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 ¼ 1 for a pure de Vaucouleurs profile; fracDeV ¼ 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 $ À23) de Vaucouleurs galaxies and the flattest are modestly bright (M r $ À18) exponential galaxies. When the axis ratio q 25 is estimated from the axis ratio of the 25 mag arcsec À2 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 , 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 À21:84, fracDeV > 0:9) have a distribution of q am that is consistent with triaxiality in the range 0:4 P T P 0:8, with mean axis ratio 0:66 P hi P 0:69. The fainter de Vaucouleurs galaxies are best fit with prolate spheroids (T ¼ 1) with mean axis ratio hi % 0:51.

Core Shapes and Orientations of Core-Sérsic Galaxies

“…Our ellipticities, inner and mid , are broadly consistent with the results from previous works which showed that massive, luminous early-type galaxies, most of which are presumably core-Sérsic galaxies, tend to be rounder ( 0.25) than their less luminous counterparts, i.e., the Sérsic galaxies, 0.25 (e.g., Davies et al 1983;Jaffe et al 1994;Ferrarese et al 1994;Faber et al 1997;Ryden et al 2001;Alam & Ryden 2002;Vincent & Ryden 2005;Lauer et al 2005;Ferrarese et al 2006;Emsellem et al 2007Emsellem et al , 2011Holden et al 2009;Cappellari et al 2011). Overall, our core-Sérsic galaxies have inner and outer ellipticity distributions with median ellipticities of med = 0.13 ± 0.01, and med = 0.20 ± 0.01, respectively ( Figure 5).…”

“…Hao et al (2006) also arrived at similar conclusions regarding the relation between the ellipticities of early-type galaxies and their local density. Likewise, Ryden et al (2001) studied the relation between galaxy shape and age while Vincent & Ryden (2005; see also Alam & Ryden 2002) examined the dependence of galaxy shapes on the luminosities. These studies concluded that brighter galaxies with old stellar populations, such as the core-Sérsic galaxies, are rounder than their fainter counterparts which have young stellar populations (see also, e.g., Lauer et al 2005;Ferrarese et al 2006).…”

The Intrinsic Shape of Galaxy Bulges

“…They found that the AARD of elliptical galaxies shows no dependence on colour, suggesting that dust extinction is not important for this sample. The full population of elliptical galaxies was well characterized by a Gaussian distribution in the equatorial ellipticity with mean Q = 0.89 and a lognormal distribution of the flattening with mean F = 0.43, which corresponds to slightly oblate ellipsoids in agreement with Vincent & Ryden (2005). In a recent paper, Rodríguez & Padilla (2013) have used the SDSS-DR8 (Aihara et al, 2011) and the morphological information from Galaxy Zoo (Lintott et al, 2011) finding that elliptical galaxies have a mean value of F = 0.58 (Figure 2, right panels).…”

“…All these improvements allowed to study the dependence of the intrinsic shape with other galaxy properties such as the luminosity, colour, physical size, and environment. Using data from the SDSS-DR3 (Abazajian et al, 2005) Vincent & Ryden (2005 found that bright galaxies (M r ≤ −21.84) with a de Vaucouleurs profile have an AARD consistent with a triaxiality parameter in the range 0.4 < T < 0.8, where Kimm & Yi (2007) were able to reproduce the AARD by using a combination of oblate, prolate, and triaxial galaxy populations. Following the early work of Tremblay & Merritt (1996), they assumed each population having a Gaussian distribution of their intrinsic axis ratios.…”

Self‐consistent Models of Cuspy Triaxial Galaxies with Dark Matter Halos

“…Even if simplified, these simulations make clear predictions about the structure of baryon-free dark matter halos and the dependence of halo properties on mass and formation redshift (Navarro et al 2004;Diemand et al 2004). With regard to shapes, observations of elliptical galaxies yield only the projected isophotes, and thus determination of the intrinsic shape distribution requires either statistical analyses based on large samples (e.g., Ryden 1992Ryden , 1996Tremblay & Merritt 1995, 1996Ryden et al 1993;Alam & Ryden 2002;Vincent & Ryden 2005;Plionis et al 2006) or mapping of potentials via detailed kinematical data for individual galaxies (e.g., Franx et al 1991;Statler et al 2004). Isophotal studies reveal that luminous and faint elliptical galaxies have significantly different shape distributions: the latter have c/a h i % 0:75, while the former are flatter, c/a h i % 0:65 (Tremblay & Merritt 1996).…”