Evolution of the Quasar Population at High Redshift

Abstract

High-redshift (z~6) quasars have been served as cosmological probes for studying the early universe. They provide crucial information on the growth of massive black holes (BHs) and formation of galaxies in the first billion years. I focus my dissertation work on exploring various properties of quasars at z~6.Based on our Gemini NIR observations, the metallicity in luminous z~6 quasars is supersolar with a typical value of ~4 Z_sun, and a comparison with low-redshift observations shows no strong evolution in metallicity up to z~6. Central BH masses are found to be 10^9 ~ 10^10 M_sun and Eddington luminosity ratios are of order unity. Our Spitzer observations show that most of z~6 quasars have prominent emission from hot dust. Their SEDs are similar to those of low-redshift quasars at rest-frame 0.15-3.5 um, suggesting that their accretion disks and hot-dust structures have reached maturity. However, two quasars were not detected in our deep 24 um images, and thus show no hot-dust emission, indicating different dust properties. Our ongoing survey of z~6 quasars has resulted in six quasars with z_AB<21 in the SDSS deep area. The bright-end power-law slope of the quasar luminosity function>(QLF) derived from available SDSS quasars is significantly steeper than the slope of the QLF at z~4. Based on this QLF, the quasar population cannot provide enough photons to ionize the IGM at z~6 unless the IGM is very homogeneous and the luminosity at which the QLF power law breaks is very low.My work also includes interesting topics of quasars at lower redshifts. Our deep spectroscopic survey of quasars at z<4 shows that the quasar density at>M_g<-22.5 peaks at z~2, which is later in cosmic time than the peak of z~2.5 found from surveys of more luminous quasars (M_g<-26). This confirms `cosmic downsizing' in quasar evolution. Using a sample of more than 30,000 quasars from SDSS, we find that the radio-loud fraction (RLF) in optically-selected quasars decreases rapidly with increasing redshift and decreasing optical luminosity. This result presents an important constraint on the radio emission mechanism and its cosmic evolution. We obtain 293 quasars with strong NIV] lambda 1486 or NIII] lambda 1750 emission lines from SDSS. These nitrogen-rich (N-rich) objects comprise ~1.1% of the SDSS quasars, and have high overall nitrogen abundances. They share many common properties with other quasars. However, the RLF in the N-rich quasars is much higher than that in other quasars with similar redshift and luminosity. This implies that the high nitrogen abundance in N-rich quasars could be an indicator of a special quasar evolution stage, in which the radio activity is also strong.