Isophotal shapes of elliptical/lenticular galaxies from the Sloan Digital Sky Survey

Abstract

We revisit the shapes of isophotes for elliptical (E) and lenticular (S0) galaxies by studying 847 nearby early-type galaxies selected from the Sloan Digital Sky Survey (SDSS) Data Release 4 (DR4) with velocity dispersions above 200 km s −1 . The IRAF task ELLIPSE was used to derive the deviations of the isophotes from pure ellipses (Fourier coefficients a 3/a and a 4/a), position angles (PAs) and ellipticities as a function of radius. We show the statistical distributions of the a 3/a and a 4/a parameters as a function of velocity dispersion, magnitude and colour. The a 4/a parameter is correlated with the ellipticity and absolute Petrosian magnitude of galaxies. No significant correlation was found between the a 4/a parameter with colour and velocity dispersion. A cross-correlation between the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey and the SDSS data reveals a lack of powerful radio emitters in discy E/S0s, as previously found by Bender et al. We also find that boxy E/S0s favour denser environments while discy E/S0s favour more isolated environments. The median values of changes in the ellipticity and PA between one and one and a half Petrosian half-light radii in the isophotes are about −0.023 and 1. ◦ 61, respectively. The average change in the PA is much larger, about 4. 12, due to an extended tail. The change in ellipticity is weakly correlated with the ellipticity itself, with an increasing ellipticity for galaxies with higher ellipticity as the radius increases. The isophote parameters for the 847 galaxies are available online.