Ian G. McCarthy

The EAGLE project: simulating the evolution and assembly of galaxies and their environments

We introduce the Virgo Consortiums EAGLE project, a suite of hydrodynamical simulations that follow the formation of galaxies and black holes in representative volumes. We discuss the limitations of such simulations in light of their finite resolution and poorly constrained subgrid physics, and how these affect their predictive power. One major improvement is our treatment of feedback from massive stars and AGN in which thermal energy is injected into the gas without the need to turn off cooling or hydrodynamical forces, allowing winds to develop without predetermined speed or mass loading factors. Because the feedback efficiencies cannot be predicted from first principles, we calibrate them to the z~0 galaxy stellar mass function and the amplitude of the galaxy-central black hole mass relation, also taking galaxy sizes into account. The observed galaxy mass function is reproduced to ≲0.2 dex over the full mass range, 108<M∗/M⊙≲1011, a level of agreement close to that attained by semi-analytic models, and unprecedented for hydrodynamical simulations. We compare our results to a representative set of low-redshift observables not considered in the calibration, and find good agreement with the observed galaxy specific star formation rates, passive fractions, Tully-Fisher relation, total stellar luminosities of galaxy clusters, and column density distributions of intergalactic CIV and OVI. While the mass-metallicity relations for gas and stars are consistent with observations for M∗≳109M⊙, they are insufficiently steep at lower masses. The gas fractions and temperatures are too high for clusters of galaxies, but for groups these discrepancies can be resolved by adopting a higher heating temperature in the subgrid prescription for AGN feedback. EAGLE constitutes a valuable new resource for studies of galaxy formation.

The physics driving the cosmic star formation history

We investigate the physics driving the cosmic star formation (SF) history using the more than 50 large, cosmological, hydrodynamical simulations that together comprise the OverWhelmingly Large Simulations project. We systematically vary the parameters of the model to determine which physical processes are dominant and which aspects of the model are robust. Generically, we find that SF is limited by the build-up of dark matter haloes at high redshift, reaches a broad maximum at intermediate redshift and then decreases as it is quenched by lower cooling rates in hotter and lower density gas, gas exhaustion and self-regulated feedback from stars and black holes. The higher redshift SF is therefore mostly determined by the cosmological parameters and to a lesser extent by photoheating from reionization. The location and height of the peak in the SF history, and the steepness of the decline towards the present, depend on the physics and implementation of stellar and black hole feedback. Mass loss from intermediate-mass stars and metal-line cooling both boost the SF rate at late times. Galaxies form stars in a self-regulated fashion at a rate controlled by the balance between, on the one hand, feedback from massive stars and black holes and, on the other hand, gas cooling and accretion. Paradoxically, the SF rate is highly insensitive to the assumed SF law. This can be understood in terms of self-regulation: if the SF efficiency is changed, then galaxies adjust their gas fractions so as to achieve the same rate of production of massive stars. Self-regulated feedback from accreting black holes is required to match the steep decline in the observed SF rate below redshift 2, although more extreme feedback from SF, for example in the form of a top-heavy initial stellar mass function at high gas pressures, can help.

Ram pressure stripping the hot gaseous haloes of galaxies in groups and clusters

We use a large suite of carefully controlled full hydrodynamic simulations to study the ram pressure stripping of the hot gaseous haloes of galaxies as they fall into massive groups and clusters. The sensitivity of the results to the orbit, total galaxy mass, and galaxy structural properties is explored. For typical structural and orbital parameters, we find that ∼30 per cent of the initial hot galactic halo gas can remain in place after 10 Gyr. We propose a physically simple analytic model that describes the stripping seen in the simulations remarkably well. The model is analogous to the original formulation of Gunn & Gott, except that it is appropriate for the case of a spherical (hot) gas distribution (as opposed to a face-on cold disc) and takes into account that stripping is not instantaneous but occurs on a characteristic time-scale. The model reproduces the results of the simulations to within ≈10 per cent at almost all times for all the orbits, mass ratios, and galaxy structural properties we have explored. The one exception involves unlikely systems where the orbit of the galaxy is highly non-radial and its mass exceeds about 10 per cent of the group or cluster into which it is falling (in which case the model underpredicts the stripping following pericentric passage). The proposed model has several interesting applications, including modelling the ram pressure stripping of both observed and cosmologically simulated galaxies and as a way to improve present semi-analytic models of galaxy formation. One immediate consequence is that the colours and morphologies of satellite galaxies in groups and clusters will differ significantly from those predicted with the standard assumption of complete stripping of the hot coronae.

The case for AGN feedback in galaxy groups

The relatively recent insight that energy input from supermassive black holes (BHs) can have a substantial effect on the star formation rates (SFRs) of galaxies motivates us to examine the effects of BH feedback on the scale of galaxy groups. At present, groups contain most of the galaxies and a significant fraction of the overall baryon content of the universe and, along with massive clusters, they represent the only systems for which it is possible to measure both the stellar and gaseous baryonic components directly. To explore the effects of BH feedback on groups, we analyse two high resolution cosmological hydrodynamic simulations from the OverWhelmingly Large Simulations (OWLS) project. While both include galactic winds driven by supernovae, only one of the models includes feedback from accreting BHs. We compare the properties of the simulated galaxy groups to a wide range of observational data, including the entropy and temperature profiles of the intragroup medium, hot gas mass fractions, the luminosity temperature and mass temperature scaling relations, the K-band luminosity of the group and its central brightest galaxy (CBG), star formation rates and ages of the CBG, and gas- and stellar-phase metallicities. Both runs yield entropy distributions similar to the data, while the run without AGN feedback yields highly peaked temperature profiles, in discord with the observations. Energy input from supermassive BHs significantly reduces the gas mass fractions of galaxy groups with masses less than a few times 10 14 M� , yielding a gas mass fraction and X-ray luminosity scaling with system temperature that is in excellent agreement with the data, although the detailed scatter in the L T relation is not quite correct. The run without AGN feedback suffers from the well known overcooling problem — the resulting stellar mass fractions are several times larger than observed and present-day cooling flows operate uninhibitedly. By contrast, the run that includes BH feedback yields stellar mass fractions, SFRs, and stellar age distributions in excellent agreement with current estimates, thus resolving the long standing ‘cooling crisis’ of simulations on the scale of groups. Both runs yield very similar gas-phase metal abundance profiles that match X-ray measurements, but they predict very different stellar metallicities. Based on the above, galaxy groups provide a compelling case that feedback from supermassive BHs is a crucial ingredient in the formation of massive galaxies.

The accretion of galaxies into groups and clusters

We use the galaxy stellar mass and halo merger tree information from the semi-analyticmodel galaxy catalogue of Font et al. (2008) to examine the accretion of galaxies into a large sample of groups and clusters, covering a wide range in halo mass (1012.9 to 1015.3 h−1 M⊙), and selected from each of four redshift epochs (z=0, 0.5, 1.0 and 1.5). We find that clusters at all examined redshifts have accreted a significant fraction of their final galaxy populations through galaxy groups. A 1014.5 h−1 M⊙ mass cluster at z=0 has, on average, accreted_ 40% of its galaxies (Mstellar > 109 h−1 M⊙) from halos with masses greater than 1013 h−1 M⊙. Further, the galaxies which are accreted through groups are more massive, on average, than galaxies accreted through smaller halos or from the field population. We find that at a given epoch, the fraction of galaxies accreted from isolated environments is independent of the final cluster or group mass. In contrast, we find that observing a cluster of the same halo mass at each redshift epoch implies different accretion rates of isolated galaxies, from 5-6 % per Gyr at z=0 to 15% per Gyr at z=1.5. We find that combining the existence of a Butcher Oemler effect at z=0.5 and the observations that galaxies within groups display significant environmental effects with galaxy accretion histories justifies striking conclusions. Namely, that the dominant environmental process must begin to occur in halos of 1012 – 1013 h−1 M⊙, and act over timescales of > 2 Gyrs. This argues in favor of a mechanism like “strangulation”, in which the hot halo of a galaxy is stripped upon infalling into a more massive halo . This simple model predicts that by z=1.5 galaxy groups and clusters will display little to no environmental effects. This conclusion may limit the effectiveness of red sequence cluster finding methods at high redshift.

Cosmological simulations of the formation of the stellar haloes around disc galaxies

We use the Galaxies-Intergalactic Medium Interaction Calculation (GIMIC) suite of cosmological hydrodynamical simulations to study the formation of stellar spheroids of Milky Way mass disc galaxies. The simulations contain accurate treatments of metal-dependent radiative cooling, star formation, supernova feedback and chemodynamics, and the large volumes that have been simulated yield an unprecedentedly large sample of ≈400 simulated ∼L∗ disc galaxies. The simulated galaxies are surrounded by low-mass, low surface brightness stellar haloes that extend out to ∼100 kpc and beyond. The diffuse stellar distributions bear a remarkable resemblance to those observed around the Milky Way, M31 and other nearby galaxies, in terms of mass density, surface brightness and metallicity profiles. We show that in situ star formation typically dominates the stellar spheroids by mass at radii of r 30 kpc, whereas accretion of stars dominates at larger radii and this change in origin induces a change in the slope of the surface brightness and metallicity profiles, which is also present in the observational data. The system-to-system scatter in the in situ mass fractions of the spheroid, however, is large and spans over a factor of 4. Consequently, there is a large degree of scatter in the shape and normalization of the spheroid density profile within r 30 kpc (e.g. when fitted by a spherical power-law profile, the indices range from −2.6 to −3.4). We show that the in situ mass fraction of the spheroid is linked to the formation epoch of the system. Dynamically, older systems have, on average, larger contributions from in situ star formation, although there is significant system-to-system scatter in this relationship. Thus, in situ star formation likely represents the solution to the long-standing failure of pure accretion-based models to reproduce the observed properties of the inner spheroid.

Quantifying the effect of baryon physics on weak lensing tomography

We use matter power spectra from cosmological hydrodynamic simulations to quantify the effect of baryon physics on the weak gravitational lensing shear signal. The simulations consider a number of processes, such as radiative cooling, star formation, supernovae and feedback from active galactic nuclei (AGN). Van Daalen et al. (2011) used the same simulations to show that baryon physics, in particular the strong feedba ck that is required to solve the overcooling problem, modifies the matter power spectrum on s cales relevant for cosmological weak lensing studies. As a result, the use of power spectra fr om dark matter simulations can lead to significant biases in the inferred cosmological para meters. We show that the typical biases are much larger than the precision with which future missions aim to constrain the dark energy equation of state, w0. For instance, the simulation with AGN feedback, which reproduces X-ray and optical properties of groups of galaxies, gi ves rise to a � 40% bias in w0. We also explore the effect of baryon physics on constraints on m, σ8, the running of the spectral index, the mass of the neutrinos and models of warm dark matter. We demonstrate that the modification of the power spectrum is dominated by groups and clusters of galaxies, the effect of which can be modelled. We consider an approach based on the popular halo model and show that simple modifications can capture the main features of baryonic feedback. Despite its simplicity, we find that our model, when calibrated on the simulations, is able to reduce the bias in w0 to a level comparable to the size of the statistical uncertai nties for a Euclid-like mission. While observations of the gas and stellar fraction s as a function of halo mass can be used to calibrate the model, hydrodynamic simulations will likely still be needed to extend the observed scaling relations down to halo masses of 10 12 h −1 M⊙.

Gas expulsion by quasar-driven winds as a solution to the overcooling problem in galaxy groups and clusters

Galaxy groups are not scaled down versions of massive galaxy clusters - the hot gas in groups (known as the intragroup medium, IGrM for short) is, on average, less dense than the intracluster medium, implying that one or more non-gravitational processes (e.g., radiative cooling, star formation, and/or feedback) has had a relatively larger effect on groups. In the present study, we compare a number of cosmological hydrodynamic simulations that form part of the OverWhelmingly Large Simulations project to isolate and quantify the effects of cooling and feedback from supernovae (SNe) and active galactic nuclei (AGN) on the gas. This is achieved by comparing Lagrangian thermal histories of the gas in the different runs, which were all started from identical initial conditions. While radiative cooling, star formation, and SN feedback are all necessary ingredients, only runs that also include AGN feedback are able to successfully reproduce the optical and X-ray properties of groups and low-mass clusters. We isolate how, when, and exactly what gas is heated by AGN. Interestingly, we find that the gas that constitutes the present-day IGrM is that which was not strongly heated by AGN. Instead, the low median density/high median entropy of the gas in present-day groups is achieved by the ejection of lower entropy gas from low-mass progenitor galaxies at high redshift (primarily 2 . z . 4). This corresponds to the epoch when supermassive black holes accreted most of their mass, typically at a rate that is close to the Eddington limit (i.e., when the black holes are in a ‘quasar mode’).

Baryon effects on the internal structure of ΛCDM haloes in the EAGLE simulations

We investigate the internal structure and density profiles of haloes of mass 1010–1014 M⊙ in the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations. These follow the formation of galaxies in a Λ cold dark matter Universe and include a treatment of the baryon physics thought to be relevant. The EAGLE simulations reproduce the observed present-day galaxy stellar mass function, as well as many other properties of the galaxy population as a function of time. We find significant differences between the masses of haloes in the EAGLE simulations and in simulations that follow only the dark matter component. Nevertheless, haloes are well described by the Navarro–Frenk–White density profile at radii larger than ∼5 per cent of the virial radius but, closer to the centre, the presence of stars can produce cuspier profiles. Central enhancements in the total mass profile are most important in haloes of mass 1012–1013 M⊙, where the stellar fraction peaks. Over the radial range where they are well resolved, the resulting galaxy rotation curves are in very good agreement with observational data for galaxies with stellar mass M* < 5 × 1010 M⊙. We present an empirical fitting function that describes the total mass profiles and show that its parameters are strongly correlated with halo mass.

Evolution of galaxy stellar masses and star formation rates in the EAGLE simulations

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