Extreme isolated elliptical galaxies

Abstract

This dissertation presents the results of a detailed search for and the photometric and spectroscopic analysis of nine elliptical galaxies isolated from nearest neighbors of M V < ?16.5 by distances in excess of 2.5 megaparsec. The extreme isolation of these galaxies makes them ideal laboratories for the study of galaxy evolution, independent of the continual galaxy-galaxy interaction in the clusters within which most ellipticals reside. A comparative density analysis technique was developed to analyse the local environments around the elliptical galaxies with the objective of validating that these objects indeed reside in extremely isolated environments. Global and surface photometry are used to verify the elliptical morphology of the candidate galaxies and to look for both isophotal deviations from a perfectly elliptical shape and overall deviations from the deVaucoulers R 1/4 surface brightness profile expected in an elliptical galaxy. Two-dimensional residual maps produced by model galaxy subtractions are used to search for fine structure in the galaxies. Clues provided by these analyses are used to make preliminary estimates of the past evolutionary history of the galaxies, and to eliminate galaxies in the study that do not show evidence of elliptical morphology. Stellar population modelling and spectral synthesis techniques are used to determine luminosity-weighted mean ages for the galaxies' stellar populations and to reveal starforming histories. The highlights of the research are summarized as follows: of the thirteen original candidate galaxies, only six are determined to be true isolated elliptical galaxies. Furthermore, all of the isolated ellipticals possess a population of young stars. Because these stars are about 2 Gyr of age, the conclusion is that these galaxies have experienced merger activity in the last 2¿3 Gyr. None of the ellipticals appears to be a ¿pristine¿ elliptical (defined in the dissertation to describe a hypothetical galaxy that formed as an elliptical and has avoided subsequent interaction with other galaxies). The very low occurrence of shells is evidence that these merger signatures are relatively short lived in highly isolated environments, and do not persist for more than about 2¿3 Gyr.