| In this thesis I study two important classes of emission-line galaxies in the Sloan Digital Sky Survey (SDSS): Active Galactic Nuclei (AGN) and metal-poor galaxies. A large sample of 1,543 broad-line (Seyfert 1s) and 3,035 narrow-line (Seyfert 2s) AGN are identified from about 90,000 galaxy spectra selected over 1151 square degrees from the SDSS spectroscopic data. Broad-line AGN are selected if their measured Hα Full-Width Half-Maximum (FWHM) values are larger than 1,200 kms−1, a criterion developed from the bimodal distribution of the Hα FWHM of emission-line galaxies. Narrow-line AGN are identified using standard emission line ratio diagnostic diagrams.; This AGN sample is used to estimate the AGN luminosity function. The Hα luminosity functions for all AGN cover a luminosity range of a factor of 104 and the shapes are well fit by broken power laws. We compare the narrow-line Hα, [OIII] and [OII] luminosity functions for Seyfert 1s and Seyfert 2s; we find that the numbers of Seyfert 1s and Seyfert 2s are comparable at low luminosity, while at high luminosity, Seyfert 1s outnumber Seyfert 2s by a factor of 2-4. In making the luminosity function measurements, we also conclude that the nuclear luminosity is independent to the host galaxy luminosity. This suggests that the main variable that determines the AGN luminosity is the Eddington ratio, not the black hole mass.; On a separate topic, we measured the oxygen abundances for 18,461 emission-line galaxies selected from the SDSS and connect them with the distance to their nth nearest neighbor. We find that there is a clear deficiency of metal-poor galaxies in very dense environments. In moderately dense environment, metal-poor galaxies are systematically more isolated than are metal-rich galaxies, which are yet more isolated than pure absorption-line galaxies. However, for more isolated galaxies the normalized fraction of metal-poor and metal-rich galaxies are about the same, both exceeding that of pure absorption-line galaxies. This is in agreement with the biased galaxy formation model (Dekel & Silk 1986), in which the majority of dwarf galaxies originate as low-mass primordial density fluctuations. |
