Paper detail

A Direct Detection of Gas Accretion: The Lyman Limit System in 3C 232

The gas added and removed from galaxies over cosmic time greatly affects their stellar populations and star formation rates. QSO absorption studies in close QSO/galaxy pairs create a unique opportunity to study the physical conditions and kinematics of this gas. Here we present new Hubble Space Telescope (HST) images of the QSO/galaxy pair 3C 232/NGC 3067. The quasar spectrum contains a Lyman-limit absorption system (LLS) due to NGC 3067 at cz = 1421 km/s that is associated with the nearby SAB galaxy NGC 3067. Previous work identifies this absorber as a high-velocity cloud (HVC) in NGC 3067 but the kinematics of the absorbing gas, infalling or outflowing, were uncertain. The HST images presented here establish the orientation of NGC 3067 and so establish that the LLS/HVC is infalling. Using this system as a prototype, we extend these results to higher-z Mg II/LLS to suggest that Mg II/LLSs are a sight line sampling of the so-called "cold mode accretion" (CMA) infalling onto luminous galaxies. But to match the observed Mg II absorber statistics, the CMA must be more highly ionized at higher redshifts. The key observations needed to further the study of low-z LLSs is HST/UV spectroscopy, for which a new instrument, the Cosmic Origins Spectrograph, has just been installed greatly enhancing our observational capabilities.