We introduce the THE THREE HUNDRED project, an endeavour to model 324 large galaxy clusters with full-physics hydrodynamical re-simulations. Here we present the data set and study the differences to observations for fundamental galaxy cluster properties and scaling relations. We find that the modelled galaxy clusters are generally in reasonable agreement with observations with respect to baryonic fractions and gas scaling relations at redshift z = 0. However, there are still some (model-dependent) differences, such as central galaxies being too massive, and galaxy colours (g - r) being bluer (about 0.2 dex lower at the peak position) than in observations. The agreement in gas scaling relations down to 10^{13} h^{-1} M_{\odot} between the simulations indicates that particulars of the sub-grid modelling of the baryonic physics only has a weak influence on these relations. We also include - where appropriate - a comparison to three semi-analytical galaxy formation models as applied to the same underlying dark-matter-only simulation. All simulations and derived data products are publicly available.
We take advantage of the largest high-resolution simulation of cosmic structure growth ever carried out — the Millennium Simulation of the concordance Λ cold dark matter (CDM) cosmogony — to study how the star formation histories, ages and metallicities of elliptical galaxies depend on environment and on stellar mass. We concentrate on a galaxy formation model which is tuned to fit the joint luminosity/colour/morphology distribution of low-redshift galaxies. Massive ellipticals in this model have higher metal abundances, older luminosity-weighted ages and shorter star formation time-scales, but lower assembly redshifts, than less massive systems. Within clusters the typical masses, ages and metal abundances of ellipticals are predicted to decrease, on average, with increasing distance from the cluster centre. We also quantify the effective number of progenitors of ellipticals as a function of present stellar mass, finding typical numbers below two for M* < 1011 M⊙, rising to approximately five for the most massive systems. These findings are consistent with recent observational results that suggest 'down-sizing' or 'antihierarchical' behaviour for the star formation history of the elliptical galaxy population, despite the fact that our model includes all the standard elements of hierarchical galaxy formation and is implemented on the standard, ΛCDM cosmogony.
We use a semi-analytic galaxy catalogue constructed from the Millennium Simulation (MS) to study the satellites of isolated galaxies in the Λ cold dark matter (ΛCDM) cosmogony. The large volume surveyed by the MS (5003 h−3 Mpc3), together with its unprecedented numerical resolution, enable the compilation of a large sample of ∼80 000 bright (Mr < −20.5) primaries, surrounded by ∼178 000 satellites down to the faint magnitude limit (Mr=−17) of our catalogue. This sample allows the characterization, with minimal statistical uncertainty, of the dynamical properties of satellite/primary galaxy systems in a ΛCDM universe. The details of this characterization are sensitive to the details of the modelling, such as its assumptions on galaxy merging and dynamical friction time-scales, but many of its general predictions should be applicable to hierarchical formation models such as ΛCDM. We find that, overall, the satellite population traces the dark matter rather well: its spatial distribution and kinematics may be approximated by a Navarro, Frenk & White profile with a mildly anisotropic velocity distribution. Their spatial distribution is also mildly anisotropic, with a well-defined 'anti-Holmberg' effect that reflects the misalignment between the major axis and angular momentum of the host halo. Our analysis also highlights a number of difficulties afflicting studies that rely on satellite velocities to constrain the primary halo mass. These arise from variations in the star formation efficiency and assembly history of isolated galaxies, which result in a scatter of up to approximately two decades in halo mass at a fixed primary luminosity. Our isolation criterion (primaries may only have companions at least 2 mag fainter within 1 h−1 Mpc) contributes somewhat to the scatter, since it picks not only galaxies in sparse environments, but also a number of primaries at the centre of 'fossil' groups. We find that the abundance and luminosity function of these unusual systems are in reasonable agreement with the few available observational constraints. Much tighter halo mass–luminosity relations are found when splitting the sample by colour: red primaries inhabit haloes more than twice as massive as those surrounding blue primaries, a difference that vanishes, however, when considering stellar mass instead of luminosity. The large scatter in the halo mass–luminosity relation hinders the interpretation of the velocity dispersion of satellites stacked according to the luminosity of the primary. We find L∝σ3 (the natural scaling expected for ΛCDM) for truly isolated primaries, that is, systems where the central galaxy contributes more than 85 per cent of the total luminosity within its virial radius. Less-strict primary selection, however, leads to substantial modification of the scaling relation: blindly stacking satellites of all primaries results in a much shallower L–σ relation that is only poorly approximated by a power law.
This paper is the first in a series in which we perform an extensive comparison of various galaxy-based cluster mass estimation techniques that utilise the positions, velocities and colours of galaxies. Our primary aim is to test the performance of these cluster mass estimation techniques on a diverse set of models that will increase in complexity. We begin by providing participating methods with data from a simple model that delivers idealised clusters, enabling us to quantify the underlying scatter intrinsic to these mass estimation techniques. The mock catalogue is based on a Halo Occupation Distribution (HOD) model that assumes spherical Navarro, Frenk and White (NFW) haloes truncated at R_200, with no substructure nor colour segregation, and with isotropic, isothermal Maxwellian velocities. We find that, above 10^14 M_solar, recovered cluster masses are correlated with the true underlying cluster mass with an intrinsic scatter of typically a factor of two. Below 10^14 M_solar, the scatter rises as the number of member galaxies drops and rapidly approaches an order of magnitude. We find that richness-based methods deliver the lowest scatter, but it is not clear whether such accuracy may simply be the result of using an over-simplistic model to populate the galaxies in their haloes. Even when given the true cluster membership, large scatter is observed for the majority non-richness-based approaches, suggesting that mass reconstruction with a low number of dynamical tracers is inherently problematic.
We introduce a simple model to self-consistently connect the growth of galaxies to the formation history of their host dark matter haloes. Our model is defined by two simple functions: the "baryonic growth function" which controls the rate at which new baryonic material is made available for star formation, and the "physics function" which controls the efficiency with which this material is converted into stars. Using simple, phenomenologically motivated forms for both functions that depend only on a single halo property, we demonstrate the model's ability to reproduce the z=0 red and blue stellar mass functions. Furthermore, by adding redshift as a second input variable to the physics function we show that the reproduction of the global stellar mass function out to z=3 is improved. We conclude by discussing the general utility of our new model, highlighting its usefulness for creating mock galaxy samples which have a number of key advantages over those generated by other techniques.
The influence of a galaxy's environment on its evolution has been studied and compared extensively in the literature, although differing techniques are often used to define environment. Most methods fall into two broad groups: those that use nearest neighbours to probe the underlying density field and those that use fixed apertures. The differences between the two inhibit a clean comparison between analyses and leave open the possibility that, even with the same data, different properties are actually being measured. In this work we apply twenty published environment definitions to a common mock galaxy catalogue constrained to look like the local Universe. We find that nearest neighbour-based measures best probe the internal densities of high-mass haloes, while at low masses the inter-halo separation dominates and acts to smooth out local density variations. The resulting correlation also shows that nearest neighbour galaxy environment is largely independent of dark matter halo mass. Conversely, aperture-based methods that probe super-halo scales accurately identify high-density regions corresponding to high mass haloes. Both methods show how galaxies in dense environments tend to be redder, with the exception of the largest apertures, but these are the strongest at recovering the background dark matter environment. We also warn against using photometric redshifts to define environment in all but the densest regions. When considering environment there are two regimes: the 'local environment' internal to a halo best measured with nearest neighbour and 'large-scale environment' external to a halo best measured with apertures. This leads to the conclusion that there is no universal environment measure and the most suitable method depends on the scale being probed.
Galaxy environment is frequently discussed, but inconsistently defined. It is especially difficult to measure at high redshift where only photometric redshifts are available. With a focus on early forming protoclusters, we use a semi-analytical model of galaxy formation to show how the environment measurement around high-redshift galaxies is sensitive to both scale and metric, as well as to cluster viewing angle, evolutionary state and the availability of either spectroscopic or photometric data. We use two types of environment metrics (nearest-neighbour and fixed aperture) at a range of scales on simulated high-z clusters to see how ‘observed’ overdensities compare to ‘real’ overdensities. We also ‘observationally’ identify z = 2 protocluster candidates in our model and track the growth histories of their parent haloes through time, considering in particular their final state at z = 0. Although the measured environment of early forming clusters is critically dependent on all of the above effects (and in particular the viewing angle), we show that such clusters are very likely ( ≳ 90 per cent) to remain overdense at z = 0, although many will no longer be among the most massive. Object-to-object comparisons using different methodologies and different data, however, require much more caution.
We present a simple model of how quasars occupy dark matter halos from z=0 to z=5 using the observed mBH-sigma relation and quasar luminosity functions. This provides a way for observers to statistically infer host halo masses for quasar observations using luminosity and redshift alone. Our model is deliberately simple and sidesteps any need to explicitly describe the physics. In spite of its simplicity, the model reproduces many key observations and has predictive power: 1) model quasars have the correct luminosity function (by construction) and spatial clustering (by consequence); 2) we predict high redshift quasars of a given luminosity live in less massive dark matter halos than the same luminosity quasars at low redshifts; 3) we predict a factor of ~5 more 10^8.5Msun black holes at z~2 than is currently observed; 4) we predict a factor of ~20 evolution in the amplitude of the mBH-Mhalo relation between z=5 and the present day; 5) we expect luminosity dependent quasar lifetimes of between tQ~10^(7-8)yr, but which may become as short as 10^(5-6)yr for quasars brighter than L*; 6) while little luminosity dependent clustering evolution is expected at z<1, increasingly strong evolution is predicted for L>L* quasars at higher redshifts. These last two results arise from the narrowing distribution of halo masses that quasars occupy as the Universe ages. We also deconstruct both "downsizing" and "upsizing" trends predicted by the model at different redshifts and space densities. Importantly, this work illustrates how current observations cannot distinguish between more complicated physically motivated quasar models and our simple phenomenological approach. It highlights the opportunities such methodologies provide.
We use EAGLE to quantify the effect galaxy mergers have on the stellar specific angular momentum of galaxies, $j_{\rm stars}$. We split mergers into: dry (gas-poor)/wet (gas-rich), major/minor, and different spin alignments and orbital parameters. Wet (dry) mergers have an average neutral gas-to-stellar mass ratio of $1.1$ ($0.02$), while major (minor) mergers are those with stellar mass ratios $\ge 0.3$ ($0.1-0.3$). We correlate the positions of galaxies in the $j_{\rm stars}$-stellar mass plane at $z=0$ with their merger history, and find that galaxies of low spins suffered dry mergers, while galaxies of normal/high spins suffered predominantly wet mergers, if any. The radial $j_{\rm stars}$ profiles of galaxies that went through dry mergers are deficient by $\approx 0.3$~dex at $r\lesssim 10\,r_{50}$ (with $r_{50}$ being the half-stellar mass radius), compared to galaxies that went through wet mergers. Studying the merger remnants reveals that dry mergers reduce $j_{\rm stars}$ by $\approx 30$\%, while wet mergers increase it by $\approx 10$\%, on average. The latter is connected to the build-up of the bulge by newly formed stars of high rotational speed. Moving from minor to major mergers accentuates these effects. When the spin vectors of the galaxies prior to the dry merger are misaligned, $j_{\rm stars}$ decreases to a greater magnitude, while in wet mergers co-rotation and high orbital angular momentum efficiently spun-up galaxies. We predict what would be the observational signatures in the $j_{\rm stars}$ profiles driven by dry mergers: (i) shallow radial profiles and (ii) profiles that rise beyond $\approx 10\,r_{50}$, both of which are significantly different from spiral galaxies.
There are strong correlations between the three structural properties of elliptical galaxies -stellar mass, velocity dispersion and size -in the form of a tight "fundamental plane" and a "scaling relation" between each pair.Major mergers of disk galaxies are assumed to be a mechanism for producing ellipticals, but semi-analytic galaxy formation models (SAM) have encountered apparent difficulties in reproducing the observed slope and scatter of the size-mass relation.We study the scaling relations of merger remnants using progenitor properties from two SAMs.We apply a simple merger model that includes gas dissipation and star formation based on theoretical considerations and simulations.Combining the SAMs and the merger model allows calculation of the structural properties of the remnants of major mergers that enter the population of elliptical galaxies at a given redshift.Without tuning the merger model parameters for each SAM, the results roughly match the slope and scatter in the observed scaling relations and their evolution in the redshift range z = 0 -3.Within this model, the observed scaling relations, including the tilt of the fundamental plane relative to the virial plane, result primarily from the decrease of gas fraction with increasing progenitor mass.The scatter in the size-mass relation of the remnants is reduced from that of the progenitors because of a correlation between progenitor size and gas fraction at a given mass.
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