Modeling galaxy formation in a cosmological context presents one of the greatest challenges in astrophysics today due to the vast range of scales and numerous physical processes involved. Here we ...review the current status of models that employ two leading techniques to simulate the physics of galaxy formation: semianalytic models and numerical hydrodynamic simulations. We focus on a set of observational targets that describe the evolution of the global and structural properties of galaxies from roughly cosmic high noon (
z
∼ 2-3) to the present. Although minor discrepancies remain, overall, models show remarkable convergence among different methods and make predictions that are in qualitative agreement with observations. Modelers have converged on a core set of physical processes that are critical for shaping galaxy properties. This core set includes cosmological accretion, strong stellar-driven winds that are more efficient at low masses, black hole feedback that preferentially suppresses star formation at high masses, and structural and morphological evolution through merging and environmental processes. However, all cosmological models currently adopt phenomenological implementations of many of these core processes, which must be tuned to observations. Many details of how these diverse processes interact within a hierarchical structure formation setting remain poorly understood. Emerging multiscale simulations are helping to bridge the gap between stellar and cosmological scales, placing models on a firmer, more physically grounded footing. Concurrently, upcoming telescope facilities will provide new challenges and constraints for models, particularly by directly constraining inflows and outflows through observations of gas in and around galaxies.
ABSTRACT
We present predictions for the evolution of the galaxy dust-to-gas ratio (DGR) and dust-to-metal ratio (DTM) from z = 0 → 6, using a model for the production, growth, and destruction of dust ...grains implemented into the simba cosmological hydrodynamic galaxy formation simulation. In our model, dust forms in stellar ejecta, grows by the accretion of metals, and is destroyed by thermal sputtering and supernovae. Our simulation reproduces the observed dust mass function at z = 0, but modestly underpredicts the mass function by ∼×3 at z ∼ 1–2. The z = 0 DGR versus metallicity relationship shows a tight positive correlation for star-forming galaxies, while it is uncorrelated for quenched systems. There is little evolution in the DGR–metallicity relationship between z = 0 and 6. We use machine learning techniques to search for the galaxy physical properties that best correlate with the DGR and DTM. We find that the DGR is primarily correlated with the gas-phase metallicity, though correlations with the depletion time-scale, stellar mass, and gas fraction are non-negligible. We provide a crude fitting relationship for DGR and DTM versus the gas-phase metallicity, along with a public code package that estimates the DGR and DTM given a set of galaxy physical properties.
The evolution of the galaxy stellar mass–star formation rate relationship (M*–SFR) provides key constraints on the stellar mass assembly histories of galaxies. For star-forming galaxies, M*–SFR is ...observed to be fairly tight with a slope close to unity from z∼ 0 → 2, and it evolves downwards roughly independently of M*. Simulations of galaxy formation reproduce these trends, broadly independent of modelling details, owing to the generic dominance of smooth and steady cold accretion in these systems. In contrast, the observed amplitude of the M*–SFR relation evolves markedly differently than in models, indicating either that stellar mass assembly is poorly understood or that observations have been misinterpreted. Stated in terms of a star formation activity parameter αsf≡ (M*/SFR)/(tHubble–1 Gyr), models predict a constant αsf∼ 1 out to redshifts z∼ 4+, while the observed M*–SFR relation indicates that αsf increases by approximately three times from z∼ 2 until today. The low αsf (i.e. rapid star formation) at high z not only conflicts with models, but also difficult to reconcile with other observations of high-z galaxies, such as the small scatter in M*–SFR, the slow evolution of star-forming galaxies at z∼ 2–4 and the modest passive fractions in mass-selected samples. Systematic biases could significantly affect measurements of M* and SFR, but detailed considerations suggest that none are obvious candidates to reconcile the discrepancy. A speculative solution is considered in which the stellar initial mass function (IMF) evolves towards more high-mass star formation at earlier epochs. Following Larson, a model is investigated in which the characteristic mass where the IMF turns over increases with redshift. Population synthesis models are used to show that the observed and predicted M*–SFR evolution may be brought into broad agreement if out to z∼ 2. Such IMF evolution matches recent observations of cosmic stellar mass growth, and the resulting z= 0 cumulative IMF is similar to the ‘paunchy’ IMF favoured by Fardal et al. to reconcile the observed cosmic star formation history with present-day fossil light measures.
Using cosmological hydrodynamic simulations that dynamically incorporate enriched galactic outflows together with analytical modelling, we study the origin of the stellar mass–gas-phase metallicity ...relation (MZR). We find that metallicities are driven by an equilibrium between the rate of enrichment owing to star formation and the rate of dilution owing to infall of unenriched gas. This equilibrium is in turn governed by the outflow strength. As such, the MZR provides valuable insights and strong constraints on galactic outflow properties across cosmic time. We compare three outflow models: no outflows, a ‘constant-wind model that emulates the popular Dekel & Silk scenario, and a ‘momentum-driven wind’ model that best reproduces z≳ 2 intergalactic medium metallicity data. Only the momentum-driven wind scaling simulation is able to reproduce the observed z∼ 2 MZR's slope, amplitude, and scatter. In order to understand why, we construct a one-zone chemical evolution model guided by simulations. This model shows that the MZR in our outflow simulations can be understood in terms of three parameters: (i) the equilibrium metallicityZg,eq= yṀSFR/ṀACC (where y= net yield), reflecting the enrichment balance between star formation rate ṀSFR and gas accretion rate ṀACC; (ii) the dilution time, representing the time-scale for a galaxy to return to Zg,eq after a metallicity-perturbing interaction; and (iii) the blowout massMblowout, which is the galaxy stellar mass above which winds can escape its halo. Without outflows, galaxy metallicities exceed observations by approximately two to three times, although the slope of the MZR is roughly correct owing to greater star formation efficiencies in larger galaxies. When outflows with mass-loading factor ηW are present, galaxies below Mblowout obey Zg,eq≈y/(1 +ηW), while above Mblowout, Zg,eq→ y. Our constant-wind model has Mblowout∼ 1010 M⊙, which yields a sharp upturn in the MZR above this scale and a flat MZR with large scatter below it, in strong disagreement with observations. Our momentum-driven wind model naturally reproduces the observed Zg∝M0.3* because Zg,eq∝η−1W∝M1/3* when ηW≫ 1 (i.e. at low masses). The flattening of the MZR at M*≳ 1010.5 M⊙ observed by Tremonti et al. is reflective of the mass-scale where ηW∼ 1 rather than a characteristic outflow speed; in fact, the outflow speed plays little role in the MZR except through Mblowout. The tight observed MZR scatter is ensured when td≲ dynamical time, which is only satisfied at all masses in our momentum-driven wind model. We also discuss secondary effects on the MZR, such as baryonic stripping from neighbouring galaxies' outflows.
ABSTRACT
We study the spin alignment of galaxies and haloes with respect to filaments and walls of the cosmic web, identified with DisPerSE , using the Simba simulation from z = 0 − 2. Massive ...haloes’ spins are oriented perpendicularly to their closest filament’s axis and walls, while low-mass haloes tend to have their spins parallel to filaments and in the plane of walls. A similar mass-dependent spin flip is found for galaxies, albeit with a weaker signal particularly at low mass and low-z, suggesting that galaxies’ spins retain memory of their larger scale environment. Low-z star-forming and rotation-dominated galaxies tend to have spins parallel to nearby filaments, while quiescent and dispersion-dominated galaxies show preferentially perpendicular orientation; the star formation trend can be fully explained by the stellar mass correlation, but the morphology trend cannot. There is a dependence on HI mass, such that high-HI galaxies tend to have parallel spins while low-HI galaxies are perpendicular, suggesting that HI content may trace anisotropic infall more faithfully than the stellar component. Finally, at fixed stellar mass, the strength of spin alignments correlates with the filament’s density, with parallel alignment for galaxies in high density environments. These findings are consistent with conditional tidal torque theory, and highlight a significant correlation between galactic spin and the larger scale tides that are important e.g., for interpreting weak lensing studies. Simba allows us to rule out numerical grid locking as the cause of previously-seen low mass alignment.
We present an analytic formalism that describes the evolution of the stellar, gas and metal content of galaxies. It is based on the idea, inspired by hydrodynamic simulations, that galaxies live in a ...slowly evolving equilibrium between inflow, outflow and star formation. We argue that this formalism broadly captures the behaviour of galaxy properties evolving in simulations. The resulting equilibrium equations for the star formation rate, gas fraction and metallicity depend on three key free parameters that represent ejective feedback, preventive feedback and reaccretion of ejected material. We schematically describe how these parameters are constrained by models and observations. Galaxies perturbed off the equilibrium relations owing to inflow stochasticity tend to be driven back towards equilibrium, such that deviations in star formation rate at a given mass are correlated with gas fraction and anticorrelated with metallicity. After an early gas accumulation epoch, quiescently star-forming galaxies are expected to be in equilibrium over most of cosmic time. The equilibrium model provides a simple intuitive framework for understanding the cosmic evolution of galaxy properties, and centrally features the cycle of baryons between galaxies and surrounding gas as the driver of galaxy growth.
We present the MUFASA suite of cosmological hydrodynamic simulations, which employs the GIZMO meshless finite mass (MFM) code including H2-based star formation, nine-element chemical evolution, ...two-phase kinetic outflows following scalings from the Feedback in Realistic Environments zoom simulations, and evolving halo mass-based quenching. Our fiducial (50 h super( -1) Mpc) super( 3) volume is evolved to z = 0 with a quarter billion elements. The predicted galaxy stellar mass functions (GSMFs) reproduces observations from ... to ... in cosmic variance, providing an unprecedented match to this key diagnostic. The cosmic star formation history and stellar mass growth show general agreement with data, with a strong archaeological downsizing trend such that dwarf galaxies form the majority of their stars after z ~ 1. We run 25 and 12.5 h super( -1) Mpc volumes to z = 2 with identical feedback prescriptions, the latter resolving all hydrogen-cooling haloes, and the three runs display fair resolution convergence. The specific star formation rates broadly agree with data at z = 0, but are underpredicted at z ~ 2 by a factor of 3, re-emphasizing a longstanding puzzle in galaxy evolution models. We compare runs using MFM and two flavours of smoothed particle hydrodynamics, and show that the GSMF is sensitive to hydrodynamics methodology at the ~x2 level, which is sub-dominant to choices for parametrizing feedback. (ProQuest: ... denotes formulae/symbols omitted.)
ABSTRACT
We present a new end-to-end pipeline for Mock Observations of X-ray Haloes and Analysis (moxha) for hydrodynamic simulations of massive haloes, and use it to investigate X-ray scaling ...relations and hydrostatic mass bias in the simba cosmological hydrodynamic simulation for haloes with M500 ∼ 1013−15M⊙. moxha ties together existing yT-based software packages and adds new functionality to provide an end-to-end pipeline for generating mock X-ray halo data from large-scale or zoom simulation boxes. We compare moxha-derived halo properties in simba to their emission-weighted counterparts, and forecast the systematic mass bias in mock Athena observations. Overall, we find inferred hydrostatic masses are biased low compared to true simba values. For simple mass-weighting, we find $b_\text{MW} = 0.15^{+0.15}_{-0.14}$ (16–84 per cent range), while emission-weighting increases this to $b_\text{LW}=0.30^{+0.19}_{-0.10}$. The larger bias versus mass-weighted values we attribute to the spectroscopic and emission-weighted temperatures being biased systematically lower than mass-weighted temperatures. The full moxha pipeline recovers the emission-weighted hydrostatic masses at R500 reasonably well, yielding $b_\text{X}=0.33^{+0.28}_{-0.34}$. moxha-derived halo X-ray scalings are in very good agreement with observed scaling relations, with the inclusion of lower mass groups significantly steepening the LX − M500, M500 − TX, and LX − TX relations. This indicates the strong effect the simba feedback model has on low-mass haloes, which strongly evacuates poor groups but still retains enough gas to reproduce observations. We find similar trends for analogous scaling relations measured at R500, as expected for halo-wide gas evacuation.
Black hole – Galaxy correlations in simba Thomas, Nicole; Davé, Romeel; Anglés-Alcázar, Daniel ...
Monthly Notices of the Royal Astronomical Society,
08/2019, Volume:
487, Issue:
4
Journal Article
Peer reviewed
Open access
ABSTRACT
We examine the co-evolution of galaxies and supermassive black holes in the simba cosmological hydrodynamic simulation. simba grows black holes via gravitational torque-limited accretion ...from cold gas and Bondi accretion from hot gas, while feedback from black holes is modelled in radiative and jet modes depending on the Eddington ratio (fEdd). simba shows generally good agreement with local studies of black hole properties, such as the black hole mass–stellar velocity dispersion (MBH–σ) relation, the black hole accretion rate versus star formation rate (BHAR–SFR), and the black hole mass function. MBH–σ evolves such that galaxies at a given MBH have higher σ at higher redshift, consistent with no evolution in MBH–M⋆. For $M_{\rm BH}\lesssim 10^8\, {\rm M}_{\odot }$, fEdd is anticorrelated with MBH since the BHAR is approximately independent of MBH, while at higher masses fEdd–MBH flattens and has a larger scatter. BHAR versus SFR is invariant with redshift, but fEdd drops steadily with time at a given MBH, such that all but the most massive black holes are accreting in a radiatively efficient mode at $z\gtrsim 2$. The black hole mass function amplitude decreases with redshift and is locally dominated by quiescent galaxies for MBH > 108 M⊙, but for $z\gtrsim 1$ star-forming galaxies dominate at all MBH. The z = 0 fEdd distribution is roughly lognormal with a peak at $f_{\rm Edd}\lesssim 0.01$ as observed, shifting to higher fEdd at higher redshifts. Finally, we study the dependence of black hole properties with H i content and find that the correlation between gas content and SFR is modulated by black hole properties, such that higher SFR galaxies at a given gas content have smaller black holes with higher fEdd.
ABSTRACT
We examine the evolution of intragroup gas rest-frame X-ray scaling relations for group-sized haloes (M500 = 1012.3–1015 M⊙) in the Simba galaxy formation simulation. X-ray luminosity LX ...versus M500 shows increasing deviation from self-similarity from z = 3 → 0, with M500 < 1013.5 M⊙ haloes exhibiting a large reduction in LX and slight increase in X-ray luminosity-weighted temperature TX. These shifts are driven by a strong drop in fgas with time for these haloes, and coincides with the onset of Simba’s black hole (BH) jet feedback, occurring when MBH > 107.5 M⊙ and Eddington ratio <0.2, in group haloes at z ∼ 1.5. The connection with BH feedback is corroborated by fBH ≡ MBH/M500 in M500 < 1013.5 M⊙ haloes being strongly anticorrelated with LX and fgas at $z\lesssim 1.5$. This is further reflected in the scatter of LX − TX: haloes with small fBH lie near self-similarity, while those with the highest fBH lie furthest below. Turning off jet feedback results in mostly self-similar behaviour down to z = 0. For the X-ray weighted metallicity ZX, stellar feedback impacts the enrichment of halo gas. Finally, halo profiles show that jet feedback flattens the electron density and entropy profiles, and introduces a core in X-ray surface brightness, particularly at M500 < 1013.5 M⊙. This argues that in Simba, intragroup X-ray evolution is largely driven by jet feedback removing hot gas from the cores of massive groups, and expelling gas altogether in less massive groups.
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