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Register a new userOn the galaxy-halo connection in the EAGLE simulation
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Empirical models of galaxy formation require assumptions about the correlations between galaxy and halo properties. These may be calibrated against observations or inferred from physical models such as hydrodynamical simulations. In this Letter, we use the EAGLE simulation to investigate the correlation of galaxy size with halo properties. We motivate this analysis by noting that the common assumption of angular momentum partition between baryons and dark matter in rotationally supported galaxies overpredicts both the spread in the stellar mass-size relation and the anticorrelation of size and velocity residuals, indicating a problem with the galaxy-halo connection it implies. We find the EAGLE galaxy population to perform significantly better on both statistics, and trace this success to the weakness of the correlations of galaxy size with halo mass, concentration and spin at fixed stellar mass. Using these correlations in empirical models will enable fine-grained aspects of galaxy scalings to be matched.
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We investigate the dependence of the galaxy properties on cosmic web environments using the most up-to-date hydrodynamic simulation: Evolution and Assembly of Galaxies and their Environments (EAGLE). The baryon fractions in haloes and the amplitudes of the galaxy luminosity function decrease going from knots to filaments to sheets to voids. Interestingly, the value of L$^*$ varies dramatically in different cosmic web environments. At z = 0, we find a characteristic halo mass of $10^{12} h^{-1}rm M_{odot}$, below which the stellar-to-halo mass ratio is higher in knots while above which it reverses. This particular halo mass corresponds to a characteristic stellar mass of $1.8times 10^{10} h^{-1}rm M_{odot}$. Below the characteristic stellar mass central galaxies have redder colors, lower sSFRs and higher metallicities in knots than those in filaments, sheets and voids, while above this characteristic stellar mass, the cosmic web environmental dependences either reverse or vanish. Such dependences can be attributed to the fact that the active galaxy fraction decreases along voids, sheets, filaments and knots. The cosmic web dependences get weaker towards higher redshifts for most of the explored galaxy properties and scaling relations, except for the gas metallicity vs. stellar mass relation.
We use the EAGLE hydrodynamical simulation to trace the quenching history of galaxies in its 10 most massive clusters. We use two criteria to identify moments when galaxies suffer significant changes in their star formation activity: {it i)} the instantaneous star formation rate (SFR) strongest drop, $Gamma_{rm SFR}^{rm SD}$, and {it ii)} a quenching criterion based on a minimum threshold for the specific SFR of $lesssim$ 10$^{-11}rm yr^{-1}$. We find that a large fraction of galaxies ($gtrsim 60%$) suffer their $Gamma_{rm SFR}^{rm SD}$ outside the clusters R$_{200}$. This ``pre-processed population is dominated by galaxies that are either low mass and centrals or inhabit low mass hosts ($10^{10.5}$M$_{odot} lesssim$ M$_{rm host} lesssim 10^{11.0}$M$_{odot}$). The host mass distribution is bimodal, and galaxies that suffered their $Gamma_{rm SFR}^{rm SD}$ in massive hosts ($10^{13.5}rm M_{odot} lesssim M_{host} lesssim 10^{14.0}M_{odot}$) are mainly processed within the clusters. Pre-processing mainly limits the total stellar mass with which galaxies arrive in the clusters. Regarding quenching, galaxies preferentially reach this state in high-mass halos ($10^{13.5}rm M_{odot} lesssim M_{host} lesssim 10^{14.5}M_{odot}$). The small fraction of galaxies that reach the cluster already quenched has also been pre-processed, linking both criteria as different stages in the quenching process of those galaxies. For the $z=0$ satellite populations, we find a sharp rise in the fraction of quenched satellites at the time of first infall, highlighting the role played by the dense cluster environment. Interestingly, the fraction of pre-quenched galaxies rises with final cluster mass. This is a direct consequence of the hierarchical cosmological model used in these simulations.
The difference in shape between the observed galaxy stellar mass function and the predicted dark matter halo mass function is generally explained primarily by feedback processes. Feedback can shape the stellar-halo mass (SHM) relation by driving gas out of galaxies, by modulating the first-time infall of gas onto galaxies (i.e., preventative feedback), and by instigating fountain flows of recycled wind material. We present a novel method to disentangle these effects for hydrodynamical simulations of galaxy formation. We build a model of linear coupled differential equations that by construction reproduces the flows of gas onto and out of galaxies and haloes in the EAGLE cosmological simulation. By varying individual terms in this model, we isolate the relative effects of star formation, ejection via outflow, first-time inflow and wind recycling on the SHM relation. We find that for halo masses $M_{200} < 10^{12} , mathrm{M_odot}$ the SHM relation is shaped primarily by a combination of ejection from galaxies and haloes, while for larger $M_{200}$ preventative feedback is also important. The effects of recycling and the efficiency of star formation are small. We show that if, instead of $M_{200}$, we use the cumulative mass of dark matter that fell in for the first time, the evolution of the SHM relation nearly vanishes. This suggests that the evolution is due to the definition of halo mass rather than to an evolving physical efficiency of galaxy formation. Finally, we demonstrate that the mass in the circum-galactic medium is much more sensitive to gas flows, especially recycling, than is the case for stars and the interstellar medium.
We analyse the mass assembly of central galaxies in the EAGLE hydrodynamical simulations. We build merger trees to connect galaxies to their progenitors at different redshifts and characterize their assembly histories by focusing on the time when half of the galaxy stellar mass was assembled into the main progenitor. We show that galaxies with stellar mass $M_*<10^{10.5}M_{odot}$ assemble most of their stellar mass through star formation in the main progenitor (`in-situ star formation). This can be understood as a consequence of the steep rise in star formation efficiency with halo mass for these galaxies. For more massive galaxies, however, an increasing fraction of their stellar mass is formed outside the main progenitor and subsequently accreted. Consequently, while for low-mass galaxies the assembly time is close to the stellar formation time, the stars in high-mass galaxies typically formed long before half of the present-day stellar mass was assembled into a single object, giving rise to the observed anti-hierarchical downsizing trend. In a typical present-day $M_*geq10^{11}M_{odot}$ galaxy, around $20%$ of the stellar mass has an external origin. This fraction decreases with increasing redshift. Bearing in mind that mergers only make an important contribution to the stellar mass growth of massive galaxies, we find that the dominant contribution comes from mergers with galaxies of mass greater than one tenth of the main progenitors mass. The galaxy merger fraction derived from our simulations agrees with recent observational estimates.
We investigate the connection between the star formation rate (SFR) of galaxies and their central black hole accretion rate (BHAR) using the EAGLE cosmological hydrodynamical simulation. We find, in striking concurrence with recent observational studies, that the <SFR>--BHAR relation for an AGN selected sample produces a relatively flat trend, whilst the <BHAR>--SFR relation for a SFR selected sample yields an approximately linear trend. These trends remain consistent with their instantaneous equivalents even when both SFR and BHAR are time-averaged over a period of 100~Myr. There is no universal relationship between the two growth rates. Instead, SFR and BHAR evolve through distinct paths that depend strongly on the mass of the host dark matter halo. The galaxies hosted by haloes of mass M200 $lesssim 10^{11.5}$Msol grow steadily, yet black holes (BHs) in these systems hardly grow, yielding a lack of correlation between SFR and BHAR. As haloes grow through the mass range $10^{11.5} lesssim$ M200 $lesssim 10^{12.5 }$Msol BHs undergo a rapid phase of non-linear growth. These systems yield a highly non-linear correlation between the SFR and BHAR, which are non-causally connected via the mass of the host halo. In massive haloes (M200 $gtrsim 10^{12.5}$Msol) both SFR and BHAR decline on average with a roughly constant scaling of SFR/BHAR $sim 10^{3}$. Given the complexity of the full SFR--BHAR plane built from multiple behaviours, and from the large dynamic range of BHARs, we find the primary driver of the different observed trends in the <SFR>--BHAR and <BHAR>--SFR relationships are due to sampling considerably different regions of this plane.
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