We investigate the evolution of galaxy masses and star formation rates in the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations. These comprise a suite of hydrodynamical simulations in a $Lambda$CDM cosmogony with subgrid m
odels for radiative cooling, star formation, stellar mass loss, and feedback from stars and accreting black holes. The subgrid feedback was calibrated to reproduce the observed present-day galaxy stellar mass function and galaxy sizes. Here we demonstrate that the simulations reproduce the observed growth of the stellar mass density to within 20 per cent. The simulation also tracks the observed evolution of the galaxy stellar mass function out to redshift z = 7, with differences comparable to the plausible uncertainties in the interpretation of the data. Just as with observed galaxies, the specific star formation rates of simulated galaxies are bimodal, with distinct star forming and passive sequences. The specific star formation rates of star forming galaxies are typically 0.2 to 0.4 dex lower than observed, but the evolution of the rates track the observations closely. The unprecedented level of agreement between simulation and data makes EAGLE a powerful resource to understand the physical processes that govern galaxy formation.
Feedback from energy liberated by gas accretion onto black holes (BHs) is an attractive mechanism to explain the exponential cut-off at the massive end of the galaxy stellar mass function (SMF). Semi-analytic models of galaxy formation in which this
form of feedback is assumed to suppress cooling in haloes where the gas cooling time is large compared to the dynamical time do indeed achieve a good match to the observed SMF. Furthermore, hydrodynamic simulations of individual halos in which gas is assumed to accrete onto the central BH at the Bondi rate have shown that a self-regulating regime is established in which the BH grows just enough to liberate an amount of energy comparable to the thermal energy of the halo. However, this process is efficient at suppressing the growth not only of massive galaxies but also of galaxies like the Milky Way, leading to disagreement with the observed SMF. The Bondi accretion rate, however, is inappropriate when the accreting material has angular momentum. We present an improved accretion model that takes into account the circularisation and subsequent viscous transport of infalling material and include it as a subgrid model in hydrodynamic simulations of the evolution of halos with a wide range of masses. The resulting accretion rates are generally low in low mass ($lsim 10^{11.5} msun$) halos, but show outbursts of Eddington-limited accretion during galaxy mergers. During outbursts these objects strongly resemble quasars. In higher mass haloes, gas accretion occurs continuously, typically at $~10$ % of the Eddington rate, which is conducive to the formation of radio jets. The resulting dependence of the accretion behaviour on halo mass induces a break in the relation between galaxy stellar mass and halo mass in these simulations that matches observations.
The observed stellar mass function (SMF) is very different to the halo mass function predicted by Lambda-CDM, and it is widely accepted that this is due to energy feedback from supernovae and black holes. However, the strength and form of this feedba
ck is not understood. In this paper, we use the phenomenological model GALFORM to explore how galaxy formation depends on the strength and halo mass dependence of feedback. We focus on expulsion models in which the wind mass loading, beta, is proportional to 1/vdisk^n, with n=0,1,2 and contrast these models with the successful Bower et al. 2008 model (B8W7). A crucial development is that our code explicitly accounts for the recapture of expelled gas as the systems halo mass (and thus gravitational potential) increases. We find that a model with modest wind speed but high mass loading matches the flat portion of the SMF. When combined with AGN feedback, the model provides a good description of the observed SMF above 10^9 h^-1 Msol. However, in the expulsion models, the brightest galaxies are assembled more recently than in B8W7, and the specific star formation rates of galaxies decrease strongly with decreasing stellar mass. The expulsion models also tend to have a cosmic star formation density that is dominated by lower mass galaxies at z=1-3, and dominated high mass galaxies at low redshift. These trends are in conflict with observational data, but the comparison highlights some deficiencies of the B8W7 model also. The experiments in this paper give us important physical insight to the impact of the feedback process on the formation histories of galaxies, but the strong mass dependence of feedback adopted in B8W7 still appears to provide the most promising description of the observed universe.
Recent wide-field imaging observations of the X-ray luminous cluster RDCSJ1252.9-2927 at z=1.24 uncovered several galaxy groups that appear to be embedded in filamentary structure extending from the cluster core. We make a spectroscopic study of the
galaxies in these groups using GMOS on Gemini-South and FORS2 on VLT with the aim of determining if these galaxies are physically associated to the cluster. We find that three groups contain galaxies at the cluster redshift and that they are probably bound to the cluster. This is the first confirmation of filamentary structure as traced by galaxy groups at z>1. We then use several spectral features in the FORS2 spectra to determine the star formation histories of group galaxies. We find a population of relatively red star-forming galaxies in the groups that are absent from the cluster core. While similarly red star forming galaxies can also be found in the field, the average strength of the hd line is systematically weaker in group galaxies. Interestingly, these groups at z=1.2 are in an environment in which the on-going build-up of red sequence is happening. The unusual line strengths can be explained by star formation that is heavily obscured by dust. We hypothesize that galaxy-galaxy interactions, which is more efficient in the group environment, is the mechanism that drives these dust obscured star formation. The hypothesis can be tested by obtaining spectral observations in the near-IR, high resolution imaging observations and observations in the mid-IR.
