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

TL;DR

This study investigates the stellar kinematics and photometric properties of low-luminosity early-type galaxies in clusters, revealing their formation history, stellar components, and constraints on merger scenarios through combined spectroscopic and imaging data.

Contribution

It provides new detailed kinematic and photometric profiles of early-type galaxies at large radii, constraining their formation mechanisms and merger history.

Findings

Galaxies host a cold, old stellar component dominating their dynamics.

Discy isophotes and ellipticity variations suggest gradual gas dissipation.

Major mergers are unlikely; minor mergers at high redshift are possible.

Abstract

We present the results of a combined analysis of the kinematic and photometric properties at large galactocentric radii of a sample of 14 low-luminosity early-type galaxies in the Fornax and Virgo clusters. From Gemini South GMOS long-slit spectroscopic data we measure radial profiles of the kinematic parameters v_{rot}, sigma, h_{3}, and h_{4} out to ~ 1 - 3 effective radii. Multi-band imaging data from the HST/ACS are employed to evaluate surface brightness profiles and isophotal shape parameters of ellipticity, position angle and discyness/boxiness. The galaxies are found to host a cold and old stellar component which extend to the largest observed radii and that is the dominant source of their dynamical support. The prevalence of discy-shaped isophotes and the radial variation of their ellipticity are signatures of a gradual gas dissipation. An early star-forming collapse appears to be the main mechanism acting in the formation of these objects. Major mergers are unlikely to have occurred in these galaxies. We can not rule out a minor merging origin for these galaxies, but a comparison of our results with model predictions of different merger categories places some constraints on the possible merger progenitors. These merger events are required to happen at high-redshift (i.e., z > 1), between progenitors of different 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 galaxies were originally late-type galaxies, whose star formation has been truncated by removal of gas and subsequently the disc has been dynamically heated by high speed encounters in the cluster environment.