Observed structural parameters of EAGLE galaxies: reconciling the mass-size relation in simulations with local observations

We use mock images of z = 0.1 galaxies in the 100 Mpc EAGLE simulation to establish the differences between the sizes and morphologies inferred from the stellar mass distributions and the optical light distributions. The optical, r-band images used were constructed with a radiative transfer method to account for the effects of dust, and we measure galaxy structural parameters by fitting Sérsic models to the images with GALFIT. We find that the derived half-light radii differ systematically from the stellar half-mass radii, as the r-band sizes are typically 0.1 dex larger, and can deviate by as much as ≈0.5 dex, depending on the dust attenuation and star formation activity, as well as the measurement method used. Consequently, we demonstrate that the r-band sizes significantly improve the agreement between the simulated and observed stellar mass-size relation: star-forming and quiescent galaxies in EAGLE are typically only slightly larger than observed (by 0.1 dex), and the slope and scatter of the local relation are reproduced well for both populations. Finally, we compare the obtained morphologies with measurements from the GAMA survey, finding that too few EAGLE galaxies have bulge-like light profiles (Sérsic indices of n ∼4). Despite the presence of a significant population of triaxial systems among the simulated galaxies, the surface brightness and stellar mass density profiles tend to be closer to exponential discs (n ∼1-2). Our results highlight the need to measure the sizes and morphologies of simulated galaxies using common observational methods in order to perform a meaningful comparison with observations.