The origin, evolution and classification of field and cluster galaxy populations

In this thesis I address the formation and evolution of lowmass galaxies and investigate their importance in the general galaxy evolution/formation process. Currently favored models of galaxy formation, such as cold dark matter (CDM), suggest that the first galaxies to form in the universe have low masses and sizes, similar to modern dwarf galaxies, that later merge to form more massive systems. The most common galaxies in the nearby universe are dwarf ellipticals in clusters that could be descendants of these early galaxies. This thesis addresses two questions related to these objects: (1) Are the dwarf elliptical galaxies seen in clusters an old primordial population that survived since their initial formation, or are these dwarfs the end product of some evolutionary dynamical process? (2) Do galaxy mergers occur in the universe, and if so, how does this merging evolve through time? To determine whether dwarf elliptical galaxies are an old cluster population, kinematic, structural, and photometric information for these objects was obtained using the WisconsinIndiana-Yale-NOAO 3.5 m telescope. Radial velocities of dwarf elliptical galaxies in the Virgo Cluster suggest that these objects are not an old cluster population but were accreted sometime in the past (C. J. Conselice, J. S. Gallagher III, & R. F. G. Wyse 2001, ApJ, 559, 791). Kinematics also demonstrate that these objects could have originated from a more massive progenitor. Photometry of dwarf elliptical galaxies in the Perseus Cluster further suggests that the stellar populations of these objects are old, with little to no recent star formation or population gradients. The broadband UBR colors of these objects are also consistent with color differences originating from metallicity. Low-mass galaxies in the Perseus Cluster however do not follow the color-magnitude relationship found for the giant elliptical galaxies, with some too red and others too blue for their magnitudes. Through comparisons with LCDM models of galaxy formation, I show that the blue lowmass cluster objects in Perseus are consistent with being old CDM galaxies. Low-mass cluster galaxies that are redder than the color-magnitude relationship are shown to be composed of stars with high metallicities. These red low-mass cluster galaxies must have very high mass-to-light ratios near 250 in solar units or must be the remnants of higher mass galaxies whose stellar material was stripped during high-speed impulsive interactions with other cluster galaxies. To address the issue of galaxy mergers, I develop and use a morphological system to identify robustly, and unambiguously, objects undergoing major mergers (C. J. Conselice, M. A. Bershady, & A. Jangren 2000, ApJ, 529, 886; C. J. Conselice, M. A. Bershady, & J. S. Gallagher III 2000, A&A, 354, L21). This system uses an asymmetry index, such that galaxies undergoing mergers have asymmetry values , with the A 1 0.35 range of possible values being 0–2. When this is applied to the galaxies in the Hubble Deep Field–North, an increase with redshift of galaxies undergoing major mergers is found. Depending on the method of computation, the merger fraction evolution out to can be characterized as , where m m z ∼ 3.5 f p (1 z) ranges from 1 to 2.5. The best fit gives a variable m, such that . m p 2.96 0.32z I also investigate the properties of a nearby massive central cluster galaxy, NGC 1275 in the Perseus Cluster, whose unusual properties and structural aspects can be partially accounted for by recent and past mergers (C. J. Conselice, J. S. Gallagher III, & R. F. G. Wyse 2001, AJ, 122, 2281). Finally, after summarizing the results of this thesis, as well as previous work, I present a new galaxy classification system according to which all galaxies can be fitted based on the three most salient properties that drive galaxy evolution: dissipation, star formation, and galaxy interactions.