Peter Anninos

Modeling primordial gas in numerical cosmology

Abstract We have reviewed the chemistry and cooling behavior of low-density (n ≲ 10 4 cm −3 ) primordial gas and devised a model which involves 19 collisional and 9 radiative processes and is applicable for temperatures in the range 1 K T 8 K. In a companion paper (Anninos et al., 1997)[NewA, 2, 209] numerical methods are presented that unify the modeling of non-equilibrium primordial gas chemistry and cooling dicussed here with cosmological hydrodynamics. We derived new fits of rate coefficients for the photo-attachment of neutral hydrogen, the formation of molecular hydrogen via H − , charge exchange beween H 2 and H + , electron detachment of H − by neutral hydrogen, dissociative recombination of H 2 + with slow electrons, photodissociation of H 2 + , and photodissociation of H 2 . Furthermore it was found that the molecular hydrogen produced through the gas-phase processes, H 2 + + H → H 2 + H + , and H − + H → H 2 + e − , is likely to be converted into its para configuration on a faster time scale than the formation time. We have tested the model extensively and shown it to agree well with former studies. We further studied the chemical kinetics in great detail and devised a minimal model which is substantially simpler than the full reaction network but predicts correct abundances. This minimal model shows convincingly that 12 collisional processes are sufficient to model the H, He, H + , H − , He + , He ++ , and H 2 abundances in low density primordial gas for applications with no radiation fields.

A Multispecies Model for Hydrogen and Helium Absorbers in Lyman-Alpha Forest Clouds

We have performed a three-dimensional multispecies hydrodynamical simulation of the formation and evolution of Ly? clouds in a flat cold dark matter (CDM) dominated universe with an external flux of ionizing radiation. We solve the fully coupled nonequilibrium rate equations for the following species: H, H+, H-, H2, H -->2+, He, He+, He++, and e-. The statistical properties of the distribution and evolution of both hydrogen and helium absorption lines are extracted and compared to observed data. We find excellent agreement for the following neutral hydrogen data: the distribution of column densities is fitted well by a power law with exponent ? = 1.55, with a possible deficiency of lines above column density 1015 cm-2; the integrated distribution matches observed data over a broad range of column densities, 1013-1017 cm-2; a Gaussian statistical fit to the Doppler parameter distribution yields a median of 35.6 km s-1; the evolution of the number of clouds with column densities larger than 1014 cm-2 follows a power law with exponent ? = 2.22. Analogous calculations are presented for He II absorption lines, and we find the ratio of Doppler parameters bHe II/bH I = 0.87. Our data also suggest that Ly? clouds may belong to two morphologically different groups: small column density clouds which tend to reside in sheets or filamental structures and are very elongated and/or flattened, and the large column density clouds which are typically found at the intersections of these caustic structures and are quasi-spherical.

Cosmological hydrodynamics with multi-species chemistry and nonequilibrium ionization and cooling

Abstract We have developed a method of solving for multi-species chemical reaction flows in nonequilibrium and self-consistently with the hydrodynamic equations in an expanding FLRW universe. The method is based on a backward differencing scheme for the required stability when solving stiff sets of equations and is designed to be efficient for three-dimensional calculations without sacrificing accuracy. In all, 28 kinetic reactions are solved including both collisional and radiative processes for the following nine separate species: H, H + , He, He + , He ++ , H − , H 2 + , H 2 , and e − . The method identifies those reactions (involving H − and H 2 + ) occurring on the shortest time scales, decoupling them from the rest of the network and imposing equilibrium concentrations to good accuracy over typical cosmological dynamical times. Several tests of our code are presented, including radiative shock waves, cosmological sheets, conservation constraints, and fully three-dimensional simulations of CDM cosmological evolutions in which we compare our method to results obtained when the packaged routine LSODAR is substituted for our algorithms.

Global General Relativistic Magnetohydrodynamic Simulation of a Tilted Black Hole Accretion Disk

This paper presents a continuation of our efforts to numerically study accretion disks that are misaligned (tilted) with respect to the rotation axis of a Kerr black hole. Here we present results of a global numerical simulation which fully incorporates the effects of the black hole spacetime as well as magnetorotational turbulence that is the primary source of angular momentum transport in the flow. This simulation shows dramatic differences from comparable simulations of untilted disks. Accretion onto the hole occurs predominantly through two opposing plunging streams that start from high latitudes with respect to both the black hole and disk midplanes. This is due to the aspherical nature of the gravitational spacetime around the rotating black hole. These plunging streams start from a larger radius than would be expected for an untilted disk. In this regard, the tilted black hole effectively acts like an untilted black hole of lesser spin. Throughout the duration of the simulation, the main body of the disk remains tilted with respect to the symmetry plane of the black hole; thus, there is no indication of a Bardeen-Petterson effect in the disk at large. The torque of the black hole instead principally causes a global precession of the main disk body. In this simulation, the precession has a frequency of 3(M☉/M) Hz, a value consistent with many observed low-frequency quasi-periodic oscillations. However, this value is strongly dependent on the size of the disk, so this frequency can be expected to vary over a large range.

First Structure Formation. I. Primordial Star-forming Regions in Hierarchical Models

We investigate the formation of the first primordial star clusters from high-σ perturbations in a cold dark matter-dominated universe. For this purpose, we have developed a powerful two-level hierarchical cosmological code with a realistic and robust treatment of multispecies primordial gas chemistry, paying special attention to the formation and destruction of H2 molecules, nonequilibrium ionization, and cooling processes. We performed three-dimensional simulations at small scales and at high redshifts and find that, analogous to simulations of large-scale structure, a complex system of voids, filaments, sheets, and spherical knots form at the intersections of filaments. On the total mass scales covered by our simulations (1 × 105 to 1 × 109 M☉), which collapse at redshifts z > 25, we find that only within the spherical knots can enough H2 be formed (n

Spectral Analysis of the Lyα Forest in a Cold Dark Matter Cosmology

{r H}2

Cosmos++ : Relativistic magnetohydrodynamics on unstructured grids with local adaptive refinement

-->/n

Collision of two black holes

{r H}

Hydrodynamic Simulations of Tilted Thick-Disk Accretion onto a Kerr Black Hole

-->5×10 -->−4) to cool the gas appreciably. The time dependence of the formation of H2 molecules and the final H2 fraction in the simulations agree with the theoretical predictions of Abel and Tegmark et al. remarkably well. Using a different H2 cooling function (that of Lepp & Shull), we repeat the calculations of Tegmark et al. We find a minimum mass that is able to collapse and cool via H2 for a given redshift that is an order of magnitude lower than that found by Tegmark et al. Furthermore, we discuss the possible implications for theories of primordial star formation from the extensive merging of small structure inherent in hierarchical models. In our simulation, typically only 5%-8% percent of the total baryonic mass in the collapsing structures is found to cool significantly. Assuming the Padoan model for star formation, our results would predict the very first stellar systems to be as small as ~50 M☉. Some implications for primordial globular cluster formation scenarios are also discussed.

THREE-DIMENSIONAL MOVING-MESH SIMULATIONS OF GALACTIC CENTER CLOUD G2

We simulate the Lyα forest in a standard cold dark matter universe using a two-level hierarchical grid code to evolve the dark and baryonic matter components self-consistently. We solve the time-dependent ionization equations for hydrogen and helium, adopting Haardt & Madaus recent estimate for the metagalactic UV radiation background. We compare our simulation results with the measured properties of the Lyα forest by constructing synthetic spectra and analyzing them using an automated procedure to identify, deblend, and fit Voigt line profiles to the absorption features. The H I column density and Doppler parameter distributions that we obtain agree closely with those measured by the Keck Observatorys High Resolution Eschelle Spectrograph (HIRES) and earlier high spectral resolution observations over the column density range 1012 cm-2 3.5). We also compare with measured values of the intergalactic He II opacity. Our results require an He II ionizing background lower than the Haardt & Madau estimate by a factor of 4, corresponding to a soft intrinsic QSO spectrum, with αQ ≈ 1.8-2.