Type II quasars from the Sloan Digital Sky Survey

Type II quasars are luminous Active Galactic Nuclei (AGN) whose central regions are shielded from the observer by large amounts of gas and dust. Until recently only a few dozen candidate type II quasars were known. In this thesis I present a sample of 291 type II AGN candidates selected from the spectroscopic database of the Sloan Digital Sky Survey, based on their emission lines properties. About 50% of the objects have [OIII]lambda5007A line luminosities in the range 3 x 108 - 1010 L⊙ , comparable to those of luminous (-27 < MB < -23) quasars, suggesting that the objects in the luminous subsample are type II quasars. I study the multi-wavelength properties of type II quasars using multi-wavelength surveys and pointed observations of objects in this sample. I use X-ray data from RASS, Chandra and XMM-Newton to probe absorption by neutral gas. These observations indicate that significant column densities (NH > 1022 cm-2) of intervening material are present. I use mid- and far-IR data from the IRAS survey and the Spitzer Space Telescope to study the spectral energy distribution of type II AGN. I find that the apparent IR luminosities of the objects in the sample range between 1044 and 1046 erg sec-1. Furthermore, the data indicate that AGN emission is anisotropic even at mid-IR wavelengths. I also present spectropolarimetry observations of a luminous subsample of objects, finding that type II quasar candidates are highly polarized and that they contain typical UV-excess broad-line AGN in their centers. Furthermore, 3-band HST images were obtained for nine type II quasars. These high quality images unencumbered by bright nuclei reveal a wealth of details about the structure of the host galaxies and their environments. This work confirms the utility of optical selection of type II quasars and demonstrates how this technique complements and independently tests selection methods developed at other wavelengths.