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

TL;DR

This paper uses the EAGLE simulation to study how galaxy morphologies, especially discs and spheroids, form and evolve over cosmic time, revealing the dynamic processes shaping the Hubble sequence.

Contribution

It provides a detailed analysis of the formation and evolution of morphological components in galaxies within the EAGLE simulation, highlighting the changing contributions of discs and spheroids.

Findings

Most galaxies are asymmetric at high redshift.

By redshift 1.5, the Hubble sequence is established.

Spheroids dominate the stellar mass budget at low redshift.

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 kinematically defined disc and spheroidal structures form and evolve in the EAGLE hydrodynamic simulation of galaxy formation. At high redshift most galaxies of all masses are asymmetric. By redshift $z\simeq 1.5$ 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 $z\simeq 0.5$ but overall remains subdominant at all times although discs contribute most of the stellar mass in systems of mass $M^*\sim 10^{10.5}{\rm M_\odot}$ at $z \leq 1.5$. 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.