Surface photometry of early-type galaxies in rich clusters

Abstract

This thesis investigates the morphology of early-type galaxies in two rich clusters using 2D surface photometry. In particular, the amount of light in the 'disk' component is focussed upon, as the presence of a disk is the main morphological criterion in distinguishing between the traditional 'elliptical' and 'S0' classes. Extensive and photometric E-band CCD observations of continuous areas of the Coma and Abell 1367 clusters were obtained at the 2.5 m Isaac Newton telescope, La Palma during March 1994. A subset of this large data-set has been used in this study, comprising a magnitude-limited (to R = 15.6) sample of 153 galaxies in the two clusters. Surface photometry measurements, including surface brightness profiles and isophotal shapes, have been made for the sample. Atmospheric seeing is a major problem when measuring light profiles at the distance of Coma from ground-based telescopes. Typical seeing at La Palma (FWHM~1.2") is a significant fraction of the effective radius of many Coma/Abell 1367 galaxies (r(_e)~3" for small ellipticals). An iterative algorithm was developed to deconvolve the effects of seeing from surface brightness profiles. The result of the algorithm is to extend the range of useful surface photometry inwards to within 2 times the FWHM. In order to parametrise the surface brightness profiles and discriminate between different profile-types, further software was developed to fit one- and two-component model profiles to the seeing-corrected data. The following parameters were measured and tabulated for each of the 153 galaxies: total magnitude M(_t); half-light parameters r (_1/2) and (μ)(_1/2); SB at half-light radius μ(r(_1/2)); photometric diameter D(_19.23) (equivalent to D(_n)); ellipticity at R = 21.5 isophote ϵ(_21.5); averaged isophote high-order terms (C(_3)), (S(_3)), (C(_4)) and (S(_4)); effective radii and surface brightnesses of 5 single power-law r(^1)(_n) models, r"e and (^)"^ (n = 1,2,3,4,5); best-fitting power-law index n; bulge effective radii and surface brightnesses from the two-component fit and (/^) disk effective parameters r'^e and {nY^] and disk-to-bulge luminosity ratio DjB. The measured parameters have been used to investigate various aspects of early-type galaxy morphology. The conclusions are outlined below. Firstly, a two-component r? plus exponential model is a better fit to most galaxies than a single component law fit. Secondly, the traditional division of early-type galaxies into 'elliptical' and 'SO' classes is severely biased by the viewing angle. In fact, it appears that early-type galaxies comprise a population of objects with smoothly varying bulge-to-disk ratio - although a few ellipticals (less than 13%) do not appear to have a exponential component. Finally, there is a general correlation (with much scatter) between the size and the profile shapes of early-type galaxies. The interpretation is that smaller galaxies are more disk-dominated than larger galaxies, which can be linked to the merging process in rich clusters. [brace not closed]