Extended, regular H I structures around early-type galaxies

Abstract

We discuss the morphology and kinematics of the H I of a sample of 30 southern gas-rich early-type galaxies selected from the H I Parkes All-Sky Survey (HIPASS). This is the largest collection of high-resolution H I data of a homogeneously selected sample. Given the sensitivity of HIPASS, these galaxies represent the most H I-rich early-type galaxies. In two-thirds of the galaxies, we find the H I to be in a large, regular disk- or ring-like structure that in some cases is strongly warped. In the remaining cases we find the H I distributed in irregular tails or clouds offset from the galaxy. The giant, regular H I structures can be up to ∼200 kpc in diameter and contain up to 1010 M⊙ of H I. The incidence of irregular H I structures appears to be somewhat higher in elliptical galaxies, but the large, regular structures are observed in both elliptical and S0 galaxies and are not strictly connected to the presence of a stellar disk. If these two types of galaxies are the result of different formation paths, this is not strongly reflected in the characteristics of the H I. The size and the regular kinematics of the H I structures imply that the neutral hydrogen must have settled in these galaxies several Gyr ago. Merging as well as gas accretion from the IGM are viable explanations for the origin of the gas in these galaxies. The average column density of the H I is low so that little star formation is expected to occur and these early-type galaxies can remain gas rich for very long periods of time. The large H I structures likely represent key structures for tracing the origin and evolution of these galaxies. © ESO 2007.