Tailoring galaxies: Size–luminosity–surface brightness relations of bulges and disks along the morphological sequence
Authors: Quilley L. , de Lapparent V. 
Published: October 2023

Aims. We revisit the scaling relations between size, luminosity, and surface brightness as a function of morphology, for the bulge and disk components of the 3106 weakly inclined galaxies of the “Extraction de Formes Idéalisées de Galaxies en Imagerie” (EFIGI) sample, in the nearby Universe.

Methods. The luminosity profiles from the Sloan Digital Sky Survey (SDSS) gri images were modeled as the sum of a Sérsic (bulge) and an exponential (disk) component for cD, elliptical (E), lenticular, and spiral galaxies, or as a single Sérsic profile for cD, E, dE, and irregular (Im) galaxies, by controlled profile fitting with the SourceXtractor++ software.

Results. For the EFIGI sample, we remeasured the Kormendy (1977, ApJ, 218, 333) relation between effective surface brightness ⟨μ⟩e and effective radius Re of elliptical galaxies, and show that it is also valid for the bulges (or Sérsic components) of galaxy types Sb and earlier. In contrast, there is a progressive departure toward fainter and smaller bulges for later Hubble types, as well as with decreasing bulge-to-total ratios (B/T) and Sérsic indices. This depicts a continuous transition from pseudo-bulges to classical ones, which we suggest to occur for absolute g magnitudes Mg between −17.8 and −19.1. We also obtain partial agreement with the Binggeli et al. (1984, AJ, 89, 64) relations between effective radius and Mg (known as “size–luminosity” relations, in log–log scale) for E and dE galaxies. There is a convex size–luminosity relation for the bulges of all EFIGI types. Both ⟨μ⟩e − Re and Re − Mg scaling relations are projections of a plane in which bulges are located according to their value of B/T, which partly determines the morphological type. Analogous scaling relations were derived for the disks of lenticular and spiral types, and the irregulars. The curvature of the size–luminosity relation for disks is 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 lenticular and spiral galaxies increase as power laws of B/T, with a steeper increase for the bulges. Both bulges and disks of lenticular galaxies have a similar and largely steeper increase with B/T than those for spirals. These relations propagate into a single scaling relation for the disk-to-bulge ratio of effective radii across ∼2 orders of magnitude in B/T, and for all types. We provide the parameters of all of these relations that can be used to build realistic mock images of nearby galaxies. The new convex size–luminosity relations are more reliable estimates of bulge, disk, and galaxy sizes at all magnitudes in the nearby Universe.

Conclusions. This analysis describes the joint size and luminosity variations of bulges and disks along the Hubble sequence. The characteristics of the successive phases of disk and bulge size growth strengthen a picture of morphological evolution in which irregulars and late spirals merge to form earlier spirals, lenticulars, and eventually ellipticals.