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

Abstract

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 $r$-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 $\approx 0.5$ 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 (Sérsic 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 ($n\sim 1-2$). 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.

22 pages, 18 figures; accepted for publication in MNRAS. The Sersic model parameters are available through the public EAGLE database (see http://icc.dur.ac.uk/Eagle/database.php )

References in corpus (26)

• The EAGLE project: Simulating the evolution and assembly of galaxies and their environments
• Introducing the Illustris Project: Simulating the coevolution of dark and visible matter in the Universe
• The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies
• The EAGLE simulations of galaxy formation: calibration of subgrid physics and model variations
• 3D-HST+CANDELS: The Evolution of the Galaxy Size-Mass Distribution since $z=3$
• A simple model to interpret the ultraviolet, optical and infrared emission from galaxies
• Simba: Cosmological Simulations with Black Hole Growth and Feedback
• Compaction and Quenching of High-z Galaxies in Cosmological Simulations: Blue and Red Nuggets
• The Evolution of Galaxy Structure over Cosmic Time
• Baryon effects on the internal structure of LCDM halos in the EAGLE simulations
• The Size Evolution of Star-forming and Quenched Galaxies in the IllustrisTNG simulation
• SKIRT: an Advanced Dust Radiative Transfer Code with a User-Friendly Architecture
• Sunrise: Polychromatic Dust Radiative Transfer in Arbitrary Geometries
• Galaxy And Mass Assembly (GAMA): the G02 field, Herschel-ATLAS target selection and Data Release 3
• Modelling the Pan-Spectral Energy Distribution of Starburst Galaxies: IV The Controlling Parameters of the Starburst SED
• A catalogue of 2D photometric decompositions in the SDSS-DR7 spectroscopic main galaxy sample: preferred models and systematics
• Geometry of Star-Forming Galaxies from SDSS, 3D-HST and CANDELS
• Feedback and the Structure of Simulated Galaxies at redshift z=2
• GALAPAGOS: From Pixels to Parameters
• Half-mass radii for ~7,000 galaxies at 1.0 < z < 2.5: most of the evolution in the mass-size relation is due to color gradients
• Far-infrared and dust properties of present-day galaxies in the EAGLE simulations
• Energy equipartition between stellar and dark matter particles in cosmological simulations results in spurious growth of galaxy sizes
• 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
• Connection Between Stellar Mass Distributions Within Galaxies and Quenching Since z =2
• Galaxies in the Illustris simulation as seen by the Sloan Digital Sky Survey - I: Bulge+disc decompositions, methods, and biases
• Importance of Mock Observations in Validating Galaxy Properties for Cosmological Simulations