Paper detail

Tailoring galaxies: size-luminosity-surface brightness relations of bulges and disks along the morphological sequence

We measure the scaling relations of the bulges and disks of the EFIGI galaxies in the nearby Universe versus morphology, using bulge and disk decomposition of SDSS gri images with SourceXtractor++. The Kormendy (1977) relation between effective surface brightness and effective radius of E galaxies extends to the bulges of types S0 to Sb, whereas fainter and smaller bulges of later Hubble types depart from it, with decreasing bulge-to-total ratio (B/T) and Sérsic indices. There is a continuous transition from pseudo-bulges to classical ones, proposed to occur for g magnitudes between -17.8 to -19.1. The size-luminosity relations for E and dE types are steeper and similar to those from Binggeli et al. (1984), resp., below which EFIGI lenticular and spiral bulges display a curved relation. The disks and irregulars also follow a continuous curved size-luminosity relation such that while they grow, they first brighten and then stabilize in surface brightness. Moreover, we obtain the unprecedented result that the effective radii of both the bulges and disks of spirals increase as power-laws of B/T, with a steeper increase for the bulges. The increase with B/T is much steeper and similar for the bulges and disks of lenticulars. The ratio of disk-to-bulge effective radii varies accordingly across 2 orders of magnitude in B/T for all lenticular and spiral types, with a mean disk-to-bulge ratio decreasing from ~15 for Sbc to Scd types to ~6 for S0. We tabulate all derived scaling relations, so that they can be used to build realistic mock images of nearby galaxies. The new curved size-luminosity relations will prevent over or under estimates of bulge, disk and galaxy sizes at all magnitudes. These results complement the analysis of Quilley & de Lapparent (2022) by providing the joint size and luminosity variations of bulges and disks, as they evolve reversely along the Hubble sequence.