Extended, regular  structures around early-type galaxies

¹ Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands e-mail: oosterloo@astron.nl
² Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
³ School of Physics, University of Sydney, NSW 2006, Australia

Received: 12 September 2006
Accepted: 13 January 2007

Abstract

We discuss the morphology and kinematics of the  of a sample of 30 southern gas-rich early-type galaxies selected from the  Parkes All-Sky Survey (HIPASS). This is the largest collection of high-resolution  data of a homogeneously selected sample. Given the sensitivity of HIPASS, these galaxies represent the most -rich early-type galaxies. In two-thirds of the galaxies, we find the  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  distributed in irregular tails or clouds offset from the galaxy. The giant, regular  structures can be up to ~200 kpc in diameter and contain up to 10¹⁰ of . The incidence of irregular  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 . The size and the regular kinematics of the  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  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  structures likely represent key structures for tracing the origin and evolution of these galaxies.