THE SDSS-III BARYON OSCILLATION SPECTROSCOPIC SURVEY: QUASAR TARGET SELECTION FOR DATA RELEASE NINE

Abstract

The SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), a five-year\nspectroscopic survey of 10,000 deg^2, achieved first light in late 2009. One of\nthe key goals of BOSS is to measure the signature of baryon acoustic\noscillations in the distribution of Ly-alpha absorption from the spectra of a\nsample of ~150,000 z>2.2 quasars. Along with measuring the angular diameter\ndistance at z\\approx2.5, BOSS will provide the first direct measurement of the\nexpansion rate of the Universe at z > 2. One of the biggest challenges in\nachieving this goal is an efficient target selection algorithm for quasars over\n2.2 < z < 3.5, where their colors overlap those of stars. During the first year\nof the BOSS survey, quasar target selection methods were developed and tested\nto meet the requirement of delivering at least 15 quasars deg^-2 in this\nredshift range, out of 40 targets deg^-2. To achieve these surface densities,\nthe magnitude limit of the quasar targets was set at g <= 22.0 or r<=21.85.\nWhile detection of the BAO signature in the Ly-alpha absorption in quasar\nspectra does not require a uniform target selection, many other astrophysical\nstudies do. We therefore defined a uniformly-selected subsample of 20 targets\ndeg^-2, for which the selection efficiency is just over 50%. This "CORE"\nsubsample will be fixed for Years Two through Five of the survey. In this paper\nwe describe the evolution and implementation of the BOSS quasar target\nselection algorithms during the first two years of BOSS operations. We analyze\nthe spectra obtained during the first year. 11,263 new z>2.2 quasars were\nspectroscopically confirmed by BOSS. Our current algorithms select an average\nof 15 z > 2.2 quasars deg^-2 from 40 targets deg^-2 using single-epoch SDSS\nimaging. Multi-epoch optical data and data at other wavelengths can further\nimprove the efficiency and completeness of BOSS quasar target selection.\n[Abridged]\n