Selections of High Redshift Quasars with Multi-wavelength Data

Abstract

High redshift quasars (z > 5) hold keys to understanding the evolution of the universe in its early stage. Yet, the number of high redshift quasars uncovered from previous studies is relatively small (70 or so), and are concentrated mostly in a limited redshift range (z ~ 6). To understand the early mass growth of supermassive black holes and the final stage of the cosmic reionization, it is important to find a statistically meaningful sample of quasars with various physical properties. Here we present a survey for high redshift quasars, which is divided into three sub-surveys: quasar selections at z ~ 5, 6, and 7. We conducted a quasar survey at z ~ 5 using SDSS and UKIDSS LAS data and supporting observations using the CQUEAN instrument. The search for quasars in this redshift range is known to be challenging due to limitations of filter sets used in previous studies. We adopted custom-designed filters, Is and Iz, and carried out imaging follow-up observations with CQUEAN. Additional data, such as WISE catalogs, were used to assign priorities to candidates. Using these new filters and a new selection technique, we were able to reduce the number of interlopers. After optical spectroscopy, we discovered six new quasars. We obtained NIR spectra of 3 of these 6 quasars and measured their physical properties. We constructed the completeness distribution to derive the selection efficiency. The quasar survey at z ~ 6 also used SDSS and UKIDSS LAS data, to select relatively bright quasars over a wide survey area. We developed a new selection method to eliminate contamination, and found that our new selection method reduced interloper efficiently. After conducting imaging and spectroscopic follow-up observations, we confirmed two new quasars. For future works, IMS data can be used for selection of additional z ~ 6 quasars, searching for relatively faint sources. To select quasars at z ~ 7, we carried out an NIR imaging survey, the Infrared Medium-deep Survey (IMS), covering a ~120 deg^2 area with 5 sigma depths of Y = 22.3 Vega and J = 22.0 Vega using UKIRT and WFCAM for 5 years. Seven extragalactic fields with deep optical data were imaged. We conducted data reduction, detection, and photometry on these data, and created catalogs of these IMS fields, which contain photometric information of ~ 7×10^6 sources. We used two selection methods for the z ~ 7 quasar selection, adopting Y -band or infrared band data, SWIRE data, in addition to deep J-band data (IMS and UKIDSS DXS) and optical data. This survey covered ~110 deg^2 area and found ~30 candidates, but we have not yet spectroscopically identified any quasars at z ~ 7. Spectroscopic follow-ups are still ongoing. We estimated black hole masses and Eddington ratios from the observed optical and NIR spectra, and found that the newly discovered quasars are growing via accretion more vigorous than those of their lower redshift counterparts. We estimated the quasar number densities from our discoveries for each redshift range, and compared them to those expected from the quasar luminosity functions in literature. We found that the number density at z ~ 5, estimated from our discovered quasars, shows values similar to those estimated in literature. However the observed number density of quasars at z ~ 7 shows values lower than what is expected, even after considering an extrapolated number density evolution. We conclude that the quasar number density at z ~ 7 declines toward higher redshift, steeper than the empirically expected evolution. Since we were unable to find quasars at 5.5 < z < 6.0 due to the low selection efficiency, we present an alternative selection method for this redshift range, using medium-band filters attached to SQUEAN. The chi^2 fitting of SEDs from these medium-band filters estimates photometric redshifts with a 1 sigma error of 0.15. Therefore we suggest that the medium-band observations can be used for a powerful selection method for robust candidates at z ~ 5.5.