We calculate the colours and luminosities of redshift z = 0.1 galaxies from the EAGLE simulation suite using the GALAXEV population synthesis models. We take into account obscuration by dust in birth clouds and diffuse ISM using a two-component scree
n model, following the prescription of Charlot and Fall. We compare models in which the dust optical depth is constant to models where it depends on gas metallicity, gas fraction and orientation. The colours of EAGLE galaxies for the more sophisticated models are in broad agreement with those of observed galaxies. In particular, EAGLE produces a red sequence of passive galaxies and a blue cloud of star forming galaxies, with approximately the correct fraction of galaxies in each population and with g-r colours within 0.1 magnitudes of those observed. Luminosity functions from UV to NIR wavelengths differ from observations at a level comparable to systematic shifts resulting from a choice between Petrosian and Kron photometric apertures. Despite the generally good agreement there are clear discrepancies with observations. The blue cloud of EAGLE galaxies extends to somewhat higher luminosities than in the data, consistent with the modest underestimate of the passive fraction in massive EAGLE galaxies. There is also a moderate excess of bright blue galaxies compared to observations. The overall level of agreement with the observed colour distribution suggests that EAGLE galaxies at z = 0.1 have ages, metallicities and levels of obscuration that are comparable to those of observed galaxies.
We investigate the internal structure and density profiles of halos of mass $10^{10}-10^{14}~M_odot$ in the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations. These follow the formation of galaxies in a $Lambda$CDM 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 halos in the EAGLE simulations and in simulations that follow only the dark matter component. Nevertheless, halos are well described by the Navarro-Frenk-White (NFW) density profile at radii larger than ~5% 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 halos of mass $10^{12}-10^{13}M_odot$, 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 $M_*<5times10^{10}M_odot$. We present an empirical fitting function that describes the total mass profiles and show that its parameters are strongly correlated with halo mass.
We use the Evolution and assembly of galaxies and their environments (EAGLE) cosmological simulation to investigate the effect of baryons on the density profiles of rich galaxy clusters. We focus on EAGLE clusters with $M_{200}>10^{14}~M_odot$ of whi
ch we have six examples. The central brightest cluster galaxies (BCGs) in the simulation have steep stellar density profiles, $rho_*(r) propto r^{-3}$. Stars dominate the mass density for $r<10~rm{kpc}$, and, as a result, the total mass density profiles are steeper than the Navarro-Frenk-White (NFW) profile, in remarkable agreement with observations. The dark matter halo itself closely follows the NFW form at all resolved radii ($rgtrsim3.0~rm{kpc}$). The EAGLE BCGs have similar surface brightness and line-of-sight velocity dispersion profiles as the BCGs in the sample of Newman et al., which have the most detailed measurements currently available. After subtracting the contribution of the stars to the central density, Newman et al. infer significantly shallower slopes than the NFW value, in contradiction with the EAGLE results. We discuss possible reasons for this discrepancy, and conclude that an inconsistency between the kinematical model adopted by Newman et al. for their BCGs, which assumes isotropic stellar orbits, and the kinematical structure of the EAGLE BCGs, in which the orbital stellar anisotropy varies with radius and tends to be radially biased, could explain at least part of the discrepancy.
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 resolutio
n 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 $lesssim 0.2$ dex over the full mass range, $10^8 < M_*/M_odot lesssim 10^{11}$, 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 $M_* gtrsim 10^9 M_odot$, 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 baryon cycle of galaxies is a dynamic process involving the intake, consumption and ejection of vast quantities of gas. In contrast, the conventional picture of satellite galaxies has them methodically turning a large gas reservoir into stars unt
il this reservoir is forcibly removed due to external ram pressure. This picture needs revision. Our modern understanding of the baryon cycle suggests that in some regimes the simple interruption of the fresh gas supply may quench satellite galaxies long before stripping events occur, a process we call overconsumption. We compile measurements from the literature of observed satellite quenching times at a range of redshifts to determine if satellites are principally quenched through orbit-based gas stripping events -- either direct stripping of the disk (ram pressure stripping) or the extended gas halo (strangulation) -- or from internally-driven star formation outflows via overconsumption. The observed timescales show significant deviation from the evolution expected for gas stripping mechanisms and suggest that either ram pressure stripping is much more efficient at high redshift, or that secular outflows quench satellites before orbit-based stripping occurs. Given the strong redshift evolution of star formation rates, at high redshift (z > 1.5) even moderate outflow rates will lead to extremely short quenching times with the expectation that such satellites will be quenched almost immediately following the cessation of cosmological inflow, regardless of stripping events. Observations of high redshift satellites give an indirect but sensitive measure of the outflow rate with current measurements suggesting that outflows are no larger than 2.5 times the star formation rate for galaxies with a stellar mass of 10^{10.5} solar masses.
We analyse the stellar and hot gas content of 18 nearby, low-mass galaxy clusters, detected in redshift space and selected to have a dynamical mass 3E14<M/Msun<6E14, as measured from the 2dF Galaxy Redshift Survey. We combine X-ray measurements from
both Chandra and XMM with ground-based near-infrared observations from CTIO, AAT and CFHT to compare the mass in hot gas and stars to the dynamical mass and state of the clusters. Only 13 of the clusters are detected in X-ray emission, and for these systems we find that a range of 7-20 per cent of their baryonic mass, and <3 per cent of their dynamical mass, is detected in starlight, similar to what is observed in more massive clusters. In contrast, the five undetected clusters are underluminous in X-ray emission, by up to a factor 10, given their stellar mass. Although the velocity distribution of cluster members in these systems is indistinguishable from a Gaussian, all show subtle signs of being unrelaxed: either they lack a central, dominant galaxy, or the bright galaxy distribution is less concentrated and/or more elongated than the rest of the sample. Thus we conclude that low-mass clusters and groups selected from the velocity distribution of their galaxies exhibit a dichotomy in their hot gas properties. Either they are detected in X-ray, in which case they generally lie on the usual scaling relations, or they are completely undetected in X-ray emission. The non-detections may be partly related to the apparently young dynamical state of the clusters, but it remains a distinct possibility that some of these systems are exceptionally devoid of hot emitting gas as the result of its expulsion or rarefaction.
We use the galaxy stellar mass and halo merger tree information from the semi-analytic model galaxy catalogue of Font et al. (2009) to examine the accretion of galaxies into a large sample of groups and clusters, covering a wide range in halo mass (1
0E12.9 to 10E15.3 Msun/h), 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 10E14.5 Msun/h mass cluster at z=0 has, on average, accreted ~ 40% of its galaxies (Mstellar > 10E9 Msun/h) from halos with masses greater than 10E13 Msun/h. 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 10E12 -- 10E13 Msun/h, 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.
Current models of galaxy formation predict satellite galaxies in groups and clusters that are redder than observed. We investigate the effect on the colours of satellite galaxies produced by the ram pressure stripping of their hot gaseous atmospheres
as the satellites orbit within their parent halo. We incorporate a model of the stripping process based on detailed hydrodynamic simulations within the Durham semi-analytic model of galaxy formation. The simulations show that the environment in groups and clusters is less aggressive than previously assumed. The main uncertainty in the model is the treatment of gas expelled by supernovae. With reasonable assumptions for the stripping of this material, we find that satellite galaxies are able to retain a significant fraction of their hot gas for several Gigayears, thereby replenishing their reservoirs of cold, star forming gas and remaining blue for a relatively long period of time. A bimodal distribution of galaxy colours, similar to that observed in SDSS data, is established and the colours of the satellite galaxies are in good agreement with the data. In addition, our model naturally accounts for the observed dependence of satellite colours on environment, from small groups to high mass clusters.
(Abridged) We demonstrate that the tenet of hierarchical structure growth leads directly to a robust, falsifiable prediction for the correlation between stellar fraction (fstar) and total system mass (M500) of galaxy groups and clusters. This predict
ion is relatively insensitive to the details of baryonic physics or cosmological parameters. In particular, if the fstar-M500 relation is fixed and does not evolve with redshift, CDM models predict the logarithmic slope of this relation to be b>-0.3. This constraint can be weakened if the fstar-M500 relation evolves strongly, but this implies more stars must be formed in situ in groups at low redshift. Conservatively requiring that at least half the stars in groups were formed by z=1, the constraint from evolution models is b>-0.35. Since the most massive clusters (M500=1E15 Msun) are observed to have fstar=0.01, this means that groups with M500=5E13 Msun must have fstar<0.03. Recent observations by Gonzalez et al. (2007) indicate a much steeper relation, with fstar>0.04 in groups, leading to b=-0.64. If confirmed, this would rule out hierarchical structure formation models: todays clusters could not have been built from todays groups, or even from the higher-redshift progenitors of those groups. We perform a careful analysis of these and other data to identify the most important systematic uncertainties in their measurements. Although correlated uncertainties on stellar and total masses might explain the steep observed relation, the data are only consistent with theory if the observed group masses are systematically underestimated.
