ABSTRACT
We present the first implementation of hyperbolic thermal conduction in smoothed particle hydrodynamics. Hyperbolic conduction is a physically motivated alternative to traditional, parabolic ...conduction. It incorporates a relaxation time, which ensures that heat propagates no faster than a physical signal speed. This allows for larger, Courant-like, time-steps for explicit schemes. Numerical solutions of the hyperbolic conduction equations require added dissipation to remain stable at discontinuities and we present a novel scheme for this. Test cases include a simple step, the Sod Shock Tube, the Sedov–Taylor blast, and a super bubble. We demonstrate how longer relaxation times limit conduction, recovering the purely hydrodynamical results, while short relaxation times converge on the parabolic conduction result. We demonstrate that our scheme is stable with explicit Courant-like time-steps and can be orders of magnitude faster than explicit parabolic conduction, depending on the application.
A study of metal enrichment of the intergalactic medium (IGM) using a series of smooth particle hydrodynamic (SPH) simulations is presented, employing models for metal cooling and the turbulent ...diffusion of metals and thermal energy. An adiabatic feedback mechanism was adopted where gas cooling was prevented on the time-scale of supernova bubble expansion to generate galactic winds without explicit wind particles. The simulations produced a cosmic star formation history (SFH) that is broadly consistent with observations until z∼ 0.5, and a steady evolution of the universal neutral hydrogen fraction () that compares reasonably well with observations. The evolution of the mass and metallicities in stars and various gas phases was investigated. At z= 0, about 40 per cent of the baryons are in the warm–hot intergalactic medium (WHIM), but most metals (80–90 per cent) are locked in stars. At higher redshifts the proportion of metals in the IGM is higher due to more efficient loss from galaxies. The results also indicate that IGM metals primarily reside in the WHIM throughout cosmic history, which differs from simulations with hydrodynamically decoupled explicit winds. The metallicity of the WHIM lies between 0.01 and 0.1 solar with a slight decrease at lower redshifts. The metallicity evolution of the gas inside galaxies is broadly consistent with observations, but the diffuse IGM is under enriched at z∼ 2.5. Galactic winds most efficiently enrich the IGM for haloes in the intermediate mass range 1010–1011 M⊙. At the low-mass end gas is prevented from accreting on to haloes and has very low metallicities. At the high-mass end, the fraction of halo baryons escaped as winds declines along with the decline of stellar mass fraction of the galaxies. This is likely because of the decrease in star formation activity and decrease in wind escape efficiency. Metals enhance cooling which allows WHIM gas to cool on to galaxies and increases star formation. Metal diffusion allows winds to mix prior to escape, decreasing the IGM metal content in favour of gas within galactic haloes and star-forming gas. Diffusion significantly increases the amount of gas with low metallicities and changes the density–metallicity relation.
We introduce the Making Galaxies In a Cosmological Context (MAGICC) programme of smoothed particle hydrodynamics simulations. We describe a parameter study of galaxy formation simulations of an L* ...galaxy that uses early stellar feedback combined with supernova feedback to match the stellar mass-halo mass relationship. While supernova feedback alone can reduce star formation enough to match the stellar mass-halo mass relationship, the galaxy forms too many stars before z = 2 to match the evolution seen using abundance matching. Our early stellar feedback is purely thermal and thus operates like an ultraviolet ionization source as well as providing some additional pressure from the radiation of massive, young stars. The early feedback heats gas to >106 K before cooling it to 104 K. The pressure from this hot gas creates a more extended disc and prevents more star formation prior to z = 1 than supernova feedback alone. The resulting disc galaxy has a flat rotation curve, an exponential surface brightness profile, and matches a wide range of disc scaling relationships. The disc forms from the inside-out with an increasing exponential scale length as the galaxy evolves. Overall, early stellar feedback helps to simulate galaxies that match observational results at low and high redshifts.
Chaos and variance in galaxy formation Keller, B W; Wadsley, J W; Wang, L ...
Monthly notices of the Royal Astronomical Society,
01/2019, Letnik:
482, Številka:
2
Journal Article
ABSTRACT
We present numerical simulations of dust clumping and planetesimal formation initiated by the streaming instability (SI) with self-gravity. We examine the variability in the planetesimal ...formation process by employing simulation domains with large radial and azimuthal extents and a novel approach of re-running otherwise identical simulations with different random initializations of the dust density field. We find that the planetesimal mass distribution and the total mass of dust that is converted into planetesimals can vary substantially between individual small simulations and within the domains of larger simulations. Our results show that the non-linear nature of the developed SI introduces substantial variability in the planetesimal formation process that has not been previously considered and suggests larger scale dynamics may affect the process.
For simulations of fluid dynamics in astrophysics, physical viscosity and diffusion are typically neglected. However, in this high Reynolds number regime, real fluids become highly turbulent and ...turbulent processes mediate substantial transport of momentum and heat that is diffusive in nature. In the absence of models for these processes, code-dependent numerical effects dominate how diffusion operates and may lead to physically incorrect simulation results. We highlight the qualitative difference in these numerical effects for smooth particle hydrodynamics (SPH) and grid-based Eulerian codes using two test problems: a buoyant gas bubble and gas in a galaxy cluster. Grid codes suffer from numerical diffusion in the absence of explicit terms, and small-scale diffusion of heat is completely absent in the Lagrangian SPH method. We find that SPH with heat diffusion added at a level similar to that expected from turbulence diffusion generates more physically appealing results. These results suggest, but do not confirm, that a flat entropy core is to be expected for gas in an idealized galaxy cluster (i.e. one without physics beyond that of a non-radiating gas). A goal of this work is thus to draw attention to the as yet unfulfilled need for models of turbulent diffusive processes in compressible gases in astrophysics.
We use cosmological hydrodynamic simulations to consistently compare the assembly of dwarf galaxies in both Λ dominated, cold dark matter (CDM) and self-interacting dark matter (SIDM) models. The ...SIDM model adopts a constant cross-section of 2 cm2 g−1, a relatively large value to maximize its effects. These are the first SIDM simulations that are combined with a description of stellar feedback that naturally drives potential fluctuations able to create dark matter (DM) cores. Remarkably, SIDM fails to significantly lower the central DM density within the central 500 pc at halo peak velocities V
max < 30 km s−1. This is due to the fact that the central regions of very low mass field haloes have relatively low central velocity dispersion and densities, leading to time-scales for SIDM collisions greater than a Hubble time. CDM haloes with V
max < 30 km s−1 have inefficient star formation, and hence weak supernova feedback. At a fixed 2 cm2 g−1 SIDM cross-section, the DM content of very low mass CDM and SIDM haloes differs by no more than a factor of 2 within 100–200 pc. At larger halo masses (∼1010 M⊙), the introduction of baryonic processes creates field dwarf galaxies with DM cores and central DM+baryon distributions that are effectively indistinguishable between CDM and SIDM. Both models are in broad agreement with observed Local Group field galaxies across the range of masses explored. To significantly differentiate SIDM from CDM at the scale of faint dwarf galaxies, a velocity-dependent cross-section that rapidly increases to values larger than 2 cm2 g−1 for haloes with V
max < 25–30 km s−1 needs to be introduced.
ABSTRACT
Kilometre-sized planetesimals form from pebbles of a range of sizes. We present the first simulations of the streaming instability (SI) that begin with a realistic, peaked size distribution, ...as expected from grain growth predictions. Our 3D numerical simulations directly form planetesimals via the gravitational collapse of pebble clouds. Models with multiple grain sizes show spatially distinct dust populations. The smallest grains in the size distribution do not participate in the formation of filaments or the planetesimals that are formed by the remaining ∼80 per cent of the dust mass. This implies a size cutoff for pebbles incorporated into asteroids and comets. Disc observations cannot resolve this dust clumping. However, we show that clumping, combined with optical depth effects, can cause significant underestimates of the dust mass, with 20–80 per cent more dust being present even at moderate optical depths if the SI is active.
Within a fully cosmological hydrodynamical simulation, we form a galaxy which rotates at 140 km s−1, and it is characterized by two loose spiral arms and a bar, indicative of a Hubble-type SBc/d ...galaxy. We show that our simulated galaxy has no classical bulge, with a pure disc profile at z = 1, well after the major merging activity has ended. A long-lived bar subsequently forms, resulting in the formation of a secularly formed 'pseudo-'bulge, with the final bulge-to-total light ratio of 0.21. We show that the majority of gas which loses angular momentum and falls to the central region of the galaxy during the merging epoch is blown back into the hot halo, with much of it returning later to form stars in the disc. We propose that this mechanism of redistribution of angular momentum via a galactic fountain, when coupled with the results from our previous study which showed why gas outflows are biased to have low angular momentum, can solve the angular momentum/bulgeless disc problem of the cold dark matter paradigm.
PDF
