Quantitative morphology of galaxies from the SDSS

I. Luminosity in bulges and discs

¹ Laboratoire d’Astrophysique de Marseille, CNRS-Université d’Aix-Marseille, 38 rue Frédéric Joliot Curie, 13388 Marseille Cedex 13, France
e-mail: lida.tasca@oamp.fr
² INAF-IASF, via Bassini 15, 20133 Milano, Italy
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching b. München, Germany

Received: 8 November 2009
Accepted: 11 April 2011

Abstract

In the first paper of this series we use the publicly available code Gim2D to model the r- and i-band images of all galaxies in a magnitude-limited (r ≤ 15.9) sample of roughly 1800 morphologically classified bright galaxies with absolute magnitudes M_i ranging from  −15.2 to  −23.7 and up to a redshift z = 0.1 (median 0.05), taken from the Sloan Digital Sky Survey. The model is a concentric superposition of two components, each with elliptical isophotes with constant flattening and position angle. The disc luminosity profile is assumed exponential, while the bulge is assumed to have a de Vaucouleurs or a Sérsic profile. We find that the parameters returned by Gim2D depend little on the waveband or bulge profile used; their formal uncertainties are usually small. Nevertheless, for bright galaxies the measured distribution of the apparent disc component flattening (b/a), deviates strongly from the expected uniform distribution, showing that the “disc” identified by the code frequently corresponds to an intrinsically 3-dimensional structure rather than to a true thin disc. We correct for this systematic problem using the observed statistics of the b/a distribution and estimate, as a function of absolute magnitude, the mean fractions of galaxy light in discs and in “pure bulge” systems (those with no detectable disc). For the brightest galaxies the disc light fraction is about 10% and about 80% are “pure bulge” systems. For fainter galaxies most of the light is in discs and we do not detect a “pure bulge” population. Averaging over the galaxy population as a whole, we find that 54 ± 2% of the local cosmic luminosity density at both r and i comes from discs and 32 ± 2% from “pure bulge” systems. The remaining 14 ± 2% comes in half from the light in the bulges and from the other half from light in bars of systems with detectable discs. These measurements offer a reference to future studies tracing the evolution of the fraction of light in discs and bulges as a clue of their formation process.