The Homogeneous Properties of Halpha-Selected Galaxies at (0.05<z<0.15)
  We show that the Halpha line (6563 Angstrom) 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 Halpha equivalent width EW(Halpha)<0 Angstrom (i.e.
no emission); 2) galaxies with morphological Sersic index n>2
(bulge-dominated); and 3) galaxies with n>2 that are also red in (g'-r'). We
find that the Halpha-selected galaxies consistently have the smallest color
scatter: for example, at z~0.05 the intrinsic scatter in apparent (g'-r') for
the Halpha sample is only 0.0287+/-0.0007 compared to 0.0682+/-0.0014 for the
Sersic sample. Applying a color-cut to the n>2 sample does decrease the color
scatter to 0.0313+/-0.0007, but there remains a measurable fraction of
star-forming and/or AGN galaxies (up to 9.3%). All of the EW(Halpha)<0 Angstrom
galaxies have n>2, i.e. they are bulge-dominated systems. The spectra for the
three samples confirm that the Halpha-selected galaxies have the highest D4000
values and are, on average, nearly twice as old as the Sersic-selected samples.
With the advent of multi-object near-infrared spectrographs, Halpha 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.
