Cosmological Hydrodynamical Simulations Research Articles

The impact of early massive mergers on the chemical evolution of Milky Way-like galaxies: insights from NIHAO-UHD simulations

ABSTRACT Recent observations of the Milky Way (MW) found an unexpected steepening of the star-forming gas metallicity gradient around the time of the Gaia–Sausage–Enceladus (GSE) merger event. Here, we investigate the influence of early (tmerger ≲ 5 Gyr) massive ($M_{\rm {gas}}^{\rm {merger}}/M_{\rm {gas}}^{\rm {main}}(t_{\rm {merger}})\gtrsim 10~{{\ \rm per\ cent}}$) merger events such as the Gaia–Sausage–Enceladus merger in the MW on the evolution of the cold gas metallicity gradient. We use the NIHAO-UHD suite of cosmological hydrodynamical simulations of MW-mass galaxies to study the frequency of massive early mergers and their detailed impact on the morphology and chemistry of the gaseous discs. We find a strong steepening of the metallicity gradient at early times for all four galaxies in our sample which is caused by a sudden increase in the cold gas disc size (up to a factor of 2) in combination with the supply of unenriched gas (∼0.75 dex lower compared to the main galaxy) by the merging dwarf galaxies. The mergers mostly affect the galaxy outskirts and lead to an increase in cold gas surface density of up to 200 per cent outside of ∼8 kpc. The addition of unenriched gas breaks the self-similar enrichment of the inter-stellar-medium and causes a dilution of the cold gas in the outskirts of the galaxies. The accreted stars and the ones formed later out of the accreted gas inhabit distinct tracks offset to lower [α/Fe] and [Fe/H] values compared to the main galaxy’s stars. We find that such mergers can contribute significantly to the formation of a second, low-α sequence as is observed in the MW.

MUSE-ALMA Haloes XI: gas flows in the circumgalactic medium

ABSTRACT The flow of gas into and out of galaxies leaves traces in the circumgalactic medium which can then be studied using absorption lines towards background quasars. We analyse 27 ${{\log [N({\textrm {H}}\, {\small {I}})/\rm {cm}^{-2}]}} > 18.0$ H i absorbers at z = 0.2 to 1.4 from the MUSE-ALMA Haloes survey with at least one galaxy counterpart within a line of sight velocity of ±500 km s−1. We perform 3D kinematic forward modelling of these associated galaxies to examine the flow of dense, neutral gas in the circumgalactic medium. From the VLT/MUSE, HST broad-band imaging, and VLT/UVES and Keck/HIRES high-resolution UV quasar spectroscopy observations, we compare the impact parameters, star-formation rates, and stellar masses of the associated galaxies with the absorber properties. We find marginal evidence for a bimodal distribution in azimuthal angles for strong H i absorbers, similar to previous studies of the Mg ii and O vi absorption lines. There is no clear metallicity dependence on azimuthal angle, and we suggest a larger sample of absorbers is required to fully test the relationship predicted by cosmological hydrodynamical simulations. A case-by-case study of the absorbers reveals that ten per cent of absorbers are consistent with gas accretion, up to 30 per cent trace outflows, and the remainder trace gas in the galaxy disc, the intragroup medium, and low-mass galaxies below the MUSE detection limit. Our results highlight that the baryon cycle directly affects the dense neutral gas required for star-formation and plays a critical role in galaxy evolution.

Probing z ≳ six massive black holes with gravitational waves

ABSTRACT We investigate the coalescence of massive black hole ($M_{\rm BH}\gtrsim 10^{6}~\rm {\rm M}_{\odot }$) binaries (MBHBs) at 6 < z < 10 by adopting a suite of cosmological hydrodynamical simulations of galaxy formation, zoomed-in on biased (>3σ) overdense regions (Mh ∼ 1012 M⊙ dark matter haloes at z = 6) of the Universe. We first analyse the impact of different resolutions and AGN feedback prescriptions on the merger rate, assuming instantaneous mergers. Then, we compute the halo bias correction factor due to the overdense simulated region. Our simulations predict merger rates that range between 3 and 15 $\rm yr^{-1}$ at z ∼6, depending on the run considered, and after correcting for a bias factor of ∼20−30. For our fiducial model, we further consider the effect of delay in the MBHB coalescence due to dynamical friction. We find that 83 per cent of MBHBs will merge within the Hubble time, and 21 per cent within 1 Gyr, namely the age of the Universe at z > 6. We finally compute the expected properties of the gravitational wave (GW) signals and find the fraction of LISA detectable events with high signal-to-noise ratio (SNR > 5) to range between 66 per cent and 69 per cent. However, identifying the electro-magnetic counterpart of these events remains challenging due to the poor LISA sky localization that, for the loudest signals ($\mathcal {M}_c\sim 10^6~{{\rm M}_{\odot }}$ at z = 6), is around 10 $\rm deg^2$.

Higgsino Dark Matter Confronts 14Years of Fermi γ-Ray Data.

Thermal Higgsino dark matter (DM), with mass around 1TeV, is a well-motivated, minimal DM scenario that arises in supersymmetric extensions of the standard model. Higgsinos may naturally be the lightest superpartners in split-supersymmetry models that decouple the scalar superpartners while keeping Higgsinos and gauginos close to the TeV scale. Higgsino DM may annihilate today to give continuum γ-ray emission at energies less than a TeV in addition to a linelike signature at energies equal to the mass. Previous searches for Higgsino DM, for example with the H.E.S.S. γ-ray telescope, have not reached the necessary sensitivity to probe the Higgsino annihilation cross section. In this work we make use of 14years of data from the Fermi Large Area Telescope at energies above ∼10 GeV to search for the continuum emission near the Galactic Center from Higgsino annihilation. We interpret our results using DM profiles from MilkyWay analog galaxies in the FIRE-2 hydrodynamic cosmological simulations. We set the strongest constraints to date on Higgsino-like DM. Our results show a mild, ∼2σ preference for Higgsino DM with a mass near the thermal Higgsino mass and, depending on the DM density profile, the expected cross section.

Hydrodynamical structure formation in Milgromian cosmology

ABSTRACT We present the first hydrodynamical cosmological simulations in the νHDM framework based on Milgromian dynamics (MOND) with light (11 eV) sterile neutrinos. νHDM can explain the expansion history, CMB anisotropies, and galaxy cluster dynamics similarly to standard cosmology while preserving MOND’s successes on galaxy scales, making this the most conservative Milgromian framework. We generate initial conditions including sterile neutrinos using camb and music and modify the publicly available code phantom of ramses to run νHDM models. The simulations start at redshift $z_e$ = 199, when the gravitational fields are stronger than $a_{_0}$ provided this does not vary. We analyse the growth of structure and investigate the impact of resolution and box size, which is at most 600 comoving Mpc. Large density contrasts arise at late times, which may explain the KBC void and Hubble tension. We quantify the mass function of formed structures at different redshifts. We show that the sterile neutrino mass fraction in these structures is similar to the cosmic fraction at high masses (consistent with MOND dynamical analyses) but approaches zero at lower masses, as expected for galaxies. We also identify structures with a low peculiar velocity comparable to the Local Group, but these are rare. The onset of group/cluster-scale structure formation at $z_e$ ≈ 4 appears to be in tension with observations of high redshift galaxies, which we discuss in comparison to prior analytical work in a MONDian framework. The formation of a cosmic web of filaments and voids demonstrates that this is not unique to standard Einstein/Newton-based cosmology.

Cosmological simulations of the same spiral galaxy: connecting the dark matter distribution of the host halo with the subgrid baryonic physics

The role of baryonic physics, star formation and stellar feedback, in shaping the galaxies and their host halos is an evolving topic. The dark matter aspects are illustrated in this work by showing distribution features in a Milky Way sized halo.We focus on the halo morphology, geometry, and profile as well as the phase space distribution using one dark matter only and five hydrodynamical cosmological high-resolution simulations of the same halo with different subgrid prescriptions for the baryonic physics (Kennicut versus multi-freefall star formation and delayed cooling versus mechanical supernovae feedback). If some general properties like the relative halo-galaxy orientation are similar, the modifications of the gravitational potential due to the presence of baryons are found to induce different dark matter distributions (rounder and more concentrated halo). The mass density profile as well as the velocity distribution are modified distinctively according to the specific resulting baryonic distribution highlighting the variability of those properties (e.g inner power index from 1.3 to 1.8, broader speed distribution). The uncertainties on those features are of paramount importance for dark matter phenomenology, particularly when dealing with dark matter dynamics or direct and indirect detection searches. As a consequence, dark matter properties and prospects using cosmological simulations require improvement on baryonic physics description. Modeling such processes is a key issue not only for galaxy formation but also for dark matter investigations.

Formation of proto-globular cluster candidates in cosmological simulations of dwarf galaxies at z > 4

ABSTRACT We perform cosmological hydrodynamical simulations to study the formation of proto-globular cluster candidates in progenitors of present-day dwarf galaxies $(M_{\rm vir} \approx 10^{10}\, {\rm M}_\odot$ at z = 0) as part of the ‘Feedback in Realistic Environment’ (FIRE) project. Compact (r1/2 < 30 pc), relatively massive (0.5 × 105 ≲ M⋆/M⊙ ≲ 5 × 105), self-bound stellar clusters form at 11 ≳ z ≳ 5 in progenitors with $M_{\rm vir} \approx 10^9\, \mathrm{M}_{\odot }$. Cluster formation is triggered when at least $10^7\, \mathrm{M}_{\odot }$ of dense, turbulent gas reaches $\Sigma _{\rm gas} \approx 10^4\, {\rm M}_\odot \, {\rm pc}^{-2}$ as a result of the compressive effects of supernova feedback or from cloud–cloud collisions. The clusters can survive for $2-3\, {\rm Gyr}$; absent numerical effects, they could possibly survive substantially longer, perhaps to z = 0. The longest lived clusters are those that form at significant distance – several hundreds of pc – from their host galaxy. We therefore predict that globular clusters forming in progenitors of present-day dwarf galaxies will be offset from any pre-existing stars within their host dark matter haloes as opposed to deeply embedded within a well-defined galaxy. Properties of the nascent clusters are consistent with observations of some of the faintest and most compact high-redshift sources in Hubble Space Telescope lensing fields and are at the edge of what will be detectable as point sources in deep imaging of non-lensed fields with JWST. By contrast, the star clusters’ host galaxies will remain undetectable.

Origin and evolution of ultradiffuse galaxies in different environments

ABSTRACT We study the formation of ultradiffuse galaxies (UDGs) using the cosmological hydrodynamical simulation TNG50 of the Illustris-TNG suite. We define UDGs as dwarf galaxies in the stellar mass range $\rm {7.5 \le log (M_{\star } / {\rm M}_{\odot }) \le 9 }$ that are in the 5 per cent most extended tail of the simulated mass–size relation. This results in a sample of UDGs with half-mass radii $\rm {r_{h \star } \gtrsim 2 \ kpc}$ and surface brightness between $\rm {24.5}$ and $\rm {28 \ mag \ arcsec^{-2}}$, similar to definitions of UDGs in observations. The large cosmological volume in TNG50 allows for a comparison of UDGs properties in different environments, from the field to galaxy clusters with virial mass $\rm {M_{200} \sim 2 \times 10^{14} ~ {\rm M}_{\odot }}$. All UDGs in our sample have dwarf-mass haloes ($\rm {M_{200}\sim 10^{11} ~ {\rm M}_{\odot } }$) and show the same environmental trends as normal dwarfs: field UDGs are star-forming and blue while satellite UDGs are typically quiescent and red. The TNG50 simulation predicts UDGs that populate preferentially higher spin haloes and more massive haloes at fixed $\rm {M_{\star }}$ compared to non-UDG dwarfs. This applies also to most satellite UDGs, which are actually ‘born’ UDGs in the field and infall into groups and clusters without significant changes to their size. We find, however, a small subset of satellite UDGs ($\lesssim 10~{{\ \rm per\ cent}}$) with present-day stellar size a factor ≥1.5 larger than at infall, confirming that tidal effects, particularly in the lower mass dwarfs, are also a viable formation mechanism for some of these dwarfs, although sub-dominant in this simulation.

The CAMELS Project: Public Data Release

The Cosmology and Astrophysics with Machine Learning Simulations (CAMELS) project was developed to combine cosmology with astrophysics through thousands of cosmological hydrodynamic simulations and machine learning. CAMELS contains 4233 cosmological simulations, 2049 N-body simulations, and 2184 state-of-the-art hydrodynamic simulations that sample a vast volume in parameter space. In this paper, we present the CAMELS public data release, describing the characteristics of the CAMELS simulations and a variety of data products generated from them, including halo, subhalo, galaxy, and void catalogs, power spectra, bispectra, Lyα spectra, probability distribution functions, halo radial profiles, and X-rays photon lists. We also release over 1000 catalogs that contain billions of galaxies from CAMELS-SAM: a large collection of N-body simulations that have been combined with the Santa Cruz semianalytic model. We release all the data, comprising more than 350 terabytes and containing 143,922 snapshots, millions of halos, galaxies, and summary statistics. We provide further technical details on how to access, download, read, and process the data at https://camels.readthedocs.io.

Origin of the differences in rotational support among early-type galaxies: The case of galaxies outside clusters

Context.Early-type galaxies (ETGs) are divided into slow and fast rotators (FRs and SRs) according to the degree of ordered rotation of their stellar populations. Cosmological hydrodynamical simulations indicate that galaxies form as FRs before their rotational support decreases, usually because of mergers.Aims.We aimed to investigate this process observationally for galaxies outside of clusters.Methods.We made use of the fact that different merger types leave different traces that have different lifetimes. We statistically analyzed multiple characteristics of galaxies that are expected to be influenced by mergers, such as tidal features, kinematically distinct cores, and stellar ages. They were taken from the MATLAS and ATLAS3Ddatabases. Through multilinear regression we identified the quantities that, at a fixed mass and environmental density of the galaxy, significantly correlate with a measure of the ordered rotation of the galaxy,λReN.Results.We found a negative correlation of the rotational support with the occurrence of tidal disturbances and kinematic substructures, and a positive correlation with metallicity and metallicity gradients. For massive galaxies, the rotational support correlates negatively with the abundance ofα-elements, and for the galaxies in low-density environments, it correlates negatively with the central photometric cuspiness. These and additional literature observational constraints are explained the easiest if the mergers that decreased the rotational support of ETGs were typically minor, wet, and happening atz ≈ 2. They did not form the currently observed tidal features. The observed frequency of tidal features implies a merging rate of 0.07–0.2 per Gyr. This is insufficient to explain the observed growth of the radii of ETGs with redshift by mergers.

The many reasons that the rotation curves of low-mass galaxies can fail as tracers of their matter distributions

ABSTRACT It is routinely assumed that galaxy rotation curves are equal to their circular velocity curves (modulo some corrections) such that they are good dynamical mass tracers. We take a visualization-driven approach to exploring the limits of the validity of this assumption for a sample of 33 low-mass galaxies ($60\lt v_\mathrm{max}/\mathrm{km}\, \mathrm{s}^{-1}\lt 120$ ) from the APOSTLE suite of cosmological hydrodynamical simulations. Only three of these have rotation curves nearly equal to their circular velocity curves at z = 0, the rest are undergoing a wide variety of dynamical perturbations. We use our visualizations to guide an assessment of how many galaxies are likely to be strongly perturbed by processes in several categories: mergers/interactions (affecting 6/33 galaxies), bulk radial gas inflows (19/33), vertical gas outflows (15/33), distortions driven by a non-spherical DM halo (17/33), warps (8/33), and winds due to motion through the intergalactic medium (5/33). Most galaxies fall into more than one of these categories; only 5/33 are not in any of them. The sum of these effects leads to an underestimation of the low-velocity slope of the baryonic Tully–Fisher relation (α ∼ 3.1 instead of α ∼ 3.9, where Mbar ∝ vα) that is difficult to avoid, and could plausibly be the source of a significant portion of the observed diversity in low-mass galaxy rotation curve shapes.

The atomic-to-molecular hydrogen transition in the TNG50 simulation: Using realistic UV fields to create spatially resolved H i maps

ABSTRACT Cold gas in galaxies provides a crucial test to evaluate the realism of cosmological hydrodynamical simulations. To extract the atomic and molecular hydrogen properties of the simulated galaxy population, post-processing methods taking the local UV field into account are required. We improve upon previous studies by calculating realistic UV fields with the dust radiative transfer code SKIRT to model the atomic-to-molecular transition in TNG50, the highest-resolution run of the IllustrisTNG suite. Comparing integrated quantities such as the H i mass function, we study to what detail the UV field needs to be modelled in order to calculate realistic cold gas properties. We then evaluate new, spatially resolved comparisons for cold gas in galaxies by exploring synthetic maps of atomic hydrogen at redshift zero and compare them to 21-cm observations of local galaxies from the WHISP survey. In terms of non-parametric morphologies, we find that TNG50 H i maps are less concentrated than their WHISP counterparts (median ΔC ≈ 0.3), due in part to central H i deficits related to the ejective character of supermassive black hole feedback in TNG. In terms of the H i column density distribution function, we find discrepancies between WHISP and IllustrisTNG that depend on the total H i abundance in these data sets as well as the post-processing method. To fully exploit the synergy between cosmological simulations and upcoming deep H i/H2 data, we advocate the use of accurate methods to estimate the UV radiation field and to generate mock maps.

Efficient Long-range Active Galactic Nuclei (AGNs) Feedback Affects the Low-redshift Lyα Forest

Active galactic nuclei (AGNs) feedback models are generally calibrated to reproduce galaxy observables such as the stellar mass function and the bimodality in galaxy colors. We use variations of the AGN feedback implementations in the IllustrisTNG (TNG) and Simba cosmological hydrodynamic simulations to show that the low-redshift Lyα forest can provide constraints on the impact of AGN feedback. We show that TNG overpredicts the number density of absorbers at column densities N HI < 1014 cm−2 compared to data from the Cosmic Origins Spectrograph (in agreement with previous work), and we demonstrate explicitly that its kinetic feedback mode, which is primarily responsible for galaxy quenching, has a negligible impact on the column density distribution (CDD) of absorbers. In contrast, we show that the fiducial Simba model, which includes AGN jet feedback, is the preferred fit to the observed CDD of the z = 0.1 Lyα forest across 5 orders of magnitude in column density. We show that the Simba results with jets produce a quantitatively better fit to the observational data than the Simba results without jets, even when the ultraviolet background is left as a free parameter. AGN jets in Simba are high speed, collimated, weakly interacting with the interstellar medium (via brief hydrodynamic decoupling), and heated to the halo virial temperature. Collectively these properties result in stronger long-range impacts on the intergalactic medium when compared to TNG’s kinetic feedback mode, which drives isotropic winds with lower velocities at the galactic radius. Our results suggest that the low-redshift Lyα forest provides plausible evidence for long-range AGN jet feedback.

The halo bispectrum as a sensitive probe of massive neutrinos and baryon physics

ABSTRACT The power spectrum has been a workhorse for cosmological studies of large-scale structure. However, the present-day matter distribution is highly non-Gaussian and significant cosmological information is also contained in higher order correlation functions. Meanwhile, baryon physics (particularly active galactic nucleus feedback) has previously been shown to strongly affect the two-point statistics but there has been limited exploration of its effects on higher order functions to date. Here, we use the bahamas suite of cosmological hydrodynamical simulations to explore the effects of baryon physics and massive neutrinos on the halo bispectrum. In contrast to matter clustering which is suppressed by baryon physics, we find that the halo clustering is typically enhanced. The strength of the effect and the scale over which it extends depends on how haloes are selected. On small scales ($k \gtrsim 1$ h Mpc−1, dominated by satellites of groups/clusters), we find that the bispectrum is highly sensitive to the efficiency of star formation and feedback, making it an excellent testing ground for galaxy formation models. We show that the effects of feedback and the effects of massive neutrinos are largely separable (independent of each other) and that massive neutrinos strongly suppress the halo bispectrum on virtually all scales up to the free-streaming length (apart from the smallest scales, where baryon physics dominates). The strong sensitivity of the bispectrum to neutrinos on the largest scales and galaxy formation physics on the smallest scales bodes well for upcoming precision measurements from the next generation of wide-field surveys.

The diversity of rotation curves of simulated galaxies with cusps and cores

ABSTRACTWe use ΛCDM cosmological hydrodynamical simulations to explore the kinematics of gaseous discs in late-type dwarf galaxies. We create high-resolution 21-cm ‘observations’ of simulated dwarfs produced in two variations of the EAGLE galaxy formation model: one where supernova-driven gas flows redistribute dark matter and form constant-density central ‘cores’, and another where the central ‘cusps’ survive intact. We ‘observe’ each galaxy along multiple sightlines and derive a rotation curve for each observation using a conventional tilted-ring approach to model the gas kinematics. We find that the modelling process introduces systematic discrepancies between the recovered rotation curve and the actual circular velocity curve driven primarily by (i) non-circular gas orbits within the discs; (ii) the finite thickness of gaseous discs, which leads to overlap of different radii in projection; and (iii) departures from dynamical equilibrium. Dwarfs with dark matter cusps often appear to have a core, whilst the inverse error is less common. These effects naturally reproduce an observed trend which other models struggle to explain: late-type dwarfs with more steeply rising rotation curves appear to be dark matter-dominated in the inner regions, whereas the opposite seems to hold in galaxies with core-like rotation curves. We conclude that if similar effects affect the rotation curves of observed dwarfs, a late-type dwarf population in which all galaxies have sizeable dark matter cores is most likely incompatible with current measurements.

Characterizing the intracluster light over the redshift range 0.2 &amp;lt; z &amp;lt; 0.8 in the DES-ACT overlap

ABSTRACT We characterize the properties and evolution of bright central galaxies (BCGs) and the surrounding intracluster light (ICL) in galaxy clusters identified in the Dark Energy Survey and Atacama Cosmology Telescope Survey (DES-ACT) overlapping regions, covering the redshift range 0.20 &amp;lt; z &amp;lt; 0.80. Over this redshift range, we measure no change in the ICL’s stellar content (between 50 and 300 kpc) in clusters with log10(M200m,SZ/M⊙) &amp;gt;14.4. We also measure the stellar mass–halo mass (SMHM) relation for the BCG+ICL system and find that the slope, β, which characterizes the dependence of M200m,SZ on the BCG+ICL stellar mass, increases with radius. The outskirts are more strongly correlated with the halo than the core, which supports that the BCG+ICL system follows a two-phase growth, where recent growth (z &amp;lt; 2) occurs beyond the BCG’s core. Additionally, we compare our observed SMHM relation results to the IllustrisTNG300-1 cosmological hydrodynamic simulations and find moderate qualitative agreement in the amount of diffuse light. However, the SMHM relation’s slope is steeper in TNG300-1 and the intrinsic scatter is lower, likely from the absence of projection effects in TNG300-1. Additionally, we find that the ICL exhibits a colour gradient such that the outskirts are bluer than the core. Moreover, for the lower halo mass clusters (log10(M200m,SZ/M⊙) &amp;lt; 14.59), we detect a modest change in the colour gradient’s slope with lookback time, which combined with the absence of stellar mass growth may suggest that lower mass clusters have been involved in growth via tidal stripping more recently than their higher mass counterparts.

VINTERGATAN-GM: The cosmological imprints of early mergers on Milky-Way-mass galaxies

ABSTRACTWe present a new suite of cosmological zoom-in hydrodynamical ($\approx 20\, \mathrm{pc}$ spatial resolution) simulations of Milky-Way mass galaxies to study how a varying mass ratio for a Gaia-Sausage-Enceladus (GSE) progenitor impacts the z = 0 chemodynamics of halo stars. Using the genetic modification approach, we create five cosmological histories for a Milky-Way-mass dark matter halo ($M_{200}\approx 10^{12} \, \mbox{M}_\mathrm{\odot }$), incrementally increasing the stellar mass ratio of a z ≈ 2 merger from 1:25 to 1:2, while fixing the galaxy’s final dynamical, stellar mass, and large-scale environment. We find markedly different morphologies at z = 0 following this change in early history, with a growing merger resulting in increasingly compact and bulge-dominated galaxies. Despite this structural diversity, all galaxies show a radially biased population of inner halo stars like the Milky-Way’s GSE which, surprisingly, has a similar magnitude, age, $\rm [Fe/H]$, and $\rm [\alpha /Fe]$ distribution whether the z ≈ 2 merger is more minor or major. This arises because a smaller ex-situ population at z ≈ 2 is compensated by a larger population formed in an earlier merger-driven starburst whose contribution to the GES can grow dynamically over time, and with both populations strongly overlapping in the $\rm [Fe/H]-\rm [\alpha /Fe]$ plane. Our study demonstrates that multiple high-redshift histories can lead to similar z = 0 chemodynamical features in the halo, highlighting the need for additional constraints to distinguish them, and the importance of considering the full spectrum of progenitors when interpreting z = 0 data to reconstruct our Galaxy’s past.

On the edge: the relation between stellar and dark matter haloes of Milky Way-mass galaxies

ABSTRACTWe investigate the build-up of the accreted stellar and dark matter haloes of Milky Way-like galaxies in APOSTLE suite of cosmological hydrodynamics simulations. We show that the stellar halo is made up primarily of stars stripped from a small number of massive dwarfs, most of which are disrupted by the present day. The dark matter halo, on the other hand, is made up primarily of small unresolved subhaloes (≲106 M⊙) and a ‘smooth’ component consisting of particles which were never bound to a subhalo. Despite these differences, the massive dwarfs that make up the majority of the stellar halo also contribute a significant fraction of the dark matter. The stars and dark matter stripped from these dwarfs are related through their kinematics and this leaves imprints in the phase-space structure of the haloes. We examine the relation between the location of features, such as caustics, in the phase space of the stars and dark halo properties. We show that the ‘edge’ of the stellar halo is a probe of dark matter halo mass and assembly history. The edges of Milky Way-mass galaxies should be visible at a surface brightness of 31–36 mag arcsec−2.

EAGLE-like simulation models do not solve the entropy core problem in groups and clusters of galaxies

ABSTRACTRecent high-resolution cosmological hydrodynamic simulations run with a variety of codes systematically predict large amounts of entropy in the intra-cluster medium at low redshift, leading to flat entropy profiles and a suppressed cool-core population. This prediction is at odds with X-ray observations of groups and clusters. We use a new implementation of the EAGLE galaxy formation model to investigate the sensitivity of the central entropy and the shape of the profiles to changes in the sub-grid model applied to a suite of zoom-in cosmological simulations of a group of mass M500 = 8.8 × 1012 M⊙ and a cluster of mass 2.9 × 1014 M⊙. Using our reference model, calibrated to match the stellar mass function of field galaxies, we confirm that our simulated groups and clusters contain hot gas with too high entropy in their cores. Additional simulations run without artificial conduction, metal cooling or active galactic nuclei (AGN) feedback produce lower entropy levels but still fail to reproduce observed profiles. Conversely, the two objects run without supernova feedback show a significant entropy increase which can be attributed to excessive cooling and star formation. Varying the AGN heating temperature does not greatly affect the profile shape, but only the overall normalization. Finally, we compared runs with four AGN heating schemes and obtained similar profiles, with the exception of bipolar AGN heating, which produces a higher and more uniform entropy distribution. Our study leaves open the question of whether the entropy core problem in simulations, and particularly the lack of power-law cool-core profiles, arise from incorrect physical assumptions, missing physical processes, or insufficient numerical resolution.

The Fundamental Signature of Star Formation Quenching from AGN Feedback: A Critical Dependence of Quiescence on Supermassive Black Hole Mass, Not Accretion Rate

We identify the intrinsic dependence of star formation quenching on a variety of galactic and environmental parameters, utilizing a machine-learning approach with Random Forest classification. We have previously demonstrated the power of this technique to isolate causality, not mere correlation, in complex astronomical data. First, we analyze three cosmological hydrodynamical simulations (Eagle, Illustris, and IllustrisTNG), selecting snapshots spanning the bulk of cosmic history from comic noon (z ∼ 2) to the present epoch, with stellar masses in the range . In the simulations, black hole mass is unanimously found to be the most predictive parameter of central galaxy quenching at all epochs. Perhaps surprisingly, black hole accretion rate (and hence the bolometric luminosity of active galactic nuclei, AGN) is found to be of little predictive power over quenching. This theoretical result is important for observational studies of galaxy quenching, as it cautions against using the current AGN state of a galaxy as a useful proxy for the cumulative impact of AGN feedback on a galactic system. The latter is traced by black hole mass, not AGN luminosity. Additionally, we explore a subset of “observable” parameters, which can be readily measured in extant wide-field galaxy surveys targeting z = 0–2, at . All three simulations predict that, in lieu of black hole mass, the stellar gravitational potential will outperform the other parameters in predicting quenching. We confirm this theoretical prediction observationally in the SDSS (at low redshifts) and in CANDELS (at intermediate and high redshifts).