The nature of flat galaxies

Disk galaxies range from bulge-dominated early-type galaxies to late types with little or no bulge. Cosmological models do not predict the formation of disk-dominated, essentially bulgeless galaxies, yet these objects exist. A particularly striking and poorly understood example of bulgeless galaxies are flat or superthin galaxies with large axis ratios. We therefore embarked on a study aimed at a better understanding of these enigmatic objects, starting by compiling a statistically meaningful sample with well-defined properties. The disk axis ratios can be most easily measured when galaxies are seen edge-on. We used data from the Sloan Digital Sky Survey (SDSS) in order to identify edge-on galaxies with disks in a uniform, reproducible, automated fashion. In the five-color photometric database of the SDSS Data Release(2099 deg2 ) we identified 3169 edge-on disk galaxies, which we subdivided into disk galaxies with bulge, intermediate types, and simple disk galaxies without any obvious bulge component. We subdivided these types further into subclasses: Sa(f), Sb(f), Sc(f), Scd(f), Sd(f), Irr(f), where the (f) indicates that these galaxies are seen edge-on. We find that approximately 15% of the edge-on disk galaxies in our catalog are flat galaxies, demonstrating that these galaxies are fairly common, especially among intermediate-mass star-forming galaxies. Bulgeless disks account for roughly one third of our galaxies when also puffy disks and edge-on irregulars are included. Interactions between galaxies can transform their morphology, and thus lead to a morphologydensity relationship even in regions where galaxies are sparsely distributed. We study environments of pure disk galaxies–the simple disk systems–in comparison to systems with bulges as a way to explore how environment relates to galaxy morphology. We study neighbors around the target disk galaxies and seek statistical relationships between local galaxy densities and galaxy structures. The observed trends are interpreted in the context of models for galaxy formation and evolution. Galaxies consisting of simple stellar disks exist in environments ranging from galaxy groups to the isolated field. This distribution overlaps with that of systems with prominent bulges, although galaxies with large bulges are systematically rare at low densities. The edge-on simple disk systems are similar in size but about one magnitude less luminous than galaxies containing bulges. Thus the morphology-density trend among edge-on systems is both one of luminosity, and thus likely stellar mass, and structure. The simple disk galaxies also are bluer, implying star formation extends over a longer time. These characteristics are discussed in terms of the roles of initial angular momentum and interactions in defining the structures of disk galaxies. The presence of simple disks in isolation and also moderate size groups suggests that groups bridge between field regions where galaxy properties are determined by very local conditions and clusters where the global environment dominates. The last part of this thesis contains a deep Ks -band imaging survey of a sample of edge-on galaxies selected from our catalog. Ks -band permits us to obtain stellar mass distributions of the nuclear and disk components on a uniform basis and fairly unimpeded by dust. We measure morphologies and structural parameters of these components in different galaxy types with special emphasis on thick disks as evolutionary tracers. The thin and thick disk properties are compared with predictions from merger models and pure infall scenarios to constrain galaxy evolution models. The combination of optical and Ks -band colors allows us to constrain the distribution of dust and the vertical color gradients in these galaxies.