Early-type galaxies at large galactocentric radii - I. Stellar kinematics and photometric properties

摘要

We present the results of a combined analysis of the kinematic andphotometric properties at large galactocentric radii of a sample of 14low-luminosity early-type galaxies in the Fornax and Virgo clusters. FromGemini South GMOS long-slit spectroscopic data we measure radial profiles ofthe kinematic parameters v_{rot}, sigma, h_{3}, and h_{4} out to ~ 1 - 3effective radii. Multi-band imaging data from the HST/ACS are employed toevaluate surface brightness profiles and isophotal shape parameters ofellipticity, position angle and discyness/boxiness. The galaxies are found tohost a cold and old stellar component which extend to the largest observedradii and that is the dominant source of their dynamical support. Theprevalence of discy-shaped isophotes and the radial variation of theirellipticity are signatures of a gradual gas dissipation. An early star-formingcollapse appears to be the main mechanism acting in the formation of theseobjects. Major mergers are unlikely to have occurred in these galaxies. We cannot rule out a minor merging origin for these galaxies, but a comparison of ourresults with model predictions of different merger categories places someconstraints on the possible merger progenitors. These merger events arerequired to happen at high-redshift (i.e., z > 1), between progenitors ofdifferent mass ratio (at least 3:1) and containing a significant amount of gas(i.e., > 10 percent). A further scenario is that the low-luminosity galaxieswere originally late-type galaxies, whose star formation has been truncated byremoval of gas and subsequently the disc has been dynamically heated by highspeed encounters in the cluster environment.