Quasars in the Cosmos Field

We obtained medium-resolution spectra of 336 quasar candidates in the COSMOS HST/Treasury field using the MMT 6.5-meter telescope and the Hectospec multi-object spectrograph. Candidates were drawn from the Sloan Digital Sky Survey (SDSS) DR1 catalog using quasar flags set by the SDSS multi-color quasar target selection algorithm. In this paper we present our discovery spectra from 1.39 square degrees (69.5% of the COSMOS field) and a discussion of the selection method and yields. We confirmed 95 quasars, including at least 2 BALs; 80 of these are new quasars that do not appear in previous quasar confirmation follow-up studies. The candidates additionally included 184 compact emission-line galaxies, a third of which are likely Type 2 AGN, and 12 stars. The quasars span a range in magnitude of 18.3 < g < 22.5 and a range in redshift of 0.2 < z < 2.3. Our results are consistent with a lower limit quasar surface density from SDSS color selection of 102 per square degree down to g = 22.5 over the entire COSMOS field. This work is the first step toward the eventual goal of setting up a grid of quasar absorption line probes of the 2 square degree field, and of conducting a complete census of supermassive black holes in this well-studied survey region. The total quasar count at the conclusion of this study is 139, making COSMOS one of the most densely-sampled regions of sky where a grid of quasar sightlines can be used to probe the intervening volume. Subject headings: quasars: general — quasars: emission lines — galaxies: emission lines — surveys