Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011a%26a...530a.106t&link_type=abstract
Astronomy & Astrophysics, Volume 530, id.A106
Astronomy and Astrophysics
Astrophysics
7
Galaxies: Fundamental Parameters, Galaxies: Bulges, Galaxies: Evolution, Galaxies: Formation
Scientific paper
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 Mi 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 (Mi ≤ssapprox -22.5) the disc light fraction is about 10% and about 80% are "pure bulge" systems. For fainter galaxies (Mi gtrapprox -21) 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.
Tasca Lidia A. M.
White Simon D. M.
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