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 S\'ersic models to the images with Galfit. We find that the derived
$r$-band 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 $\approx0.5$ dex. The magnitude of this size discrepancy depends
strongly on the dust attenuation and star formation activity within the galaxy,
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 in the GAMA survey (by
0.1 dex), and the slope and scatter of the local mass-size relation are
reproduced well for both populations. Finally, we also compare the obtained
morphologies with measurements from GAMA, finding that too few EAGLE galaxies
have light profiles that are similar to local early-type galaxies (S\'ersic
indices of $n\sim 4$). 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
($n\sim1-2$). 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.
