We introduce the Virgo Consortium's 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.

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

Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

Monthly Notices of the Royal Astronomical Society

We present a robust method to constrain average galaxy star formation rates (SFRs), star formation histories (SFHs), and the intracluster light (ICL) as a function of halo mass. Our results are consistent with observed galaxy stellar mass functions, specific star formation rates (SSFRs), and cosmic star formation rates (CSFRs) from z = 0 to z = 8. We consider the effects of a wide range of uncertainties on our results, including those affecting stellar masses, SFRs, and the halo mass function at the heart of our analysis. As they are relevant to our method, we also present new calibrations of the dark matter halo mass function, halo mass accretion histories, and halo-subhalo merger rates out to z = 8. We also provide new compilations of CSFRs and SSFRs; more recent measurements are now consistent with the buildup of the cosmic stellar mass density at all redshifts. Implications of our work include: halos near 1012 M ☉ are the most efficient at forming stars at all redshifts, the baryon conversion efficiency of massive halos drops markedly after z ~ 2.5 (consistent with theories of cold-mode accretion), the ICL for massive galaxies is expected to be significant out to at least z ~ 1-1.5, and dwarf galaxies at low redshifts have higher stellar mass to halo mass ratios than previous expectations and form later than in most theoretical models. Finally, we provide new fitting formulae for SFHs that are more accurate than the standard declining tau model. Our approach places a wide variety of observations relating to the SFH of galaxies into a self-consistent framework based on the modern understanding of structure formation in ΛCDM. Constraints on the stellar mass-halo mass relationship and SFRs are available for download online.

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The Average Star Formation Histories of Galaxies in Dark Matter Halos from z = 0-8

We introduce the Illustris Project, a series of large-scale hydrodynamical simulations of galaxy formation. The highest resolution simulation, Illustris-1, covers a volume of (106.5 Mpc)^3, has a dark mass resolution of 6.26 × 10^6 M_⊙, and an initial baryonic matter mass resolution of 1.26 × 10^6 M_⊙. At z = 0 gravitational forces are softened on scales of 710 pc, and the smallest hydrodynamical gas cells have an extent of 48 pc. We follow the dynamical evolution of 2 × 1820^3 resolution elements and in addition passively evolve 1820^3 Monte Carlo tracer particles reaching a total particle count of more than 18 billion. The galaxy formation model includes: primordial and metal-line cooling with self-shielding corrections, stellar evolution, stellar feedback, gas recycling, chemical enrichment, supermassive black hole growth, and feedback from active galactic nuclei. Here we describe the simulation suite, and contrast basic predictions of our model for the present-day galaxy population with observations of the local universe. At z = 0 our simulation volume contains about 40 000 well-resolved galaxies covering a diverse range of morphologies and colours including early-type, late-type and irregular galaxies. The simulation reproduces reasonably well the cosmic star formation rate density, the galaxy luminosity function, and baryon conversion efficiency at z = 0. It also qualitatively captures the impact of galaxy environment on the red fractions of galaxies. The internal velocity structure of selected well-resolved disc galaxies obeys the stellar and baryonic Tully–Fisher relation together with flat circular velocity curves. In the well-resolved regime, the simulation reproduces the observed mix of early-type and late-type galaxies. Our model predicts a halo mass dependent impact of baryonic effects on the halo mass function and the masses of haloes caused by feedback from supernova and active galactic nuclei.

Introducing the Illustris Project: simulating the coevolution of dark and visible matter in the Universe

We present a new statistical method to determine the relationship between the stellar masses of galaxies and the masses of their host dark matter haloes over the entire cosmic history from z � 4 to the present. This multi-epoch abundance matching (MEAM) model self-consistently takes into account that satellite galaxies first become satellites at times earlier than they are observed. We employ a redshift-dependent parameterization of the stellar-to-halo mass relation to populate haloes and subhaloes in the Millennium simulations with galaxies, requiring that the observed stellar mass functions at several redshifts be reproduced simultaneously. We show that physically meaningful growth of massive galaxies is consistent with these data only if observational mass errors are taken into account. Using merger trees extracted from the dark matter simulations in combination with MEAM, we predict the average assembly histories of galaxies, separating into star formation within the galaxies (in-situ) and accretion of stars (ex-situ). Our main results are: The peak star formation efficiency decreases with redshift from 23 per cent at z = 0 to 9 per cent at z = 4 while the corresponding halo mass increases from 10 11.8 M⊙ to 10 12.5 M⊙. The star formation rate of central galaxies peaks at a redshift which depends on halo mass; for massive haloes this peak is at early cosmic times while for low-mass galaxies the peak has not been reached yet. In haloes similar to that of the Milky-Way about half of the central stellar mass is assembled after z = 0.7. In low-mass haloes, the accretion of satellites contributes little to the assembly of their central galaxies, while in massive haloes more than half of the central stellar mass is formed ex-situ with significant accretion of satellites at z < 2. We find that our method implies a cosmic star formation history and an evolution of specific star formation rates which are consistent with those inferred directly. We present convenient fitting functions for stellar masses, star formation rates, and accretion rates as functions of halo mass and redshift.

Galactic star formation and accretion histories from matching galaxies to dark matter haloes

We extend the abundance matching technique (AMT) to infer the satellite-subhalo and central-halo mass relations (MRs) of local galaxies as well as the corresponding satellite conditional mass functions (CSMFs). We use as inputs the observed galaxy stellar mass function (GSMF) decomposed into centrals and satellites and the ΛCDM distinct halo and subhalo mass functions. We explore the effects of defining the subhalo mass, m sub, at the time of (sub)halo accretion (m acc sub) versus defining it at the time of observation (m obs sub); we also test the standard assumption that centrals and satellites follow the same MRs. We show that this assumption leads to predictions in disagreement with observations, especially when m obs sub is used. We find that when the satellite-subhalo MRs are constrained by the satellite GSMF, they are always different from the central-halo MR: The smaller the stellar mass, the less massive the subhalo of satellites as compared to the halo of centrals of the same stellar mass. This difference is more dramatic when m obs sub is used instead of m acc sub. On average, for stellar masses lower than ~2 × 1011 M ☉, the dark mass of satellites decreased by 60%-65% with respect to their masses at accretion time. We find that MRs for both definitions of subhalo mass yield CSMFs in agreement with observations. Also, when these MRs are used in a halo occupation model, the predicted two-point correlation functions at different stellar mass bins agree with observations. The average stellar-halo MR is close to the MR of central galaxies alone, and conceptually this average MR is equivalent to abundance matching the cumulative total GSMF to the halo + subhalo mass function (the standard AMT). We show that the use of m obs sub leads to less uncertain MRs than m acc sub and discuss some implications of the obtained satellite-subhalo MR. For example, we show that the tension between abundance and dynamics of Milky Way satellites in the ΛCDM cosmogony gives a value of ~ – 1.6 in the faint-end slope of the GSMF upturns.

https://iopscience.iop.org/article/10.1088/0004-637X/756/1/2/pdf

The stellar-subhalo mass relation of satellite galaxies

We explore how the slopes and scatters of the scaling relations of disk galaxies (V m-L[-M], R-L[-M], and V m-R) change when moving from B- to K-bands and to stellar and baryonic quantities. For our compiled sample of 76 normal, non-interacting, high and low-surface brightness (SB) disk-galaxies, we find important changes, which evidence evolution effects, mainly related to the gas infall and star-formation (SF) processes. We also explore correlations among the (B – K) color, stellar mass fraction, f s, mass, M, luminosity, L, and surface density (or SB), as well as correlations among the residuals of the scaling relations, and among these residuals and those of the other relations studied here. Some of our findings are the following: (i) the scale length R bar is a third parameter in the baryonic Tully-Fisher relation (TFR) and the residuals of this relation follow a trend (slope ≈–0.15) with the residuals of the R bar-M bar relation; for the stellar and K-band cases, the scale length is no longer a third parameter and the mentioned trend disappears; (ii) among the TFRs, the B-band TFR is the most scattered; in this case, the color is a third parameter, in agreement with previous works; (iii) the low-SB (LSB) galaxies break some observed trends in diagrams that include SD, color, and f s, suggesting then a threshold in the gas SD, Σ g , below which the SF efficiency becomes independent of Σ g and of the gas infall rate. Our results are interpreted and discussed in the light of Λ Cold Dark Matter (ΛCDM)-based models of disk-galaxy formation and evolution. These models are able to explain not only the baryonic scaling correlations, but also most of the processes responsible for the observed changes in the slopes, scatters, and correlations among the residuals when changing to stellar and luminous quantities. The galaxy baryon fraction, f gal, is required to be smaller than 0.05 on average. We detect some potential difficulties for the models: the observed color-M and SD-M correlations are steeper, and the intrinsic scatter in the baryonic TFR is smaller than those predicted.

https://iopscience.iop.org/article/10.1088/0004-6256/136/3/1340/pdf

On the baryonic, stellar, and luminous scaling relations of disk galaxies

We report new calculations of the cooling rate of primordial gas by the HD molecule, taking into account its ro-vibrational structure. The HD cooling function is calculated including radiative and collisional transitions for J 8 rotational levels, and for the vibrational levels v = 0, 1, 2 and 3. The ro-vibrational level population is calculated from the balance equation assuming steady state. The cooling function is evaluated in the ranges of the kinetic temperatures, T k, from 10 2 to 2 × 10 4 K and the number densities, nH, from 1 to 10 8 cm −3 .W efind that the inclusion of collisional ro-vibrational transitions increases significantly the HD cooling efficiency, in particular for high densities and temperatures. For n H 10 5 and T k ∼ 10 4 K the cooling function becomes more than an order of magnitude higher than previously reported. We give also the HD cooling rate in the presence of the cosmic microwave radiation field for radiation temperatures of 30, 85 and 276 K (redshifts of 10, 30 and 100). The tabulated cooling functions are available at http://www.cifus.uson.mx/Personal Pages/anton/DATA/HD cooling/HD cool. html. We discuss the relevance to explore the effects of including our results into models and simulations of galaxy formation, especially in the regime when gas cools down from temperatures above ∼3000 K.

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The cooling function of HD molecule revisited

One of the predictions of the standard cold dark matter model is that dark haloes have centrally divergent density profiles. An extensive body of rotation curve observations of dwarf and low surface brightness galaxies shows the dark haloes of those systems to be characterized by soft constant density central cores. Several physical processes have been proposed to produce soft cores in dark haloes, each one with different scaling properties. With the aim of discriminating among them we have examined the rotation curves of dark matter dominated dwarf and low surface brightness galaxies and the inner mass profiles of two clusters of galaxies lacking a central cD galaxy and with evidence of soft cores in the centre. The core radii and central densities of these haloes scale in a well defined manner with the depth of their potential wells, as measured through the maximum circular velocity. As a result of our analysis we identify self-interacting cold dark matter as a viable solution to the core problem, where a non-singular isothermal core is formed in the halo center surrounded by a Navarro, Frenk, & White profile in the outer parts. We show that this particular physical situation predicts core radii in agreement with observations. Furthermore, using the observed scalings, we derive an expression for the minimum cross section (�) which has an explicit dependence with the halo dispersion velocity (v). If mx is the mass of the dark matter particle: �/mx � 4 10 25 (100 kms 1 /v) cm 2 /Gev.

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Constraints on dark matter physics from dwarf galaxies through galaxy cluster haloes

Resumen en: A model for calculating the cosmic star formation rate density history in a hierarchical A Cold Dark Matter (ACDM) universe is presented and compared wit...

Vladimir Avila-Reese

Papers

The stellar-subhalo mass relation of satellite galaxies

On the baryonic, stellar, and luminous scaling relations of disk galaxies

The cooling function of HD molecule revisited

Constraints on dark matter physics from dwarf galaxies through galaxy cluster haloes

The cosmic star formation history