Systematic effects on the size-luminosity relation: dependence on model
  fitting and morphology
  We quantify the systematics in the size-luminosity relation of galaxies in
the SDSS main sample which arise from fitting different 1- and 2-component
model profiles to the images. In objects brighter than L*, fitting a single
Sersic profile to what is really a two-component SerExp system leads to biases:
the half-light radius is increasingly overestimated as n of the fitted single
component increases; it is also overestimated at B/T ~ 0.6. However, the net
effect on the R-L relation is small, except for the most luminous tail, where
it curves upwards towards larger sizes. We also study how this relation depends
on morphological type. Our analysis is one of the first to use
Bayesian-classifier derived weights, rather than hard cuts, to define
morphology. Crudely, there appear to be only two relations: one for early-types
(Es, S0s and Sa's) and another for late-types (Sbs and Scds). However, closer
inspection shows that within the early-type sample S0s tend to be 15% smaller
than Es of the same luminosity, and, among faint late types, Sbs are more than
25% smaller than Scds. Neither the early- nor the late-type relations are pure
power-laws: both show significant curvature, which we quantify. However, the
R-L relations of the bulges of early-types are almost pure power laws. Our
analysis confirms that two mass scales are special for early-type galaxies: M*
= 3e10 and 2e11 Msun. These same mass scales are also special for late types:
there is almost no correlation between R and M* below the former, and almost no
late-types above the latter. We also show that the intrinsic scatter around the
relation decreases at large luminosity and/or stellar mass; this should provide
additional constraints on models of how the most massive galaxies formed.
