Disk components in early-type galaxies.

Abstract

This thesis clarifies the role of disk components embedded in the spheroids of early type galaxies, with particular focus on the frequency and structure of disks in galaxies conventionally classified as "ellipticals". We discuss both photometric and spectroscopic means of assessing disks. Using simple photometric models, we explore what physical disk parameters result in detectable photometric signatures. We discuss in particular the deviations of the projected isophotes from perfect ellipses in disk/spheroid systems. We show that a wide range of intrinsic disk-to-spheroid ratios (D/S) can produce very similar photometric signatures, depending on viewing angle. We find the distribution of observed isophote distortions in a sample of ellipticals with published surface photometry to be consistent with the D/S hypothesis, implying that about half of the sample members could contain disks with D/S ∼ 0.25. To confront our models with a more suitable set of data, we obtained surface photometry at 0.4μ and 1.6μ for a statistical sample of about 80 galaxies, comprised of both E's and S0's. Analyzing this data set we find that in any given luminosity bin of early type galaxies, one third of the objects contain disks whose detectability depends on a favourably high inclination. This fraction was estimated independently from isophote distortions and from radial luminosity profiles. The apparent smooth transition between disk galaxies and purely spheroidal objects can be explained exclusively by changes in the viewing angle, even assuming two discrete classes of early type galaxies (either having substantial disks or none at all). There is no need to invoke continuity along the Hubble sequence from E's to S0's. For the members of this sample we find a considerable range in D/S, 0.15 < D/S < 5. However, most of that variation is caused by changes in the relative scale lengths rather than by changes in disk surface brightness. To analyze kinematic signatures of disk components we develop an optimal algorithm to extract the line-of-sight velocity distribution (LOSVD) from the broadening of absorption line spectra. Analyzing the LOSVD's in two kinematically distinct cores of elliptical galaxies, we find that they can be modelled dynamically as small disks embedded in the large spheroid. The range in rotational support, 1.3 < ν/σ < 4, of these disks suggests that some of them have formed dissipatively and others through a merger event.