The Homogeneous Properties of Halpha-Selected Galaxies at (0.05<z<0.15)

TL;DR

This study demonstrates that Halpha emission alone is a highly effective criterion for selecting homogeneous, old, bulge-dominated, and quiescent galaxies across a range of redshifts, outperforming traditional morphological and color-based methods.

Contribution

The paper introduces Halpha emission as a simple, reliable indicator for identifying uniform, quiescent galaxy populations, improving upon previous selection techniques.

Findings

Halpha-selected galaxies have minimal color scatter (~0.0287) indicating uniformity.

Halpha-selected galaxies are nearly twice as old as Sersic-selected ones.

Halpha alone can reliably identify quiescent galaxies up to z~2.

Abstract

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.