The puzzling nature of dwarf-sized gas poor disk galaxies

Abstract

Early-type dwarf galaxies were originally described as elliptical feature-less galaxies. However, later disk signatures were revealed in some of them. In fact, it is still disputed whether they follow photometric scaling relations similar to giant elliptical galaxies or whether they are rather formed in transformations of late-type galaxies induced by the galaxy cluster environment. The early-type dwarf galaxies are the most abundant galaxy type in clusters, and their low-mass make them susceptible to processes that let galaxies evolve. Therefore, they are well-suited as probes of galaxy evolution. In this thesis we explore possible relationships and evolutionary links of early-type dwarfs to other galaxy types. We observed a sample of 121 galaxies and obtained deep near-infrared images. For analyzing the morphology of these galaxies, we apply two-dimensional multicomponent fitting to the data. This is done for the first time for a large sample of early-type dwarfs. A large fraction of the galaxies is shown to have complex multicomponent structures. The photometric parameters of the inner and outer components of the dwarfs are compared to bulge and disk components of other galaxy types from studies using a similar decomposition approach. The parameters of the bulges and disks form rather tight relations of size as a function of galaxy brightness. The inner and outer components of the dwarfs are offset from the extrapolations of these scaling relations, and we conclude that their nature is different. Complementary N-body simulations illustrate that the inner and outer components of the dwarfs may indeed have been formed from the disks of late-type galaxies that are ha- rassed by the cluster environment. The multiple structure components may also explain our finding that the early-type dwarf galaxies show subtle deviations from common scaling relations with bright elliptical galaxies, i.e. the relation of size versus galaxy brightness. The dwarfs and giants would be expected to follow jointly one such relation, based on the observed, continuos variation of the light profiles, if all the galaxies followed simple one-component profiles. Altogether our results indicate that many of the early-type dwarf galaxies may be disk galaxies. This view is supported by the fact that their sizes and galaxy brightnesses place the early-type dwarfs close to the extrapolation of the scaling relation of these parameters of disks in bright galaxies, but offset from that of the bulges. A possible explanation for early-type dwarfs beings disks is that they have been transformed from late-type disk galaxies by the cluster environment. Furthermore, we demonstrate that processes that may be responsible for such transformations are also viable options for: (i) explaining that the slowly moving nucleated early-type dwarfs in the cluster center are round, while the fast moving are flat, (ii) making blue-compact dwarfs to evolve into early-type dwarfs, and (iii) forming ultra-compact dwarfs by disruption of nucleated early-type dwarfs.