[en] We present an analysis of the effects of dissipational baryonic physics on the local dark matter (DM) distribution at the location of the Sun, with an emphasis on the consequences for direct detection experiments. Our work is based on a comparative analysis of two cosmological simulations with identical initial conditions of a Milky Way halo, one of which (Eris) is a full hydrodynamic simulation and the other (ErisDark) is a DM-only one. We find that in Eris two distinct processes lead to a 30% enhancement of DM in the disk plane at the location of the Sun: the accretion and disruption of satellites resulting in a DM component with net angular momentum, and the contraction of baryons pulling the DM into the disk plane without forcing it to co-rotate. Owing to its particularly quiescent merger history for dark halos of Milky Way mass, the co-rotating dark disk in Eris is less massive than what has been suggested by previous work, contributing only 9% of the local DM density. Yet, since the simulation results in a realistic Milky Way analog galaxy, its DM halo provides a plausible alternative to the Maxwellian standard halo model (SHM) commonly used in direct detection analyses. The speed distribution in Eris is broadened and shifted to higher speeds, compared to its DM-only twin simulation ErisDark. At high speeds f(v) falls more steeply in Eris than in ErisDark or the SHM, easing the tension between recent results from the CDMS-II and XENON100 experiments. The non-Maxwellian aspects of f(v) are still present, but much less pronounced in Eris than in the DM-only runs. The weak dark disk increases the time-averaged scattering rate by only a few percent at low recoil energies. On the high velocity tail, however, the increase in typical speeds due to baryonic contraction results in strongly enhanced mean scattering rates compared to ErisDark, although they are still suppressed compared to the SHM. Similar trends are seen regarding the amplitude of the annual modulation, while the modulated fraction is increased compared to the SHM and decreased compared to ErisDark.

[en] We analyze the formation and evolution of the stellar components in ''Eris'', a 120 pc resolution cosmological hydrodynamic simulation of a late-type spiral galaxy. The simulation includes the effects of a uniform UV background, a delayed-radiative-cooling scheme for supernova feedback, and a star formation recipe based on a high gas density threshold. It allows a detailed study of the relative contributions of ''in-situ'' (within the main host) and ''ex-situ'' (within satellite galaxies) star formation to each major Galactic component in a close Milky Way analog. We investigate these two star-formation channels as a function of galactocentric distance, along different lines of sight above and along the disk plane, and as a function of cosmic time. We find that: (1) approximately 70% of today's stars formed in-situ; (2) more than two thirds of the ex-situ stars formed within satellites after infall; (3) the majority of ex-situ stars are found today in the disk and in the bulge; (4) the stellar halo is dominated by ex-situ stars, whereas in-situ stars dominate the mass profile at distances ≲ 5 kpc from the center at high latitudes; and (5) approximately 25% of the inner, r ≲ 20 kpc, halo is composed of in-situ stars that have been displaced from their original birth sites during Eris' early assembly history

[en] We study the impact that large-scale perturbations of (i) the matter density and (ii) the primordial gravitational potential with local primordial non-Gaussianity (PNG) have on galaxy formation using the IllustrisTNG model. We focus on the linear galaxy bias $b_1$ and the coefficient $b_{\mathrm{\varphi <!--\; ??\; -->}}$ of the scale-dependent bias induced by PNG, which describe the response of galaxy number counts to these two types of perturbations, respectively. We perform our study using separate universe simulations, in which the effect of the perturbations is mimicked by changes to the cosmological parameters: modified cosmic matter density for $b_1$ and modified amplitude ${\mathcal{A}}_s$ of the primordial scalar power spectrum for $b_{\mathrm{\varphi <!--\; ??\; -->}}$. We find that the widely used universality relation $b_{\mathrm{\varphi <!--\; ??\; -->}}=2{\mathrm{\delta <!--\; ??\; -->}}_c(b_1-<!--\; ???\; -->1)$ is a poor description of the bias of haloes and galaxies selected by stellar mass $M_{\ast <!--\; ???\; -->}$, which is instead described better by $b_{\mathrm{\varphi <!--\; ??\; -->}}(M_{\ast <!--\; ???\; -->})=2{\mathrm{\delta <!--\; ??\; -->}}_c(b_1(M_{\ast <!--\; ???\; -->})-<!--\; ???\; -->p)$ with $p\in <!--\; ???\; -->[0.4,0.7]$. This is explained by the different impact that matter overdensities and local PNG have on the median stellar-to-halo-mass relation. A simple model of this impact allows us to describe the stellar mass dependence of $b_1$ and $b_{\mathrm{\varphi <!--\; ??\; -->}}$ fairly well. Our results also show a nontrivial relation between $b_1$ and $b_{\mathrm{\varphi <!--\; ??\; -->}}$ for galaxies selected by color and black hole mass accretion rate. Our results provide refined priors on $b_{\mathrm{\varphi <!--\; ??\; -->}}$ for local PNG constraints and forecasts using galaxy clustering. Given that the widely used universality relation underpredicts $b_{\mathrm{\varphi <!--\; ??\; -->}}(M_{\ast <!--\; ???\; -->})$, existing analyses may underestimate the true constraining power on local PNG.

[en] The thermal Sunyaev–Zel’dovich (tSZ) and the kinematic Sunyaev–Zel’dovich (kSZ) effects trace the distribution of electron pressure and momentum in the hot universe. These observables depend on rich multiscale physics, thus, simulated maps should ideally be based on calculations that capture baryonic feedback effects such as cooling, star formation, and other complex processes. In this paper, we train deep convolutional neural networks with a U-Net architecture to map from the three-dimensional distribution of dark matter to electron density, momentum, and pressure at ∼100 kpc resolution. These networks are trained on a combination of the TNG300 volume and a set of cluster zoom-in simulations from the IllustrisTNG project. The neural nets are able to reproduce the power spectrum, one-point probability distribution function, bispectrum, and cross-correlation coefficients of the simulations more accurately than the state-of-the-art semianalytical models. Our approach offers a route to capture the richness of a full cosmological hydrodynamical simulation of galaxy formation with the speed of an analytical calculation.

[en] We discuss a novel approach to ''weighing'' the Milky Way (MW) dark matter halo, one that combines the latest samples of halo stars selected from the Sloan Digital Sky Survey (SDSS) with state of the art numerical simulations of MW analogs. The fully cosmological runs employed in the present study include ''Eris'', one of the highest resolution hydrodynamical simulations of the formation of a M_vir = 8 × 10¹¹ M_☉ late-type spiral, and the dark-matter-only M_vir = 1.7 × 10¹² M_☉ ''Via Lactea II'' (VLII) simulation. Eris provides an excellent laboratory for creating mock SDSS samples of tracer halo stars, and we successfully compare their density, velocity anisotropy, and radial velocity dispersion profiles with the observational data. Most mock SDSS realizations show the same ''cold veil'' recently observed in the distant stellar halo of the MW, with tracers as cold as σ_los ≈ 50 km s^–1 between 100 and 150 kpc. Controlled experiments based on the integration of the spherical Jeans equation as well as a particle tagging technique applied to VLII show that a ''heavy'' M_vir ≈ 2 × 10¹² M_☉ realistic host produces a poor fit to the kinematic SDSS data. We argue that these results offer added evidence for a ''light'', centrally concentrated MW halo

[en] We show that the position of the central dark matter (DM) density peak may be expected to differ from the dynamical center of the Galaxy by several hundred parsecs. In Eris, a high-resolution cosmological hydrodynamics simulation of a realistic Milky-Way-analog disk galaxy, this offset is 300-400 pc (∼3 gravitational softening lengths) after z = 1. In its dissipationless DM-only twin simulation ErisDark, as well as in the Via Lactea II and GHalo simulations, the offset remains below one softening length for most of its evolution. The growth of the DM offset coincides with a flattening of the central DM density profile in Eris inward of ∼1 kpc, and the direction from the dynamical center to the point of maximum DM density is correlated with the orientation of the stellar bar, suggesting a bar-halo interaction as a possible explanation. A DM density offset of several hundred parsecs greatly affects expectations of the DM annihilation signals from the Galactic center. It may also support a DM annihilation interpretation of recent reports by Weniger and Su and Finkbeiner of highly significant 130 GeV gamma-ray line emission from a region 1.°5 (∼200 pc projected) away from Sgr A* in the Galactic plane.

[en] To break the degeneracy among galactic stellar components, we extract kinematic structures using the framework that was described in Du et al. For example, the concept of stellar halos is generalized to weakly rotating structures that are composed of loosely bound stars, which can hence be associated to both disk and elliptical type morphologies. By applying this method to central galaxies with stellar mass 10^10−11.5 M _⊙ from the TNG50 simulation, we identify three broadly-defined types of galaxies: galaxies dominated by disk, by bulge, or by stellar halo structures. We then use the simulation to infer the underlying connection between the growth of structures and physical processes over cosmic time. By tracing galaxies back in time, we recognize three fundamental regimes: an early phase of evolution (z ≳ 2), and internal and external (mainly mergers) processes that act at later times. We find that disk- and bulge-dominated galaxies are not significantly affected by mergers since z ∼ 2. The difference in their present-day structures originates from two distinct evolutionary pathways—extended versus compact—that are likely to be determined by their parent dark matter halos (i.e., nature). In contrast, slow rotator elliptical galaxies are typically halo-dominated, forming by external processes (e.g., mergers) in the later phase (i.e., nurture). This picture challenges the general idea that elliptical galaxies are the same objects as classical bulges. In observations, both bulge- and halo-dominated galaxies are likely to be classified as early-type galaxies with compact morphology and quiescent star formation. However, here we find them to have very different evolutionary histories.

[en] Using the TNG100 (100 Mpc)³ simulation of the IllustrisTNG project, we demonstrate a strong connection between the onset of star formation quenching and the stellar size of galaxies. We do so by tracking the evolutionary history of extended and normal-size galaxies selected at z = 2 with $\mathrm{l}\mathrm{o}\mathrm{g}(M_{\ast <!--\; ???\; -->}/M_{\odot <!--\; ???\; -->})$ $=10.2\text{--}11$ and stellar-half-mass-radii above and within 1σ of the stellar size–stellar mass relation, respectively. We match the stellar mass and star formation rate distributions of the two populations. By z = 1, only 36% of the extended massive galaxies have quenched, in contrast to a quenched fraction of 69% for the normal-size massive galaxies. We find that normal-size massive galaxies build up their central stellar mass without a significant increase in their stellar size between $z=2\text{--}4$, whereas the stellar size of the extended massive galaxies almost doubles in the same time. In IllustrisTNG, lower black hole masses and weaker kinetic-mode feedback appears to be responsible for the delayed quenching of star formation in the extended massive galaxies. We show that relatively gas-poor mergers may be responsible for the lower central stellar density and weaker supermassive black hole feedback in the extended massive galaxies.

[en] We present an analysis of the angular momentum content of the circumgalactic medium (CGM) using TNG100, one of the flagship runs of the IllustrisTNG project. We focus on Milky Way–mass halos (∼10¹² M _⊙) at z = 0 but also analyze other masses and redshifts up to z = 5. We find that the CGM angular momentum properties are strongly correlated with the stellar angular momentum of the corresponding galaxy: the CGM surrounding high-angular momentum galaxies has a systematically higher angular momentum and is better aligned to the rotational axis of the galaxy itself than the CGM surrounding low-angular momentum galaxies. Both the hot and cold phases of the CGM show this dichotomy, though it is stronger for colder gas. The CGM of high-angular momentum galaxies is characterized by a large wedge of cold gas with rotational velocities at least ∼1/2 of the halo’s virial velocity, extending out to ∼1/2 of the virial radius, and by biconical polar regions dominated by radial velocities suggestive of galactic fountains; both of these features are absent from the CGM of low-angular momentum galaxies. These conclusions are general to halo masses ≲10¹² M _⊙ and for z ≲ 2, but they do not apply for more massive halos or at the highest redshift studied. By comparing simulations run with alterations to the fiducial feedback model, we identify the better alignment of the CGM to high-angular momentum galaxies as a feedback-independent effect and the galactic winds as a dominant influence on the CGM’s angular momentum.

[en] We use ZFIRE and ZFOURGE observations with the spectral energy distribution fitting tool PROSPECTOR to reconstruct the star formation histories (SFHs) of protocluster and field galaxies at z ∼ 2 and compare our results to the TNG100 run of the IllustrisTNG cosmological simulation suite. In the observations, we find that massive protocluster galaxies ($\mathrm{l}\mathrm{o}\mathrm{g}[M_{\ast <!--\; ???\; -->}/M_{\odot <!--\; ???\; -->}]$ > 10.5) form 45% ± 8% of their total stellar mass in the first 2 Gyr of the universe, compared to 31% ± 2% formed in the field galaxies. In both observations and simulations, massive protocluster galaxies have a flat/declining SFH with decreasing redshift compared to rising SFH in their field counterparts. Using IllustrisTNG, we find that massive galaxies ($\mathrm{l}\mathrm{o}\mathrm{g}[M_{\ast <!--\; ???\; -->}/M_{\odot <!--\; ???\; -->}]⩾<!--\; ???\; -->10.5$) in both environments are on average ≈190 Myr older than low-mass galaxies ($\mathrm{l}\mathrm{o}\mathrm{g}[M_{\ast <!--\; ???\; -->}/M_{\odot <!--\; ???\; -->}]=9\text{--}9.5$). However, the difference in mean stellar ages of cluster and field galaxies is minimal when considering the full range in stellar mass ($\mathrm{l}\mathrm{o}\mathrm{g}[M_{\ast <!--\; ???\; -->}/M_{\odot <!--\; ???\; -->}]⩾<!--\; ???\; -->9$). We explore the role of mergers in driving the SFH in IllustrisTNG and find that massive cluster galaxies consistently experience mergers with low gas fraction compared to other galaxies after 1 Gyr from the big bang. We hypothesize that the low gas fraction in the progenitors of massive cluster galaxies is responsible for the reduced star formation.