Elizabeth J. Tasker

Fundamental differences between SPH and grid methods

We have carried out a comparison study of hydrodynamical codes by investigating their performance in modelling interacting multiphase fluids. The two commonly used techniques of grid and smoothed particle hydrodynamics (SPH) show striking differences in their ability to model processes that are fundamentally important across many areas of astrophysics. Whilst Eulerian grid based methods are able to resolve and treat important dynamical instabilities, such as Kelvin-Helmholtz or Rayleigh-Taylor, these processes are poorly or not at all resolved by existing SPH techniques. We show that the reason for this is that SPH, at least in its standard implementation, introduces spurious pressure forces on particles in regions where there are steep density gradients. This results in a boundary gap of the size of an SPH smoothing kernel radius over which interactions are severely damped.

STAR FORMATION IN DISK GALAXIES. I. FORMATION AND EVOLUTION OF GIANT MOLECULAR CLOUDS VIA GRAVITATIONAL INSTABILITY AND CLOUD COLLISIONS

We investigate the formation and evolution of giant molecular clouds (GMCs) in a Milky-Way-like disk galaxy with a flat rotation curve. We perform a series of three-dimensional adaptive mesh refinement numerical simulations that follow both the global evolution on scales of ~20 kpc and resolve down to scales 10 pc with a multiphase atomic interstellar medium. In this first study, we omit star formation and feedback, and focus on the processes of gravitational instability and cloud collisions and interactions. We define clouds as regions with n H ≥ 100 cm-3 and track the evolution of individual clouds as they orbit through the galaxy from their birth to their eventual destruction via merger or via destructive collision with another cloud. After ~140 Myr a large fraction of the gas in the disk has fragmented into clouds with masses ~106 M ☉ and a mass spectrum similar to that of Galactic GMCs. The disk settles into a quasi-steady-state in which gravitational scattering of clouds keeps the disk near the threshold of global gravitational instability. The cloud collision time is found to be a small fraction, ~1/5, of the orbital time, and this is an efficient mechanism to inject turbulence into the clouds. This helps to keep clouds only moderately gravitationally bound, with virial parameters of order unity. Many other observed GMC properties, such as mass surface density, angular momentum, velocity dispersion, and vertical distribution, can be accounted for in this simple model with no stellar feedback.

A test suite for quantitative comparison of hydrodynamic codes in astrophysics

We test four commonly used astrophysical simulation codes, enzo, flash, gadget and hydra, using a suite of numerical problems with analytic initial and final states. Situations similar to the conditions of these tests, a Sod shock, a Sedov blast, and both a static and translating King sphere, occur commonly in astrophysics, where the accurate treatment of shocks, sound waves, supernovae explosions and collapsed haloes is a key condition for obtaining reliable validated simulations. We demonstrate that comparable results can be obtained for Lagrangian and Eulerian codes by requiring that approximately one particle exists per grid cell in the region of interest. We conclude that adaptive Eulerian codes, with their ability to place refinements in regions of rapidly changing density, are well suited to problems where physical processes are related to such changes. Lagrangian methods, on the other hand, are well suited to problems where large density contrasts occur and the physics are related to the local density itself rather than the local density gradient.

Simulating Star Formation and Feedback in Galactic Disk Models

We use a high-resolution grid-based hydrodynamics method to simulate the multiphase interstellar medium (ISM) in a quiescent Milky Way-sized disk galaxy. The models are global and three-dimensional, and they include a treatment of star formation and feedback. We examine the formation of gravitational instabilities and show that a form of the Toomre instability criterion can successfully predict where star formation will occur. Two common prescriptions for star formation are investigated. The first is based on cosmological simulations and has a relatively low threshold for star formation but also enforces a comparatively low efficiency. The second only permits star formation above a number density of 103 cm-3 but adopts a high efficiency. We show that both methods can reproduce the observed slope of the relationship between star formation and gas surface density (although at too high a rate for our adopted parameters). A run that includes feedback from Type II supernovae is successful at driving gas out of the plane, most of which falls back onto the disk. This feedback also substantially reduces the star formation rate. Finally, we examine the density and pressure distribution of the ISM and show that there is a rough pressure equilibrium in the disk, but with a wide range of pressures at a given location (and even wider for the case including feedback).

THE EFFECT OF THE INTERSTELLAR MODEL ON STAR FORMATION PROPERTIES IN GALACTIC DISKS

We studied the effect of interstellar gas conditions on global galaxy simulations by considering three different models for the ISM. Our first model included only radiative cooling down to 300 K, our second model added an additional background heating term due to photoelectric heating, and our third model uses an isothermal equation of state with a temperature of 104 K and no explicit heating or cooling. Two common prescriptions for star formation are implemented in each case. The first is based on cosmological simulations with a low threshold for star formation but also a low efficiency. The second assumes that stars form only in high-density regions but with a higher efficiency. We also explore the effects of including feedback from Type II supernovae. We find that the different ISM types produce marked differences in the structure of the disk and temperature phases present in the gas, although inclusion of feedback largely dominates these effects. In particular, the size of the star-forming clumps was increased both by background heating and by enforcing an isothermal ISM. We also looked at the one-dimensional profiles and found that a lognormal probability distribution function (PDF) provides a good fit for all our simulations over several orders of magnitude in density. Overall, despite noticeable structural differences, the star formation properties in the disk are largely insensitive to ISM type and agree reasonably well with observations.

THE ENVIRONMENTAL IMPACT OF LYMAN-BREAK GALAXIES

We perform cosmological simulations of galaxies forming at z = 3 using the hydrodynamics grid code, Enzo. By selecting the largest galaxies in the volume to correspond to Lyman break galaxies, we construct observational spectra of the H I flux distribution around these objects, as well as column densities of C IV and O VI throughout a refined region. We successfully reproduce the most recent observations of the mean H I flux in the close vicinity of Lyman break galaxies but see no evidence for the proximity effect in earlier observations. While our galaxies do return metals to the intergalactic medium, their quantity and volume appear to be somewhat less than observed. We conclude either that we do not adequately resolve galactic winds or that at least some of the intergalactic metal enrichment is by early epoch objects whose mass is smaller than our minimum resolved halo mass.

Galactic spiral generation in tidal encounters

The existence of grand design spiral galaxies in the universe is still a standing problem. The passage of a small companion is known to be able to induce spiral structures in disc galaxies, but there remains questions over how relevant this mechanism is to the galaxies observed in the real universe. Our study aims to address two key points regarding such interactions; the limiting mass companion needed to drive tidal spiral structures, and the differences between the resulting gas and stellar morphology. We find the minimum mass of a companion to be as low as 5% of the stellar mass of the galaxy, and that the arms formed in the gas and the stars display very minor dynamical and morphological differences.