Optical Selection of Galaxies at Redshifts

Few galaxies have been found between the redshift ranges z < ∼ 1 probed by magnitude-limited surveys and z > ∼ 3 probed by Lyman-break surveys. Comparison of galaxy samples at lower and higher redshift suggests that large numbers of stars were born and the Hubble sequence began to take shape at the intermediate redshifts 1 < z < 3, but observational challenges have prevented us from observing the process in much detail. We present simple and efficient strategies that can be used to find large numbers of galaxies throughout this important but unexplored redshift range. All the strategies are based on selecting galaxies for spectroscopy on the basis of their colors in ground-based images taken through a small number of optical filters: GRi for redshifts 0.85 < z < 1.15, GRz for 1 < z < 1.5, and UnGR for 1.4 < z < 2.1 and 1.9 < z < 2.7. The performance of our strategies is quantified empirically through spectroscopy of more than 2000 galaxies at 1 < z < 3.5. We estimate that more than half of the UV-luminosity density at 1 < z < 3 is produced by galaxies that satisfy our color-selection criteria. Our methodology is described in detail, allowing readers to devise analogous selection criteria for other optical filter systems. Subject headings: galaxies: formation, galaxies: high-redshift