Abstract

We show that the Hα line (6563 Å) alone is an extremely effective criterion for identifying galaxies that are uniform in color (red), luminosity-weighted age (old), and morphology (bulge-dominated). By combining the Sloan Digital Sky Survey (Data Release 6) with the New York University Value-Added Galaxy Catalog, we have photometric and spectroscopic indices for over 180,000 galaxies at (0.05 < z < 0.15). We separate the galaxies into three samples: (1) galaxies with Hα equivalent width, EW <0 Å (i.e., no emission), (2) galaxies with morphological Sérsic index n > 2 (bulge-dominated), and (3) galaxies with n > 2 that are also red in (g' - r'). We find that the Hα-selected galaxies consistently have the smallest color scatter: for example, at z ~ 0.05 the intrinsic scatter in apparent (g' - r') for the Hα sample is only 0.0287 ± 0.0007 compared to 0.0682 ± 0.0014 for the Sérsic sample. Applying a color cut to the n > 2 sample does decrease the color scatter to 0.0313 ± 0.0007, but a measurable fraction of star-forming and/or active galactic nucleus galaxies (up to 9.3%) remain. All of the EW(Hα) <0 Å galaxies have n > 2, i.e., they are bulge-dominated systems. The spectra for the three samples confirm that the Hα-selected galaxies have the highest D4000 values and are, on average, nearly twice as old as the Sérsic-selected samples. With the advent of multi-object near-infrared spectrographs, Hα alone can be used to reliably isolate truly quiescent galaxies dominated by evolved stellar populations at any epoch from z ~ 0 up to z ~ 2.