We show that the H \alpha line ( 6563Å ) 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 H \alpha equivalent width < 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 ^ { \prime } - r ^ { \prime } ) .
We find that the H \alpha -selected galaxies consistently have the smallest color scatter : for example , at z \sim 0.05 the intrinsic scatter in apparent ( g ^ { \prime } - r ^ { \prime } ) for the H \alpha sample is only 0.0287 \pm 0.0007 compared to 0.0682 \pm 0.0014 for the Sérsic sample .
Applying a color-cut to the n > 2 sample does decrease the color scatter to 0.0313 \pm 0.0007 , but there remains a measurable fraction of star-forming and/or AGN galaxies ( up to 9.3 % ) .
All of the EW ( H \alpha ) < 0 Å galaxies have n > 2 , i . e . they are bulge-dominated systems .
The spectra for the three samples confirm that the H \alpha -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 , H \alpha alone can be used to reliably isolate truly quiescent galaxies dominated by evolved stellar populations at any epoch from z \sim 0 up to z \sim 2 .