Quantitative Morphology of Galaxies from the SDSS I: Luminosity in Bulges and Disks

TL;DR

This study uses SDSS data and modeling to quantify the luminosity contributions of bulges and disks in galaxies, revealing that most light in the local universe comes from disk structures, with a systematic correction for projection effects.

Contribution

It introduces a method to accurately estimate galaxy component luminosities from SDSS images, accounting for projection effects and systematic biases in disk identification.

Findings

Over half of the local cosmic luminosity density is from disks.

Bright galaxies are mostly bulge-dominated with about 10% disk light.

Faint galaxies are predominantly disk-dominated with few pure bulge systems.

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 sample of roughly 1800 morphologically classified galaxies 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 disk luminosity profile is assumed exponential, while the bulge is assumed to have a de Vaucouleurs or a Sersic 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 b/a, the apparent disk flattening, deviates strongly from the expected uniform distribution, showing that the `disk' identified by the code frequently corresponds to an intrinsically 3-dimensional structure rather than to a true thin disk. We correct approximately 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 disks and in `pure bulge' systems (those with no detectable disk). For the brightest galaxies the disk light fraction is about 10% and about 80% are `pure bulge' systems. For faint galaxies most of the light is in disks and we do not detect a `pure bulge' population. Averaging over the galaxy population as a whole, we find that 54 \pm 2% of the local cosmic luminosity density at both r and i comes from disks and 32 \pm 2% from `pure bulge' systems. The remaining 14 \pm 2% comes from bulges in galaxies with detectable disks.