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International audienceIn 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 \textless= 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 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 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 (M-i \textless= -22.5) the disc light fraction is about 10% and about 80% are “pure bulge” systems. For fainter galaxies (M-i \textgreater= -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

Quantitative morphology of galaxies from the SDSS