ALBERT

Description: We investigate the abundance of galactic molecular hydrogen (H 2 ) in the ‘Evolution and Assembly of GaLaxies and their Environments’ (EAGLE) cosmological hydrodynamic simulations. We assign H 2 masses to gas particles in the simulations in post-processing using two different prescriptions that depend on the local dust-to-gas ratio and the interstellar radiation field. Both result in H 2 galaxy mass functions that agree well with observations in the local and high-redshift Universe. The simulations reproduce the observed scaling relations between the mass of H 2 and the stellar mass, star formation rate and stellar surface density. Towards high redshifts, galaxies in the simulations display larger H 2 mass fractions and lower H 2 depletion time-scales, also in good agreement with observations. The comoving mass density of H 2 in units of the critical density, $\Omega _{\rm H_2}$ , peaks at z   1.2–1.5, later than the predicted peak of the cosmic star formation rate activity, at z   2. This difference stems from the decrease in gas metallicity and increase in interstellar radiation field with redshift, both of which hamper H 2 formation. We find that the cosmic H 2 budget is dominated by galaxies with $M_{\rm H_2} 〉 10^9\,\rm M_{{\odot }}$ , star formation rates $ 〉 \,\!\!10\,\rm M_{{\odot }}\,\rm yr^{-1}$ and stellar masses M stellar 〉 10 10 M , which are readily observable in the optical and near-IR. The match between the H 2 properties of galaxies that emerge in the simulations and observations is remarkable, particularly since H 2 observations were not used to adjust parameters in EAGLE.

Description: We explore the galaxy formation physics governing the low-mass end of the H i mass function in the local Universe. Specifically, we predict the effects on the H i mass function of varying (i) the strength of photoionization feedback and the redshift of the end of the epoch of reionization, (ii) the cosmology, (iii) the supernovae feedback prescription and (iv) the efficiency of star formation. We find that the shape of the low-mass end of the H i mass function is most affected by the critical halo mass below which galaxy formation is suppressed by photoionization heating of the intergalactic medium. We model the redshift dependence of this critical dark matter halo mass by requiring a match to the low-mass end of the H i mass function. The best-fitting critical dark matter halo mass decreases as redshift increases in this model, corresponding to a circular velocity of ~50 km s –1 at z  = 0, ~30 km s –1 at z  ~ 1 and ~12 km s –1 at z  = 6. We find that an evolving critical halo mass is required to explain both the shape and abundance of galaxies in the H i mass function below $M_{\rm H\,\small {I}} \sim 10^{8} \,h^{-2}\,{\rm M_{{\odot }}}$ . The model makes specific predictions for the clustering strength of H i -selected galaxies with H i masses 〉10 6 and 〉10 7 h –2 M and for the relation between the H i and stellar mass contents of galaxies which will be testable with upcoming surveys with the Square Kilometre Array and its pathfinders. We conclude that measurements of the H i mass function at z  ≥ 0 will lead to an improvement in our understanding of the net effect of photoionization feedback on galaxy formation and evolution.

Description: We investigate correlations between different physical properties of star-forming galaxies in the ‘Evolution and Assembly of GaLaxies and their Environments’ (EAGLE) cosmological hydrodynamical simulation suite over the redshift range 0 ≤ z ≤ 4.5. A principal component analysis reveals that neutral gas fraction ( f gas,neutral ), stellar mass ( M stellar ) and star formation rate (SFR) account for most of the variance seen in the population, with galaxies tracing a two-dimensional, nearly flat, surface in the three-dimensional space of f gas, neutral – M stellar –SFR with little scatter. The location of this plane varies little with redshift, whereas galaxies themselves move along the plane as their f gas, neutral and SFR drop with redshift. The positions of galaxies along the plane are highly correlated with gas metallicity. The metallicity can therefore be robustly predicted from f gas, neutral , or from the M stellar and SFR. We argue that the appearance of this ‘Fundamental Plane of star formation’ is a consequence of self-regulation, with the plane's curvature set by the dependence of the SFR on gas density and metallicity. We analyse a large compilation of observations spanning the redshift range 0 z 3, and find that such a plane is also present in the data. The properties of the observed Fundamental Plane of star formation are in good agreement with EAGLE's predictions.

Description: We compare global predictions from the eagle hydrodynamical simulation, and two semi-analytic (SA) models of galaxy formation, l-galaxies and galform . All three models include the key physical processes for the formation and evolution of galaxies and their parameters are calibrated against a small number of observables at z 0. The two SA models have been applied to merger trees constructed from the eagle dark matter only simulation. We find that at z ≤ 2, both the galaxy stellar mass functions for stellar masses M * 〈 10 10.5 M and the median specific star formation rates (sSFRs) in the three models agree to better than 0.4 dex. The evolution of the sSFR predicted by the three models closely follows the mass assembly history of dark matter haloes. In both eagle and l-galaxies there are more central passive galaxies with M * 〈 10 9.5 M than in galform . This difference is related to galaxies that have entered and then left a larger halo and which are treated as satellites in galform . In the range 0 〈 z 〈 1, the slope of the evolution of the star formation rate density in eagle is a factor of 1.5 steeper than for the two SA models. The median sizes for galaxies with M * 〉 10 9.5 M differ in some instances by an order of magnitude, while the stellar mass–size relation in eagle is a factor of 2 tighter than for the two SA models. Our results suggest the need for a revision of how SA models treat the effect of baryonic self-gravity on the underlying dark matter. The treatment of gas flows in the models needs to be revised based on detailed comparison with observations to understand in particular the evolution of the stellar mass–metallicity relation.

Description: High-redshift galaxy clusters allow us to examine galaxy formation in extreme environments. Here we compile data for 15 z 〉 1 galaxy clusters to test the predictions from a state-of-the-art semi-analytical model of galaxy formation. The model gives a good match to the slope and zero-point of the cluster red sequence. The model is able to match the cluster galaxy luminosity function at faint and bright magnitudes, but underestimates the number of galaxies around the break in the cluster luminosity function. We find that simply assuming a weaker dust attenuation improves the model predictions for the cluster galaxy luminosity function, but worsens the predictions for the red sequence at bright magnitudes. Examination of the properties of the bright cluster galaxies suggests that the default dust attenuation is large due to these galaxies having large reservoirs of cold gas as well as small radii. We find that matching the luminosity function and colours of high-redshift cluster galaxies, whilst remaining consistent with local observations, poses a challenge for galaxy formation models.

Description: We present predictions for the two-point correlation function of galaxy clustering as a function of stellar mass, computed using two new versions of the galform semi-analytic galaxy formation model. These models make use of a high resolution, large volume N -body simulation, set in the 7-year Wilkinson Microwave Anisotropy Probe cosmology. One model uses a universal stellar initial mass function (IMF), while the other assumes different IMFs for quiescent star formation and bursts. Particular consideration is given to how the assumptions required to estimate the stellar masses of observed galaxies (such as the choice of IMF, stellar population synthesis model, and dust extinction) influence the perceived dependence of galaxy clustering on stellar mass. Broad-band spectral energy distribution fitting is carried out to estimate stellar masses for the model galaxies in the same manner as in observational studies. We show clear differences between the clustering signals computed using the true and estimated model stellar masses. As such, we highlight the importance of applying our methodology to compare theoretical models to observations. We introduce an alternative scheme for the calculation of the merger time-scales for satellite galaxies in galform , which takes into account the dark matter subhalo information from the simulation. This reduces the amplitude of small-scale clustering. The new merger scheme offers improved or similar agreement with observational clustering measurements, over the redshift range 0 〈  z  〈 0.7. We find reasonable agreement with clustering measurements from the Galaxy and Mass Assembly Survey, but find larger discrepancies for some stellar mass ranges and separation scales with respect to measurements from the Sloan Digital Sky Survey and the VIMOS Public Extragalactic Redshift Survey, depending on the galform model used.

Description: We examine the evolution of intrinsic u – r colours of galaxies in the EAGLE cosmological hydrodynamical simulations, which has been shown to reproduce the observed redshift z = 0.1 colour–magnitude distribution well, with a focus on z 〈 2. The median u – r of star-forming (‘blue cloud’) galaxies reddens by 1 mag from z = 2 to 0 at fixed stellar mass, as their specific star formation rates decrease with time. A red sequence starts to build-up around z = 1, due to the quenching of low-mass satellite galaxies at the faint end, and due to the quenching of more massive central galaxies by their active galactic nuclei (AGN) at the bright end. This leaves a dearth of intermediate-mass red sequence galaxies at z = 1, which is mostly filled in by z = 0. We quantify the time-scales of colour transition finding that most galaxies spend less than 2 Gyr in the ‘green valley’. We find the time-scale of transition to be independent of quenching mechanism, i.e. whether a galaxy is a satellite or hosting an AGN. On examining the trajectories of galaxies in a colour–stellar mass diagram, we identify three characteristic tracks that galaxies follow (quiescently star-forming, quenching and rejuvenating galaxies) and quantify the fraction of galaxies that follow each track.

Description: We perform automated bulge + disc decomposition on a sample of ~7500 galaxies from the Galaxy And Mass Assembly (GAMA) survey in the redshift range of 0.002 〈 z 〈 0.06 using Structural Investigation of Galaxies via Model Analysis, a wrapper around galfit 3. To achieve robust profile measurements, we use a novel approach of repeatedly fitting the galaxies, varying the input parameters to sample a large fraction of the input parameter space. Using this method, we reduce the catastrophic failure rate significantly and verify the confidence in the fit independently of 2 . Additionally, using the median of the final fitting values and the 16th and 84th percentile produces more realistic error estimates than those provided by galfit , which are known to be underestimated. We use the results of our decompositions to analyse the stellar mass – half-light radius relations of bulges, discs and spheroids. We further investigate the association of components with a parent disc or elliptical relation to provide definite z = 0 disc and spheroid $\mathcal {M_\star }\text{--}R_{\rm e}$ relations. We conclude by comparing our local disc and spheroid $\mathcal {M_\star }\text{--}R_{\rm e}$ to simulated data from eagle and high-redshift data from Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey-Ultra Deep Survey. We show the potential of using the $\mathcal {M_\star }\text{--}R_{\rm e}$ relation to study galaxy evolution in both cases but caution that for a fair comparison, all data sets need to be processed and analysed in the same manner.

Description: We present a new version of the galform semi-analytical model of galaxy formation. This brings together several previous developments of galform into a single unified model, including a different initial mass function (IMF) in quiescent star formation and in starbursts, feedback from active galactic nuclei suppressing gas cooling in massive haloes, and a new empirical star formation law in galaxy discs based on their molecular gas content. In addition, we have updated the cosmology, introduced a more accurate treatment of dynamical friction acting on satellite galaxies, and updated the stellar population model. The new model is able to simultaneously explain both the observed evolution of the K -band luminosity function and stellar mass function, and the number counts and redshift distribution of sub-mm galaxies selected at 850 μm. This was not previously achieved by a single physical model within the cold dark matter framework, but requires having an IMF in starbursts that is somewhat top-heavy. The new model is tested against a wide variety of observational data covering wavelengths from the far-UV to sub-mm, and redshifts from z = 0 to 6, and is found to be generally successful. These observations include the optical and near-infrared (IR) luminosity functions, H i mass function, fraction of early type galaxies, Tully–Fisher, metallicity–luminosity and size–luminosity relations at z = 0, as well as far-IR number counts, and far-UV luminosity functions at z ~ 3–6. Discrepancies are, however, found in galaxy sizes and metallicities at low luminosities, and in the abundance of low-mass galaxies at high- z , suggesting the need for a more sophisticated model of supernova feedback.