A New Nonparametric Approach to Galaxy Morphological Classification

Abstract

We present two new nonparametric methods for quantifying galaxy morphology: the relative distribution of the galaxy pixel flux values (the Gini coefficient or G) and the second-order moment of the brightest 20% of the galaxy's flux (M₂₀). We test the robustness of G and M₂₀ to decreasing signal-to-noise ratio (S/N) and spatial resolution and find that both measures are reliable to within 10% for images with average S/N per pixel greater than 2 and resolutions better than 1000 and 500 pc, respectively. We have measured G and M₂₀, as well as concentration (C), asymmetry (A), and clumpiness (S) in the rest-frame near-ultraviolet/optical wavelengths for 148 bright local ``normal'' Hubble-type galaxies (E-Sd) galaxies, 22 dwarf irregulars, and 73 0.05<z<0.25 ultraluminous infrared galaxies (ULIRGs). We find that most local galaxies follow a tight sequence in G-M<sub>20</sub>-C, where early types have high G and C and low M<sub>20</sub> and late-type spirals have lower G and C and higher M<sub>20</sub>. The majority of ULIRGs lie above the normal galaxy G-M<sub>20</sub> sequence because of their high G and M<sub>20</sub> values. Their high Gini coefficients arise from very bright nuclei, while the high second-order moments are produced by multiple nuclei and bright tidal tails. All of these features are signatures of recent and on-going mergers and interactions. We also find that in combination with A and S, G is more effective than C at distinguishing ULIRGs from the ``normal'' Hubble types. Finally, we measure the morphologies of 491.7<z<3.8 galaxies from HST NICMOS observations of the Hubble Deep Field North. We find that many of the z~2 galaxies possess G and A higher than expected from degraded images of local elliptical and spiral galaxies and have morphologies more like low-redshift ULIRGs.