Brian W. O’Shea

Introducing Enzo, an AMR Cosmology Application

In this paper we introduce Enzo, a 3D MPI-parallel Eulerian block-structured adaptive mesh refinement cosmology code. Enzo is designed to simulate cosmological structure formation, but can also be used to simulate a wide range of astrophysical situations. Enzo solves dark matter N-body dynamics using the particle-mesh technique. The Poisson equation is solved using a combination of fast fourier transform (on a periodic root grid) and multigrid techniques (on non-periodic subgrids). Euler’s equations of hydrodynamics are solved using a modified version of the piecewise parabolic method. Several additional physics packages are implemented in the code, including several varieties of radiative cooling, a metagalactic ultraviolet background, and prescriptions for star formation and feedback. We also show results illustrating properties of the adaptive mesh portion of the code. Information on profiling and optimizing the performance of the code can be found in the contribution by James Bordner in this volume.

The Destruction of Cosmological Minihalos by Primordial Supernovae

We present numerical simulations of primordial supernovae in cosmological minihalos at z ~ 20. We consider Type II supernovae, hypernovae, and pair instability supernovae (PISN) in halos from 6.9 × 105 to 1.2 × 107 M☉, those in which Population III stars are expected to form via H2 cooling. Our simulations are the first to follow the evolution of the blast from a free expansion on spatial scales of 10−4 pc until its approach to pressure equilibrium in the relic H II region of the progenitor, ~1000 pc. Supernovae in H II regions first expand adiabatically and then radiate strongly upon collision with baryons ejected from the halo during its photoevaporation by the progenitor. In contrast to previous findings, supernovae in neutral halos promptly emit most of their kinetic energy as X-rays, but retain enough momentum to seriously disrupt the halo. Explosions in H II regions escape into the IGM, but neutral halos confine the blast and its metals. In H II regions, a prompt second generation of stars may form in the remnant at radii of 100-200 pc. Explosions confined by massive halos instead recollapse, with infall rates in excess of 10−2 M☉ yr−1 that heavily contaminate their interior. This fallback may either fuel massive black hole growth at very high redshifts or create the first globular clusters with radii of 10-20 pc at the center of the halo. Our findings suggest that the first primitive galaxies may therefore have formed sooner, with greater numbers of stars and distinct chemical signatures, than in current models.

Cosmological Shocks in Adaptive Mesh Refinement Simulations and the Acceleration of Cosmic Rays

We present new results characterizing cosmological shocks within adaptive mesh refinement N-body/hydrodynamic simulations that are used to predict nonthermal components of large-scale structure. This represents the first study of shocks using adaptive mesh refinement. We propose a modified algorithm for finding shocks from those used on unigrid simulations that reduces the shock frequency of low Mach number shocks by a factor of ~3. We then apply our new technique to a large, (512 h−1 Mpc)3, cosmological volume and study the shock Mach number () distribution as a function of preshock temperature, density, and redshift. Because of the large volume of the simulation, we have superb statistics that result from having thousands of galaxy clusters. We find that the Mach number evolution can be interpreted as a method to visualize large-scale structure formation. Shocks with 20 generally follow accretion onto filaments and galaxy clusters, respectively. By applying results from nonlinear diffusive shock acceleration models using the first-order Fermi process, we calculate the amount of kinetic energy that is converted into cosmic-ray protons. The acceleration of cosmic-ray protons is large enough that in order to use galaxy clusters as cosmological probes, the dynamic response of the gas to the cosmic rays must be included in future numerical simulations.

THE NATURE OF THE WARM/HOT INTERGALACTIC MEDIUM. I. NUMERICAL METHODS, CONVERGENCE, AND O VI ABSORPTION

We perform a series of cosmological simulations using Enzo, an Eulerian adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the warm/hot intergalactic medium (WHIM). The WHIM may be an important component of the baryons missing observationally at low redshift. We investigate the dependence of the global star formation rate and mass fraction in various baryonic phases on spatial resolution and methods of incorporating stellar feedback. Although both resolution and feedback significantly affect the total mass in the WHIM, all of our simulations find that the WHIM fraction peaks at z ~ 0.5, declining to 35%-40% at z = 0. We construct samples of synthetic O VI absorption lines from our highest-resolution simulations, using several models of oxygen ionization balance. Models that include both collisional ionization and photoionization provide excellent fits to the observed number density of absorbers per unit redshift over the full range of column densities (1013 cm?2 N O VI 1015 cm?2). Models that include only collisional ionization provide better fits for high column density absorbers (N O VI 1014 cm?2). The distribution of O VI in density and temperature exhibits two populations: one at T ~ 105.5 K (collisionally ionized, 55% of total O VI) and one at T ~ 104.5 K (photoionized, 37%) with the remainder located in dense gas near galaxies. While not a perfect tracer of hot gas, O VI provides an important tool for a WHIM baryon census.

How the First Stars Regulated Local Star Formation. I. Radiative Feedback

We present numerical simulations of how a 120 -->M☉ primordial star regulates star formation in nearby cosmological halos at -->z ~ 20 by photoevaporation. Our models include nine-species primordial chemistry and self-consistent multifrequency conservative transfer of UV photons with all relevant radiative processes. Whether or not new stars form in halos clustered around a Population III star ultimately depends on their core densities and proximity to the star. Diffuse halos with central densities below 2-3 cm−3 are completely ionized and evaporated anywhere in the cluster. Evolved halos with core densities above 2000 cm−3 are impervious to both ionizing and Lyman-Werner flux at most distances from the star and collapse as quickly as they would in its absence. Star formation in halos of intermediate density can be either promoted or suppressed depending on how the ionization front (I-front) remnant shock compresses, deforms, and enriches the core with H2. We find that the 120 -->M☉ star photodissociates H2 in most halos in the cluster, but that catalysis by H− restores it a few hundred kiloyears after the death of the star, with little effect on star formation. Our models exhibit significant departures from previous one-dimensional, spherically symmetric simulations, which are prone to serious errors due to unphysical geometric focusing effects.

Vertical density waves in the Milky Way disc induced by the Sagittarius dwarf galaxy

Recently, Widrow and collaborators announced the discovery of vertical density waves in the Milky Way disk. Here we investigate a scenario where these waves were induced by the Sagittarius dwarf galaxy as it plunged through the Galaxy. Using numerical simulations, we find that the Sagittarius impact produces North-South asymmetries and vertical wave-like behavior that qualitatively agrees with what is observed. The extent to which vertical modes can radially penetrate into the disc, as well as their amplitudes, depend on the mass of the perturbing satellite. We show that the mean height of the disc is expected to vary more rapidly in the radial than in the azimuthal direction. If the observed vertical density asymmetry is indeed caused by vertical oscillations, we predict radial and azimuthal variations of the mean vertical velocity, correlating with the spatial structure. These variations can have amplitudes as large as 8 km/s.

PROBING THE ULTRAVIOLET LUMINOSITY FUNCTION OF THE EARLIEST GALAXIES WITH THE RENAISSANCE SIMULATIONS

In this paper, we present the first results from the Renaissance Simulations, a suite of extremely high-resolution and physics-rich AMR calculations of high-redshift galaxy formation performed on the Blue Waters supercomputer. These simulations contain hundreds of well-resolved galaxies at , and make several novel, testable predictions. Most critically, we show that the ultraviolet luminosity function of our simulated galaxies is consistent with observations of high-z galaxy populations at the bright end of the luminosity function , but at lower luminosities is essentially flat rather than rising steeply, as has been inferred by Schechter function fits to high-z observations, and has a clearly defined lower limit in UV luminosity. This behavior of the luminosity function is due to two factors: (i) the strong dependence of the star formation rate (SFR) on halo virial mass in our simulated galaxy population, with lower-mass halos having systematically lower SFRs and thus lower UV luminosities; and (ii) the fact that halos with virial masses below do not universally contain stars, with the fraction of halos containing stars dropping to zero at . Finally, we show that the brightest of our simulated galaxies may be visible to current and future ultra-deep space-based surveys, particularly if lensed regions are chosen for observation.

Signatures of minor mergers in Milky Way like disc kinematics: ringing revisited

By means of N-body simulations, we study the response of a galactic disc to a minor merger event. We find that non-self-gravitating, spiral-like features are induced in the thick disc. As we have shown in a previous work, this ‘ringing’ also leaves an imprint in velocity space (the u–v plane) in small spatial regions, such as the solar neighbourhood. As the disc relaxes after the event, clumps in the u–v plane get closer with time, allowing us to estimate the time of impact. In addition to confirming the possibility of this diagnostic, here we show that in a more realistic scenario, the infall trajectory of the perturber gives rise to an azimuthal dependence of the structure in phase space. We also find that the space defined by the energy and angular momentum of stars is a better choice than velocity space, as clumps remain visible even in large local volumes. This makes their observational detection much easier since one need not be restricted to a small spatial volume. We show that information about the time of impact, the mass of the perturber and its trajectory is stored in the kinematics of disc stars.

A Global Model for Circumgalactic and Cluster-core Precipitation

We provide an analytic framework for interpreting observations of multiphase circumgalactic gas that is heavily informed by recent numerical simulations of thermal instability and precipitation in cool-core galaxy clusters. We start by considering the local conditions required for the formation of multiphase gas via two different modes: (1) uplift of ambient gas by galactic outflows, and (2) condensation in a stratified stationary medium in which thermal balance is explicitly maintained. Analytic exploration of these two modes provides insights into the relationships between the local ratio of the cooling and freefall time scales (i.e., t_cool / t_ff), the large-scale gradient of specific entropy, and development of precipitation and multiphase media in circumgalactic gas. We then use these analytic findings to interpret recent simulations of circumgalactic gas in which global thermal balance is maintained. We show that long-lasting configurations of gas with 5 < t_cool / t_ff < 20 and radial entropy profiles similar to observations of local cool-core galaxy cluster cores are a natural outcome of precipitation-regulated feedback. We conclude with some observational predictions that follow from these models. This work focuses primarily on precipitation and AGN feedback in galaxy cluster cores, because that is where the observations of multiphase gas around galaxies are most complete. However, many of the physical principles that govern condensation in those environments apply to circumgalactic gas around galaxies of all masses.

GROWTH AND EVOLUTION OF THERMAL INSTABILITIES IN IDEALIZED GALAXY CLUSTER CORES

Heat input roughly balances radiative cooling in the gaseous cores of galaxy clusters even when the central cooling time is short, implying that cooling triggers a feedback loop that maintains thermal balance. Furthermore, cores with short cooling times tend to have multiphase structure, suggesting that the intracluster medium (ICM) becomes locally thermally unstable for cooling times < 1 Gyr. In this work, we use 2D and 3D hydrodynamic simulations to study the onset of condensation in idealized galaxy-cluster cores. In particular, we look at how the condensation process depends on the ratio of cooling time to freefall time and on the geometry of the gravitational potential. We conclude that the ICM can always evolve to a state in which condensation occurs if given enough time, but that an initial timescale ratio tcool /tff < 10 is needed for thermal instability to grow quickly enough to affect realistic cluster cores within a timescale that is relevant for cosmological structure formation. We find that instability leads to convection and that perturbations continue to grow while the gas convects. Condensation occurs when the timescale ratio in the low-entropy tail of the perturbation distribution drops below tcool /tff < 3, even if the volume-averaged timescale ratio is substantially greater. In our simulations, the geometry of the gravitational potential does not have a strong effect on thermal stability. Finally, we find that if condensation is powering feedback, a conversion efficiency of around 10^-3 for converting the condensed mass into thermal energy is sufficient to maintain thermal balance in the ICM.