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Register a new userFeedback and metal enrichment in cosmological SPH simulations - II. A multiphase model with supernova energy feedback
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Cecilia Scannapieco
Publication date
2006
fields
Physics
and research's language is
English
Authors
Cecilia Scannapieco
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We have developed a new scheme to treat a multiphase interstellar medium in smoothed particle hydrodynamics simulations of galaxy formation. This scheme can represent a co-spatial mixture of cold and hot ISM components, and is formulated without scale-dependent parameters. It is thus particularly suited to studies of cosmological structure formation where galaxies with a wide range of masses form simultaneously. We also present new algorithms for energy and heavy element injection by supernovae, and show that together these schemes can reproduce several important observed effects in galaxy evolution. Both in collapsing systems and in quiescent galaxies our codes can reproduce the Kennicutt relation between the surface densities of gas and of star formation. Strongly metal-enhanced winds are generated in both cases with ratios of mass-loss to star formation which are similar to those observed. This leads to a self-regulated cycle for star formation activity. The overall impact of feedback depends on galaxy mass. Star formation is suppressed at most by a factor of a few in massive galaxies, but in low-mass systems the effects can be much larger, giving star formation an episodic, bursty character. The larger the energy fraction assumed available in feedback, the more massive the outflows and the lower the final stellar masses. Winds from forming disks are collimated perpendicular to the disk plane, reach velocities up to 1000 km/s, and efficiently transport metals out of the galaxies. The asymptotically unbound baryon fraction drops from >95 per cent to ~30 per cent from the least to the most massive of our idealised galaxies, but the fraction of all metals ejected with this component exceeds 60 per cent regardless of mass. Such winds could plausibly enrich the intergalactic medium to observed levels.
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We discuss a model for treating chemical enrichment by SNII and SNIa explosions in simulations of cosmological structure formation. Our model includes metal-dependent radiative cooling and star formation in dense collapsed gas clumps. Metals are returned into the diffuse interstellar medium by star particles using a local SPH smoothing kernel. A variety of chemical abundance patterns in enriched gas arise in our treatment owing to the different yields and lifetimes of SNII and SNIa progenitor stars. In the case of SNII chemical production, we adopt metal-dependent yields. Because of the sensitive dependence of cooling rates on metallicity, enrichment of galactic haloes with metals can in principle significantly alter subsequent gas infall and the build up of the stellar components. Indeed, in simulations of isolated galaxies we find that a consistent treatment of metal-dependent cooling produces 25% more stars outside the central region than simulations with a primordial cooling function. In the highly-enriched central regions, the evolution of baryons is however not affected by metal cooling, because here the gas is always dense enough to cool. A similar situation is found in cosmological simulations because we include no strong feedback processes which could spread metals over large distances and mix them into unenriched diffuse gas. We demonstrate this explicitly with test simulations which adopt super-solar cooling functions leading to large changes both in the stellar mass and in the metal distributions. We also find that the impact of metallicity on the star formation histories of galaxies may depend on their particular evolutionary history. Our results hence emphasise the importance of feedback processes for interpreting the cosmic metal enrichment.
We present results from seventy-one zoom simulations of a Milky Way-sized (MW) halo, exploring the parameter space for a widely-used star formation and feedback model in the {tt Enzo} simulation code. We propose a novel way to match observations, using functional fits to the observed baryon makeup over a wide range of halo masses. The model MW galaxy is calibrated using three parameters: the star formation efficiency $left(f_*right)$, the efficiency of thermal energy from stellar feedback $left(epsilonright)$ and the region into which feedback is injected $left(r {rm and} sright)$. We find that changing the amount of feedback energy affects the baryon content most significantly. We then identify two sets of feedback parameter values that are both able to reproduce the baryonic properties for haloes between $10^{10},mathrm{M_odot}$ and $10^{12},mathrm{M_odot}$. We can potentially improve the agreement by incorporating more parameters or physics. If we choose to focus on one property at a time, we can obtain a more realistic halo baryon makeup. We show that the employed feedback prescription is insensitive to dark matter mass resolution between $10^5,{rm M_odot}$ and $10^7,{rm M_odot}$. Contrasting both star formation criteria and the corresponding combination of optimal feedback parameters, we also highlight that feedback is self-consistent: to match the same baryonic properties, with a relatively higher gas to stars conversion efficiency, the feedback strength required is lower, and vice versa. Lastly, we demonstrate that chaotic variance in the code can cause deviations of approximately 10% and 25% in the stellar and baryon mass in simulations evolved from identical initial conditions.
In this study, we present and validate a variation of recently-developed physically motivated sub-grid prescriptions for supernova feedback that account for the unresolved energy-conserving phase of the bubble expansion. Our model builds upon the implementation publicly available in the mesh-less hydrodynamic code GIZMO, and is coupled with the chemistry library KROME. Here, we test it against different setups, to address how it affects the formation/dissociation of molecular hydrogen (H$_2$). First, we explore very idealised conditions, to show that it can accurately reproduce the terminal momentum of the blast-wave independent of resolution. Then, we apply it to a suite of numerical simulations of an isolated Milky Way-like galaxy and compare it with a similar run employing the delayed-cooling sub-grid prescription. We find that the delayed-cooling model, by pressurising ad-hoc the gas, is more effective in suppressing star formation. However, to get this effect, it must maintain the gas warm/hot at densities where it is expected to cool efficiently, artificially changing the thermo-chemical state of the gas, and reducing the H$_2$ abundance even in dense gas. Mechanical feedback, on the other hand, is able to reproduce the H$_2$ column densities without altering the gas thermodynamics, and, at the same time, drives more powerful outflows. However, being less effective in suppressing star formation, it over-predicts the Kennicutt-Schmidt relation by a factor of about 2.5. Finally, we show that the model is consistent at different resolution levels, with only mild differences.
We discuss a numerical model for black hole growth and its associated feedback processes that for the first time allows cosmological simulations of structure formation to self-consistently follow the build up of the cosmic population of galaxies and active galactic nuclei. Our model assumes that seed black holes are present at early cosmic epochs at the centres of forming halos. We then track their growth from gas accretion and mergers with other black holes in the course of cosmic time. For black holes that are active, we distinguish between two distinct modes of feedback, depending on the black hole accretion rate itself. Black holes that accrete at high rates are assumed to be in a `quasar regime, where we model their feedback by thermally coupling a small fraction of their bolometric luminosity to the surrounding gas. For black holes with low accretion rates, we conjecture that most of their feedback occurs in mechanical form, where AGN-driven bubbles are injected into a gaseous environment. Using our new model, we carry out TreeSPH cosmological simulations on the scales of individual galaxies to those of massive galaxy clusters, both for isolated systems and for cosmological boxes. We demonstrate that our model produces results for the black hole and stellar mass densities in broad agreement with observational constraints. We find that the black holes significantly influence the evolution of their host galaxies, changing their star formation history, their amount of cold gas, and their colours. Also, the properties of intracluster gas are affected strongly by the presence of massive black holes in the cores of galaxy clusters, leading to shallower metallicity and entropy profiles, and to a suppression of strong cooling flows. [Abridged]
We present a new multi-phase sub-resolution model for star formation and feedback in SPH numerical simulations of galaxy formation. Our model, called MUPPI (MUlti-Phase Particle Integrator), describes each gas particle as a multi-phase system, with cold and hot gas phases, coexisting in pressure equilibrium, and a stellar component. Cooling of the hot tenuous gas phase feeds the cold gas phase. Stars are formed out of molecular gas with a given efficiency, which scales with the dynamical time of the cold phase. Our prescription for star formation is not based on imposing the Schmidt-Kennicutt relation, which is instead naturally produced by MUPPI. Energy from supernova explosions is deposited partly into the hot phase of the gas particles, and partly to that of neighboring particles. Mass and energy flows among the different phases of each particle are described by a set of ordinary differential equations which we explicitly integrate for each gas particle, instead of relying on equilibrium solutions. This system of equations also includes the response of the multi-phase structure to energy changes associated to the thermodynamics of the gas. We apply our model to two isolated disk galaxy simulations and two spherical cooling flows. MUPPI is able to reproduce the Schmidt-Kennicutt relation for disc galaxies. It also reproduces the basic properties of the inter-stellar medium in disc galaxies, the surface densities of cold and molecular gas, of stars and of star formation rate, the vertical velocity dispersion of cold clouds and the flows connected to the galactic fountains. Quite remarkably, MUPPI also provides efficient stellar feedback without the need to include a scheme of kinetic energy feedback. [abridged]
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