We have carried out a hydrodynamical code comparison study of 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 the SPH smoothing kernel over which information is not transferred.Comment: 15 pages, 13 figures, to be submitted to MNRAS. For high-resolution figures, please see http://www-theorie.physik.unizh.ch/~agertz

This paper describes the open-source code Enzo, which uses block-structured adaptive mesh refinement to provide high spatial and temporal resolution for modeling astrophysical fluid flows. The code is Cartesian, can be run in 1, 2, and 3 dimensions, and supports a wide variety of physics including hydrodynamics, ideal and non-ideal magnetohydrodynamics, N-body dynamics (and, more broadly, self-gravity of fluids and particles), primordial gas chemistry, optically-thin radiative cooling of primordial and metal-enriched plasmas (as well as some optically-thick cooling models), radiation transport, cosmological expansion, and models for star formation and feedback in a cosmological context. In addition to explaining the algorithms implemented, we present solutions for a wide range of test problems, demonstrate the code's parallel performance, and discuss the Enzo collaboration's code development methodology.

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 3D adaptive mesh refinement (AMR) 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 (ISM). 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 ∼ 10 6 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.

We performed sub-parsec (∼0.06 pc) scale simulations of two idealized molecular clouds with different masses undergoing a collision. Gas clumps with densities greater than 10 −20 g cm −3 (0.3 × 10 4 cm −3 ) were identified as pre-stellar cores and tracked throughout the simulation. The colliding system showed a partial gas arc morphology with core formation in the oblique shock front at the collision interface. These characteristics support NANTEN observations of objects suspected to be colliding giant molecular clouds (GMCs). We investigated the effect of turbulence and collision speed on the resulting core population and compared the cumulative mass distribution to cores in observed GMCs. Our results suggest that a faster relative velocity increases the number of cores formed but that cores grow via accretion predominately while in the shock front, leading to a slower shock being more important for core growth. The core masses obey a power-law relation with index γ = −1.6, in good agreement with observations. This suggests that core production through collisions should follow a similar mass distribution as quiescent formation, albeit at a higher mass range. If cores can be supported against collapse during their growth, then the estimated ram pressure from gas infall is of the right order to counter the radiation pressure and form a star of 100 M .

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.

We use a high-resolution grid-based hydrodynamics method to simulate the multi-phase interstellar medium in a Milky Way-size quiescent disk galaxy. The models are global and three-dimensional, and 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 1000 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 which 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 feedbackComment: 14 pages, 12 figures, accepted to Astrophysical Journa

We investigate the effect of star formation and diffuse photoelectric heating on the properties of giant molecular clouds (GMCs) formed in high-resolution ( 10 pc) global (∼20 kpc) simulations of isolated Milky-Way-type galaxy disks. The clouds are formed through gravitational fragmentation, and structures with densities n H,c > 100 cm −3 are identified as GMCs. Between 1000 and 1500 clouds are created in the simulations with masses M > 10 5 M and 180-240 with masses M > 10 6 M in agreement with estimates of the Milky Way's population. We find that the effect of photoelectric heating is to suppress the fragmentation of the interstellar medium, resulting in a filamentary structure in the warm gas surrounding clouds. This environment suppresses the formation of a retrograde rotating cloud population, with 88% of the clouds rotating prograde with respect to the galaxy after 300 Myr. The diffuse heating also reduces the initial star formation rate (SFR), slowing the conversation of gas into stars. We therefore conclude that the interstellar environment plays an important role in the GMC evolution. Our clouds live between 0 and 20 Myr with a high infant mortality (t < 3 Myr) due to cloud mergers and star formation. Other properties, including distributions of mass, size, and surface density, agree well with observations. Collisions between our clouds are common, occurring at a rate of ∼ 1/4 of the orbital period. It is not clear whether such collisions trigger or suppress star formation at our current resolution. Our SFR is a factor of 10 higher than observations in local galaxies. This is likely due to the absence of localized feedback in our models.

We investigate the impact of galactic environment on the properties of simulated giant molecular clouds (GMCs) formed in a M83-type barred spiral galaxy. Our simulation uses a rotating stellar potential to create the grand design features and resolves down to 1.5 pc. From the comparison of clouds found in the bar, spiral and disc regions, we find that the typical GMC is environment independent, with a mass of 5 × 10 5 M and radius 11 pc. However, the fraction of clouds in the property distribution tails varies between regions, with larger, more massive clouds with a higher velocity dispersion being found in greatest proportions in the bar, spiral and then disc. The bar clouds also show a bimodality that is not reflected in the spiral and disc clouds except in the surface density, where all three regions show two distinct peaks. We identify these features as being due to the relative proportion of three cloud types, classified via the mass-radius scaling relation, which we label A, B and C. Type A clouds have the typical values listed above and form the largest fraction in each region. Type B clouds are massive giant molecular associations (GMAs) while Type C clouds are unbound, transient clouds that form in dense filaments and tidal tails. The fraction of each clouds type depends on the cloud-cloud interactions, which cause mergers to build up the GMA Type Bs and tidal features in which the Type C clouds are formed. The number of cloud interactions is greatest in the bar, followed by the spiral, causing a higher fraction of both cloud types compared to the disc. While the cloud types also exist in lower resolution simulations, their identification becomes more challenging as they are not well separated populations on the mass-radius relation or distribution plots. Finally, we compare the results for three star formation models to estimate the star formation rate and efficiency in each galactic region.