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Register a new userThe EAGLE project: Simulating the evolution and assembly of galaxies and their environments
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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 $lesssim 0.2$ 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.
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Despite the insights gained in the last few years, our knowledge about the formation and evolution scenario for the spheroid-dominated galaxies is still incomplete. New and more powerful cosmological simulations have been developed that together with more precise observations open the possibility of more detailed study of the formation of early-type galaxies (ETGs). The aim of this work is to analyse the assembly histories of ETGs in a $Lambda$-CDM cosmology, focussing on the archeological approach given by the mass-growth histories.We inspected a sample of dispersion-dominated galaxies selected from the largest volume simulation of the EAGLE project. This simulation includes a variety of physical processes such as radiative cooling, star formation (SF), metal enrichment, and stellar and active galactic nucleus (AGN) feedback. The selected sample comprised 508 spheroid-dominated galaxies classified according to their dynamical properties. Their surface brightness profile, the fundamental relations, kinematic properties, and stellar-mass growth histories are estimated and analysed. The findings are confronted with recent observations.The simulated ETGs are found to globally reproduce the fundamental relations of ellipticals. All of them have an inner disc component where residual younger stellar populations (SPs) are detected. A fraction of this inner-disc correlates with bulge-to-total ratio. We find a relation between kinematics and shape that implies that dispersion-dominated galaxies with low $V/sigma_L$ (where $V$ is the average rotational velocity and $sigma_L$ the one dimensional velocity dispersion) tend to have ellipticity smaller than $sim 0.5$ and are dominated by old stars. Abridged
We investigate the formation of a galaxy reaching a virial mass of $~ 10^8$ solar mass at $z=10$ by carrying out a zoomed radiation-hydrodynamical cosmological simulation. This simulation traces Population~III (Pop~III) star formation, characterized by a modestly top-heavy initial mass function (IMF), and considers stellar feedback such as photoionization heating from Pop III and Population~II (Pop~II) stars, mechanical and chemical feedback from supernovae (SNe), and X-ray feedback from accreting black holes (BHs) and high-mass X-ray binaries (HMXBs). We self-consistently impose a transition in star formation mode from top-heavy Pop III to low-mass Pop~II, and find that the star formation rate in the computational box is dominated by Pop~III until $z=13$, and by Pop~II thereafter. The simulated galaxy experiences bursty star formation, with a substantially reduced gas content due to photoionization heating from Pop~III and Pop~II stars, together with SN feedback. All the gas within the simulated galaxy is metal-enriched above $10^{-5}$ solar, such that there are no remaining pockets of primordial gas. The simulated galaxy has an estimated observed flux of $~10^{-3} nJy$, which is too low to be detected by the James Webb Space Telescope (JWST) without strong lensing amplification. We also show that our simulated galaxy is similar in terms of stellar mass to Segue 2, the least luminous dwarf known in the Local Group.
We investigate the population of dwarf galaxies with stellar masses similar to the Large Magellanic Cloud (LMC) and M33 in the EAGLE galaxy formation simulation. In the field, galaxies reside in haloes with stellar-to-halo mass ratios of $1.03^{+0.50}_{-0.31}times10^{-2}$ (68% confidence level); systems like the LMC, which have an SMC-mass satellite, reside in haloes about 1.3 times more massive, which suggests an LMC halo mass at infall, $M_{200}=3.4^{+1.8}_{-1.2}times10^{11}M_odot$ (68% confidence level). The colour distribution of dwarfs is bimodal, with the red galaxies ($g-r>0.6$) being mostly satellites. The fraction of red LMC-mass dwarfs is 15% for centrals, and for satellites this fraction increases rapidly with host mass: from 10% for satellites of Milky Way (MW)-mass haloes to nearly 90% for satellites of groups and clusters. The quenching timescale, defined as the time after infall when half of the satellites have acquired red colours, decreases with host mass from ${>}5$ Gyrs for MW-mass hosts to $2.5$ Gyrs for cluster mass hosts. The satellites of MW-mass haloes have higher star formation rates and bluer colours than field galaxies. This is due to enhanced star formation triggered by gas compression shortly after accretion. Both the LMC and M33 have enhanced recent star formation that could be a manifestation of this process. After infall into their MW-mass hosts, the $g-r$ colours of LMC-mass dwarfs become bluer for the first 2 Gyrs, after which they rapidly redden. LMC-mass dwarfs fell into their MW-mass hosts only relatively recently, with more than half having an infall time of less than 3.5 Gyrs.
We examine the evolution of assembly bias using a semi-analytical model of galaxy formation implemented in the Millennium-WMAP7 N-body simulation. We consider fixed number density galaxy samples ranked by stellar mass or star formation rate. We investigate how the clustering of haloes and their galaxy content depend on halo formation time and concentration, and how these relationships evolve with redshift. At $z=0$ the dependences of halo clustering on halo concentration and formation time are similar. However, at higher redshift, halo assembly bias weakens for haloes selected by age, and reverses and increases for haloes selected by concentration. The variation of the halo occupation with concentration and formation time is also similar at $z=0$ and changes at higher redshifts. In this case, the occupancy variation with halo age stays mostly constant with redshift but decreases for concentration. Finally, we look at the evolution of assembly bias reflected in the galaxy distribution by examining the galaxy correlation functions relative to those of shuffled galaxy samples which remove the occupancy variation. This correlation functions ratio monotonically decreases with larger redshift and for lower number density samples, going below unity in some cases, leading to reduced galaxy clustering. While the halo occupation functions themselves vary, the assembly bias trends are similar whether selecting galaxies by stellar mass or star formation rate. Our results provide further insight into the origin and evolution of assembly bias. Our extensive occupation function measurements and fits are publicly available and can be used to create realistic mock catalogues.
We explore the co-evolution of the specific angular momentum of dark matter haloes and the cold baryons that comprise the galaxies within. We study over two thousand central galaxies within the reference cosmological hydrodynamical simulation of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) project. We employ a methodology within which the evolutionary history of a system is specified by the time-evolving properties of the Lagrangian particles that define it at z=0. We find a strong correlation between the evolution of the specific angular momentum of todays stars (cold gas) and that of the inner (whole) dark matter halo they are associated with. This link is particularly strong for the stars formed before the epoch of maximum expansion and subsequent collapse of the central dark matter halo (turnaround). Spheroids are typically assembled primarily from stars formed prior to turnaround, and are therefore destined to suffer a net loss of angular momentum associated with the strong merging activity during the assembly of the inner dark matter halo. Stellar discs retain their specific angular momentum since they are comprised of stars formed mainly after turnaround, from gas that mostly preserves the high specific angular momentum it acquired by tidal torques during the linear growth of the halo. Since the specific angular momentum loss of the stars is tied to the galaxys morphology today, it may be possible to use our results to predict, statistically, the assembly history of a halo given the morphology of the galaxy it hosts.
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