The Morphology, Environment, and Interstellar Medium of Early‐Type Galaxies as a Means for Studying the Evolution of Galaxies Outside of Clusters

Abstract

The main goal of this work is to study the properties of isolated early-type galaxies with the hope of learning about their formation and evolution. With this goal in mind, B and I optical and near-infrared images of four different samples Ks of early-type galaxies in contrasting galactic density environments have been secured with the Steward Observatory 90 and 61 inch telescopes. The four early-type galaxy samples consist of a sample extracted from the Catalog of Isolated Galaxies (V. E. Karachentseva 1973, Commun. Spec. Astrophys. Obs., 8, 3), a sample of isolated galaxies previously studied by G. Fasano & C. Bonoli (1989, AA most of the galaxies in this sample have been studied by T. Wiklind, F. Combes, & C. Henkel 1995, AA T. Wiklind & C. Henkel 1995, AA and W. K. Huchtmeier, L. J. Sage, & C. Henkel 1995, A&A, 300, 675). This data set is used to examine whether differences in the galactic environment are related to differences in key properties of early-type galaxies. Multicolor surface photometry analysis is performed on the four samples, and the results are used in several contexts. For most of the galaxies in the Karachentseva sample, surface photometry is reported for the first time. Likewise, data for Ks most of the objects are also reported for the first time. The light profile of the galaxies is fitted with Sersic profile functions (J. L. Sersic 1968, Atlas de 1/n I(r) p I exp { b [(r/r ) 1]} e n e Galaxias Australes [Cordoba: Observatorio Astronomico]). We have found that the correlation between the logarithm of the Sersic structure parameter n and the Sersic effective radius re previously discussed by other investigations (e.g., N. Caon, M. Cappacioli, & M. D’Onofrio 1993, MNRAS, 265, 1013 and A. Graham, T. R. Lauer, M. Colles, & M. Postman 1996, ApJ, 465, 534) appears to be valid over a fairly wide range of galaxy sizes. Furthermore, we have found that this correlation does not appear to be strongly dependent on other properties, such as environment or ISM content. Since the de Vaucouleurs profile function is a special case (when ) of the Sersic profile n p 4 function, some of the limitations of the widely accepted de Vaucouleurs function to describe the light profile of elliptical galaxies are discussed. The B I color index, ellipticity, position angle, and thirdand fourth-order Fourier coefficients are derived from the surface photometry. It is confirmed that the third-order coefficients signal the presence of dust and that the coefficient of cos 4v gives structural information on the galaxies. Furthermore, the subtraction of model elliptical galaxies has revealed peculiarities in the morphology, such as disks, spiral arms, dust lanes, multiple nuclei, merger signatures, etc. As an example of model subtraction, we present in Figure 1 the I-band image of the galaxy KIG 83 (UGC 1503) and in Figure 2 the ratio of the original image and a model image. Whereas spiral features can barely be seen in Figure 1, such features clearly stand out in Figure 2. The results of the surface photometry as well as the morphological analysis are used to compare the properties among the different samples. The largest and most relevant sample in this investigation consists of 39 isolated galaxies, and it contains ∼25% of the galaxies classified as elliptical galaxies in the Karachentseva catalog. According to our morphological analysis, only approximately one-half of the galaxies classified as elliptical galaxies in that catalog appear to be correctly classified, a result that may imply a reduction in the percentage of elliptical galaxies in the Karachentseva catalog to ∼6% of the total population of isolated galaxies. This result suggests that the overall percentage of elliptical galaxies that exist in isolated regions of space is smaller by a factor of ∼2 than currently accepted values. Most of the misclassified galaxies turned out to have spiral arms, but a significant number of perturbed galaxies or merger candidates were also found among the galaxies in the Karachentseva catalog. It is argued in the thesis that the fraction of merger candidates among isolated elliptical galaxies can be used to empirically constrain models for the evolution of compact groups into isolated galaxies (J. Saucedo & J. Bieging 2001, Ap&SS, in press). Aside from the optical and infrared studies, the results of a search for the CO(1–0) molecular line for six of the isolated early-type galaxies in the Karachentseva catalog taken at the