The star formation rate and stellar content contributions of morphological components in the EAGLE simulations

Abstract

The Hubble sequence provides a useful classification of galaxy morphology at low redshift. However, morphologies are not static, but rather evolve as the growth of structure proceeds through mergers, accretion and secular processes. We investigate how morphological structures form and evolve in the EAGLE hydrodynamic simulation of galaxy formation via their physical (rather than observable) properties, using mass distributions to classify galaxies, or kinematics to decompose individual galaxies into discs and spheroids. We focus on galaxies of mass M⋆ ≥ 109M⊙ from the largest fiducial EAGLE simulation, yielding a volume-limited sample of 13,395 systems by z = 0.1. At high redshift most galaxies of all masses are asymmetric. By redshift z ≃ 1.5 the Hubble sequence is established and after this time most of the stellar mass is in spheroids, whose contribution to the stellar mass budget continues to rise to the present day. The stellar mass fraction in discs peaks at z ≃ 0.5 but overall remains subdominant at all times, although discs contribute most of the stellar mass in systems of mass M* ∼ 1010.5M⊙ at z ≤ 1.5. Star formation occurs predominantly in disc structures throughout most of cosmic time but morphological transformations rearrange stars, thus establishing the low-redshift morphological mix. Morphological transformations are common and we quantify the rates at which they occur. The rate of growth of spheroids decreases at z < 2, while the rate of decay of discs remains roughly constant at z < 1. Finally, we find that the prograde component of galaxies becomes increasingly dynamically cold with time.