of the kinematics of components comprising the lines into the spatial distribution of gas around an absorbing galaxy. In this Letter we present Hubble Space Telescope (HST) spectra of Q1219+047, a QSO whose line of sight passes 21 h~(-1) kpc from the center of M61, through the outer regions of an extended H Ⅰ disk. We detect complex Mg Ⅱ absorption, spanning a velocity range of ≈ 300 km s~(-1), and strong C Ⅳ absorption; these are the first UV observations of a QSO absorption-line system arising in the outskirts of a disk of a nearby galaxy at low inclination. Our observations are at odds with models of galaxies in which absorbing clouds corotate with a galaxy's disk, because M61's low inclination should give rise to only a few Mg Ⅱ components spread over a small velocity range in such a model. Hence our results throw doubt on whether absorption-line profiles can be used to infer the spatial distribution of gas around absorbing galaxies.