# Galaxies in the Illustris simulation as seen by the Sloan Digital Sky Survey - II: Size-luminosity relations and the deficit of bulge-dominated galaxies in Illustris at low mass

## TL;DR

This study compares galaxy size-luminosity relations from the Illustris simulation with SDSS observations, revealing size and brightness discrepancies and a deficit of bulge-dominated galaxies at low masses in Illustris.

## Contribution

It provides a detailed comparison of simulated and observed galaxy properties, highlighting specific differences in size, brightness, and bulge-dominated galaxy populations.

## Key findings

- Illustris galaxies are larger and brighter than SDSS galaxies.
- Size-luminosity relation trends are qualitatively similar but quantitatively different.
- Significant deficit of bulge-dominated galaxies in Illustris at low stellar masses.

## Abstract

The interpretive power of the newest generation of large-volume hydrodynamical simulations of galaxy formation rests upon their ability to reproduce the observed properties of galaxies. In this second paper in a series, we employ bulge+disc decompositions of realistic dust-free galaxy images from the Illustris simulation in a consistent comparison with galaxies from the Sloan Digital Sky Survey (SDSS). Examining the size-luminosity relations of each sample, we find that galaxies in Illustris are roughly twice as large and $0.7$ magnitudes brighter on average than galaxies in the SDSS. The trend of increasing slope and decreasing normalization of size-luminosity as a function of bulge-fraction is qualitatively similar to observations. However, the size-luminosity relations of Illustris galaxies are quantitatively distinguished by higher normalizations and smaller slopes than for real galaxies. We show that this result is linked to a significant deficit of bulge-dominated galaxies in Illustris relative to the SDSS at stellar masses $\log\mathrm{M}_{\star}/\mathrm{M}_{\odot}\lesssim11$. We investigate this deficit by comparing bulge fraction estimates derived from photometry \emph{and} internal kinematics. We show that photometric bulge fractions are systematically lower than the kinematic fractions at low masses, but with increasingly good agreement as the stellar mass increases.