Galaxy mergers can initiate quenching by unlocking an AGN-driven transformation of the baryon cycle

We use zoom simulations to show how merger-driven disruption of the gas disc in a galaxy provides its central active galactic nucleus (AGN) with fuel to drive outflows that entrain and expel a significant fraction of the circumgalactic medium (CGM). This in turn suppresses replenishment of the interstellar medium, causing the galaxy to quench up to several Gyr after the merger. We start by performing a zoom simulation of a present-day star-forming disc galaxy with the EAGLE galaxy formation model. Then, we re-simulate the galaxy with controlled changes to its initial conditions, using the genetic modification technique. These modifications either increase or decrease the stellar mass ratio of the galaxy’s last significant merger, which occurs at z ≈ 0.74. The halo reaches the same present-day mass in all cases, but changing the mass ratio of the merger yields markedly different galaxy and CGM properties. We find that a merger can unlock rapid growth of the central supermassive black hole if it disrupts the co-rotational motion of gas in the black hole’s vicinity. Conversely, if a less disruptive merger occurs and gas close to the black hole is not disturbed, the AGN does not strongly affect the CGM, and consequently the galaxy continues to form stars. Our result illustrates how a unified view of AGN feedback, the baryon cycle and the interstellar medium is required to understand how mergers and quenching are connected over long timescales

The impact of free-streaming on dwarf galaxy counts in low-density regions

We study the statistics of dwarf galaxy populations as a function of environment in the cold dark matter (CDM) and warm dark matter (WDM) cosmogonies, using hydrodynamical simulations starting from initial conditions with matched phases but differing power spectra, and evolved with the EAGLE galaxy formation model. We measure the abundance of dwarf galaxies within 3~Mpc of DM haloes with a present-day halo mass similar to that of the Milky Way (MW), and find that the radial distribution of galaxies $M_{*}>10^7$\Msun is nearly identical for WDM and CDM. However, the cumulative mass function becomes shallower for WDM at lower masses, yielding 50~per~cent fewer dwarf galaxies of $M_{*}\gtrsim10^{5}$~\Msun than CDM. The suppression of low-mass halo counts in WDM relative to CDM increases significantly from high-density regions to low-density regions for haloes in the region of the half-mode mass, M_\rm{hm}. The luminous fraction in the two models also diverges from the overdense to the underdense regions for M>2M_\rm{hm}, as the increased collapse delay at small densities pushes the collapse to after the reionization threshold. However, the stellar mass--halo mass relation of WDM haloes relative to CDM increases towards lower-density regions. Finally, we conclude that the suppression of galaxies with $M_{*}\gtrsim10^5$\Msun between WDM and CDM is independent of density: the suppression of halo counts and the luminous fraction is balanced by an enhancement in stellar mass--halo mass relation.Comment: 10 pages, 6 figures, manuscript to be submitted to MNRA

On the Computation of the Cross-section Properties of Arbitrary Thin-walled Structures

In this paper, a generalized computational algorithm based on the line chain and tree models is developed for the cross section properties of arbitrarily configuration struts without closed loops. The two C++ programs for such models are developed. However, the two models cannot apply to struts with any cross-section possessing closed loops. Therefore, the further investigation should be completed

Drosophila melanogaster Set8 and L(3)mbt function in gene expression independently of histone H4 lysine 20 methylation.

Monomethylation of lysine 20 of histone H4 (H4K20me1) is catalyzed by Set8 and thought to play important roles in many aspects of genome function that are mediated by H4K20me binding proteins. We interrogated this model in a developing animal by comparing in parallel the transcriptomes of Set8 null, H4 K20R/A, and l(3)mbt mutant Drosophila melanogaster. We found that the gene expression profiles of H4 K20A and H4 K20R larvae are markedly different than Set8 null larvae despite similar reductions in H4K20me1. Set8 null mutant cells have a severely disrupted transcriptome and fail to proliferate in vivo, but these phenotypes are not recapitulated by mutation of H4 K20, indicating that the developmental defects of Set8 null animals are largely due to H4K20me1-independent effects on gene expression. Furthermore, the H4K20me1 binding protein L(3)mbt is recruited to the transcription start sites of most genes independently of H4K20me even though genes bound by L(3)mbt have high levels of H4K20me1. Moreover, both Set8 and L(3)mbt bind to purified H4K20R nucleosomes in vitro. We conclude that gene expression changes in Set8 null and H4 K20 mutants cannot be explained by loss of H4K20me1 or L(3)mbt binding to chromatin and therefore that H4K20me1 does not play a large role in gene expression

An EAGLE’s View of Ex-situ Galaxy Growth

Modern observational and analytic techniques now enable the direct measurement of star formation histories and the inference of galaxy assembly histories. However, current theoretical predictions of assembly are not ideally suited for direct comparison with such observational data. We therefore extend the work of prior examinations of the contribution of ex-situ stars to the stellar mass budget of simulated galaxies. Our predictions are specifically tailored for direct testing with a new generation of observational techniques by calculating ex-situ fractions as functions of galaxy mass and morphological type, for a range of surface brightnesses. These enable comparison with results from large FoV IFU spectrographs, and increasingly accurate spectral fitting, providing a look-up method for the estimated accreted fraction. We furthermore provide predictions of ex-situ mass fractions as functions of galaxy mass, galactocentric radius and environment. Using z = 0 snapshots from the 100cMpc3 and 25cMpc3 EAGLE simulations we corroborate the findings of prior studies, finding that ex-situ fraction increases with stellar mass for central and satellite galaxies in a stellar mass range of 2× 107 - 1.9× 1012 M⊙. For those galaxies of mass M*>5× 108M⊙, we find that the total ex-situ mass fraction is greater for more extended galaxies at fixed mass. When categorising satellite galaxies by their parent group/cluster halo mass we find that the ex-situ fraction decreases with increasing parent halo mass at fixed galaxy mass. This apparently counter-intuitive result may be due to high passing velocities within large cluster halos inhibiting efficient accretion onto individual galaxies

Hydrodynamical simulations of the galaxy population: Enduring successes and outstanding challenges

We review the progress in modeling the galaxy population in hydrodynamical simulations of the ΛCDM cosmogony. State-of-the-art simulations now broadly reproduce the observed spatial clustering of galaxies; the distributions of key characteristics, such as mass, size, and SFR; and scaling relations connecting diverse properties to mass. Such improvements engender confidence in the insight drawn from simulations. Many important outcomes, however, particularly the properties of circumgalactic gas, are sensitive to the details of the subgrid models used to approximate the macroscopic effects of unresolved physics, such as feedback processes. We compare the outcomes of leading simulation suites with observations, and with each other, to identify the enduring successes they have cultivated and the outstanding challenges to be tackled with the next generation of models. Our key conclusions include the following: ▪ Realistic galaxies can be reproduced by calibrating the ill-constrained parameters of subgrid feedback models. Feedback is dominated by stars and black holes in low-mass and high-mass galaxies, respectively. ▪ Adjusting or disabling the processes implemented in simulations can elucidate their impact on observables, but outcomes can be degenerate. ▪ Similar galaxy populations can emerge in simulations with dissimilar feedback implementations. However, these models generally predict markedly different gas flow rates into, and out of, galaxies and their halos. CGM observations are thus a promising means of breaking this degeneracy and guiding the development of new feedback models

Globular cluster metallicity distributions in the E-MOSAICS simulations

The metallicity distributions of globular cluster (GC) systems in galaxies are a critical test of any GC formation scenario. In this work, we investigate the predicted GC metallicity distributions of galaxies in the MOdelling Star cluster population Assembly In Cosmological Simulations within EAGLE (E-MOSAICS) simulation of a representative cosmological volume ($L = 34.4$ comoving Mpc). We find that the predicted GC metallicity distributions and median metallicities from the fiducial E-MOSAICS GC formation model agree well the observed distributions, except for galaxies with masses $M_\ast \sim 2 \times 10^{10}$ M$_\odot$, which contain an overabundance of metal-rich GCs. The predicted fraction of galaxies with bimodal GC metallicity distributions ($37 \pm 2$ per cent in total; $45 \pm 7$ per cent for $M_\ast > 10^{10.5}$ M$_\odot$) is in good agreement with observed fractions ($44^{+10}_{-9}$ per cent), as are the mean metallicities of the metal-poor and metal-rich peaks. We show that, for massive galaxies ($M_\ast > 10^{10}$ M$_\odot$), bimodal GC distributions primarily occur as a result of cluster disruption from initially-unimodal distributions, rather than as a result of cluster formation processes. Based on the distribution of field stars with GC-like abundances in the Milky Way, we suggest that the bimodal GC metallicity distribution of Milky Way GCs also occurred as a result of cluster disruption, rather than formation processes. We conclude that separate formation processes are not required to explain metal-poor and metal-rich GCs, and that GCs can be considered as the surviving analogues of young massive star clusters that are readily observed to form in the local Universe today.Comment: 19 pages, 16 figures. Published in MNRA

What shapes the galaxy mass function? Exploring the roles of supernova-driven winds and active galactic nuclei

The observed stellar mass function (SMF) is very different to the halo mass function predicted by Λ cold dark matter (ΛCDM), and it is widely accepted that this is due to energy feedback from supernovae and black holes. However, the strength and form of this feedback is not understood. In this paper, we use the phenomenological model galform to explore how galaxy formation depends on the strength and halo mass dependence of feedback. We focus on 'expulsion' models in which the wind mass loading, β, is proportional to 1/v^n_(disc), with n= 0, 1, 2 and contrast these models with the successful Bower et al. model (B8W7), for which β α 1/v^(3.2)_(disc). A crucial development is that our code explicitly accounts for the recapture of expelled gas as the system’s halo mass (and thus gravitational potential) increases. While models with high wind speed and mass loading result in a poor match to the observed SMF, a model with slower wind speed matches the flat portion of the SMF at M_★∼ 10^9–10^(11) h^(−1) M_⊙. When combined with active galactic nucleus feedback, the model provides a good description of the observed SMF above 10^9 h^(−1) M_⊙. In order to explore the impact of different feedback schemes further, we examine how the expulsion models compare with a further range of observational data, contrasting the results with the B8W7 model. In the expulsion models, the brightest galaxies are assembled more recently, and the specific star formation rates of galaxies decrease strongly with decreasing stellar mass. The expulsion models tend to have a cosmic star formation density that is dominated by lower mass galaxies at z= 1–3, and dominated by high-mass galaxies at low redshift. These trends are in conflict with observational data, but the comparison highlights some deficiencies of the B8W7 model also. The experiments in this paper not only give us important physical insight into the impact of the feedback process on the formation histories of galaxies, but the strong mass dependence of feedback adopted in B8W7 still appears to provide the most promising description of the observed Universe