The Dependence of Galaxy Shape on Luminosity and Surface Brightness Profile

Abstract

For a sample of 96,951 galaxies from the Sloan Digital Sky Survey Data Release 3, we study the distribution of apparent axis ratios as a function of r-band absolute magnitude and surface brightness profile type. We use the parameter fracDeV to quantify the profile type (fracDeV = 1 for a de Vaucouleurs profile; fracDeV = 0 for an exponential profile). When the apparent axis ratio q_{am} is estimated from the moments of the light distribution, the roundest galaxies are very bright (M_r \sim -23) de Vaucouleurs galaxies and the flattest are modestly bright (M_r \sim -18) exponential galaxies. When the apparent axis ratio q_{25} is estimated from the axis ratio of the 25 mag/arcsec^2 isophote, we find that de Vaucouleurs galaxies are flatter than exponential galaxies of the same absolute magnitude. For a given surface brightness profile type, very bright galaxies are rounder, on average, than fainter galaxies. We deconvolve the distributions of apparent axis ratios to find the distribution of the intrinsic short-to-long axis ratio gamma, assuming constant triaxiality T. For all profile types and luminosities, the distribution of apparent axis ratios is inconsistent with a population of oblate spheroids, but is usually consistent with a population of prolate spheroids. Bright galaxies with a de Vaucouleurs profile (M_r 0.9) have a distribution of q_{am} that is consistent with triaxiality in the range 0.4 < T < 0.8, with mean intrinsic axis ratio 0.66 < gamma < 0.69. The fainter de Vaucouleurs galaxies are best fit with prolate spheroids (T = 1) with mean axis ratio gamma = 0.51.