Dependence of Galaxy Shape on Environment in the Sloan Digital Sky Survey

Abstract

Using a sample of galaxies from the Sloan Digital Sky Survey Data (SDSS) Release 4, we study the trends relating surface brightness profile type and apparent axis ratio to the local galaxy environment. We use the SDSS parameter "fracDeV" to quantify the profile type (fracDeV = 1 for a pure de Vaucouleurs profile, fracDeV = 0 for a pure exponential profile). We find that galaxies with M r ##IMG## [http://ej.iop.org/icons/Entities/gtrsim.gif] {gtrsim} -18 are mostly described by exponential profiles in all environments. Galaxies with -21 ##IMG## [http://ej.iop.org/icons/Entities/lesssim.gif] {lesssim} M r ##IMG## [http://ej.iop.org/icons/Entities/lesssim.gif] {lesssim} -18 mainly have exponential profiles in low-density environments and de Vaucouleurs profiles in high-density environments. The most luminous galaxies, with M r ##IMG## [http://ej.iop.org/icons/Entities/lesssim.gif] {lesssim} -21, are mostly described by de Vaucouleurs profiles in all environments. For galaxies with M r ##IMG## [http://ej.iop.org/icons/Entities/lesssim.gif] {lesssim} -19, the fraction of de Vaucouleurs galaxies is a monotonically increasing function of local density, while the fraction of exponential galaxies is monotonically decreasing. For a fixed surface brightness profile type, the apparent axis ratio is frequently correlated with environment. As the local density of galaxies increases, we find that for M r ∈ [-18, - 20], galaxies of all profile types become slightly rounder, on average. For M r ∈ [-20, - 22], galaxies with mostly exponential profiles tend to become flatter, while galaxies with de Vaucouleurs profiles tend to become rounder. For M r ∈ [-22, - ∞], galaxies with mostly exponential profiles become flatter, while the de Vaucouleurs galaxies become rounder in their inner regions, yet exhibit no change in their outer regions. We comment on how the observed trends relate to the merger history of galaxies.