The Dependence of Galaxy Shape on Luminosity and Surface Brightness Profile

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 pure de Vaucouleurs profile; fracDeV = 0 for a pure 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 ~ -23) de Vaucouleurs galaxies and the flattest are modestly bright ( M r ~ -18) exponential galaxies. When the axis ratio q 25 is estimated from the axis ratio of the 25 mag arcsec -2 isophote, we find that de Vaucouleurs galaxies, at this low surface brightness, 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 γ, making the assumption of constant triaxiality T . For all profile types and luminosities, the distribution of 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 ≤ -21.84, fracDeV > 0.9) have a distribution of q am that is consistent with triaxiality in the range 0.4 ≲ T ≲ 0.8, with mean axis ratio 0.66 ≲ ⟨γ⟩ ≲ 0.69. The fainter de Vaucouleurs galaxies are best fit with prolate spheroids ( T = 1) with mean axis ratio ⟨γ⟩ ≈ 0.51.