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Cosmological Radiation Hydrodynamics with ENZO  [PDF]
Michael L. Norman,Daniel R. Reynolds,Geoffrey C. So
Physics , 2009, DOI: 10.1063/1.3250065
Abstract: We describe an extension of the cosmological hydrodynamics code ENZO to include the self-consistent transport of ionizing radiation modeled in the flux-limited diffusion approximation. A novel feature of our algorithm is a coupled implicit solution of radiation transport, ionization kinetics, and gas photoheating, making the timestepping for this portion of the calculation resolution independent. The implicit system is coupled to the explicit cosmological hydrodynamics through operator splitting and solved with scalable multigrid methods. We summarize the numerical method, present a verification test on cosmological Stromgren spheres, and then apply it to the problem of cosmological hydrogen reionization.
Chemical enrichment in cosmological, smoothed particle hydrodynamics simulations  [PDF]
Robert P. C. Wiersma,Joop Schaye,Tom Theuns,Claudio Dalla Vecchia,Luca Tornatore
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15331.x
Abstract: (Abridged) We present an implementation of stellar evolution and chemical feedback for smoothed particle hydrodynamics (SPH) simulations. We consider the timed release of individual elements by both massive (Type II supernovae and stellar winds) and intermediate mass stars (Type Ia supernovae and asymptotic giant branch stars). We illustrate the results of our method using a suite of cosmological simulations that include new prescriptions for radiative cooling, star formation, and galactic winds. Radiative cooling is implemented element-by-element, in the presence of an ionizing radiation background, and we track all 11 elements that contribute significantly to the radiative cooling. We contrast two reasonable definitions of the metallicity of a resolution element and find that while they agree for high metallicities, there are large differences at low metallicities. We argue the discrepancy is indicative of the lack of metal mixing caused by the fact that metals are stuck to particles. We argue that since this is a (numerical) sampling problem, solving it using a poorly constrained physical process such as diffusion could have undesired consequences. We demonstrate that the two metallicity definitions result in redshift z = 0 stellar masses that can differ by up to a factor of two, because of the sensitivity of the cooling rates to the elemental abundances. We find that by z = 0 most of the metals are locked up in stars. The gaseous metals are distributed over a very wide range of gas densities and temperatures. The shock-heated warm-hot intergalactic medium has a relatively high metallicity of ~ 10^-1 Z_sun that evolves only weakly and is therefore an important reservoir of metals.
Relativistic cosmological hydrodynamics  [PDF]
J. Hwang,H. Noh
Physics , 1997,
Abstract: We investigate the relativistic cosmological hydrodynamic perturbations. We present the general large scale solutions of the perturbation variables valid for the general sign of three space curvature, the cosmological constant, and generally evolving background equation of state. The large scale evolution is characterized by a conserved gauge invariant quantity which is the same as a perturbed potential (or three-space curvature) in the comoving gauge.
Using Cross-Correlations to Calibrate Lensing Source Redshift Distributions: Improving Cosmological Constraints from Upcoming Weak Lensing Surveys  [PDF]
Roland de Putter,Olivier Doré,Sudeep Das
Physics , 2013, DOI: 10.1088/0004-637X/780/2/185
Abstract: Cross-correlations between the galaxy number density in a lensing source sample and that in an overlapping spectroscopic sample can in principle be used to calibrate the lensing source redshift distribution. In this paper, we study in detail to what extent this cross-correlation method can mitigate loss of cosmological information in upcoming weak lensing surveys (combined with a CMB prior) due to lack of knowledge of the source distribution. We consider a scenario where photometric redshifts are available, and find that, unless the photometric redshift distribution p(z_{ph}|z) is calibrated very accurately a priori (bias and scatter known to ~0.002 for, e.g., EUCLID), the additional constraint on p(z_{ph}|z) from the cross correlation technique to a large extent restores the cosmological information originally lost due to the uncertainty in dn/dz(z). Considering only the gain in photo-z accuracy and not the additional cosmological information, enhancements of the dark energy figure of merit of up to a factor of 4 (40) can be achieved for a SuMIRe (Subaru Measurement of Images and Redshifts, the combination of the Hyper Suprime Cam lensing survey and the Prime Focus Spectrograph redshift survey)-like (EUCLID-like) combination of lensing and redshift surveys. However, the success of the method is strongly sensitive to our knowledge of the galaxy bias evolution in the source sample. If this bias is modeled by a free parameter in each of a large number of redshift bins, we find that a prior of order 0.01 is needed on b_i \sqrt{\Delta z} in each redshift slice (where \Delta z is the bin width and b_i the value of the galaxy bias in the i-th source bin) to optimize the gains from the cross-correlation method (i.e. to approach the cosmology constraints attainable if the bias were known exactly).[abridged]
EvoL: The new Padova T-SPH parallel code for cosmological simulations - I. Basic code: gravity and hydrodynamics  [PDF]
Emiliano Merlin,Umberto Buonomo,Tommaso Grassi,Lorenzo Piovan,Cesare Chiosi
Physics , 2009, DOI: 10.1051/0004-6361/200913514
Abstract: We present EvoL, the new release of the Padova N-body code for cosmological simulations of galaxy formation and evolution. In this paper, the basic Tree + SPH code is presented and analysed, together with an overview on the software architectures. EvoL is a flexible parallel Fortran95 code, specifically designed for simulations of cosmological structure formation on cluster, galactic and sub-galactic scales. EvoL is a fully Lagrangian self-adaptive code, based on the classical Oct-tree and on the Smoothed Particle Hydrodynamics algorithm. It includes special features such as adaptive softening lengths with correcting extra-terms, and modern formulations of SPH and artificial viscosity. It is designed to be run in parallel on multiple CPUs to optimize the performance and save computational time. We describe the code in detail, and present the results of a number of standard hydrodynamical tests.
RAMSES-RT: Radiation hydrodynamics in the cosmological context  [PDF]
Joakim Rosdahl,Jeremy Blaizot,Dominique Aubert,Timothy Stranex,Romain Teyssier
Physics , 2013, DOI: 10.1093/mnras/stt1722
Abstract: We present a new implementation of radiation hydrodynamics (RHD) in the adaptive mesh refinement (AMR) code RAMSES. The multi-group radiative transfer (RT) is performed on the AMR grid with a first-order Godunov method using the M1 closure for the Eddington tensor, and is coupled to the hydrodynamics via non-equilibrium thermochemistry of hydrogen and helium. This moment-based approach has the large advantage that the computational cost is independent of the number of radiative sources - it can even deal with continuous regions of emission such as bound-free emission from gas. As it is built directly into RAMSES, the RT takes natural advantage of the refinement and parallelization strategies already in place. Since we use an explicit advection solver for the radiative transport, the time step is restricted by the speed of light - a severe limitation that can be alleviated using the so--called "reduced speed of light" approximation. We propose a rigorous framework to assess the validity of this approximation in various conditions encountered in cosmology and galaxy formation. We finally perform with our newly developed code a complete suite of RHD tests, comparing our results to other RHD codes. The tests demonstrate that our code performs very well and is ideally suited for exploring the effect of radiation on current scenarios of structure and galaxy formation.
Cosmological Implications of a Possible Class of Particles Able to Travel Faster than Light (abridged version)  [PDF]
Luis Gonzalez-Mestres
Physics , 1995,
Abstract: Superluminal particles are not excluded by particle physics. The apparent Lorentz invariance of the laws of physics does not imply that space-time is indeed minkowskian. Matter made of solutions of Lorentz-invariant equations would feel a relativistic space-time even if the actual space-time had a quite different geometry (f.i. a galilean space-time). If Lorentz invariance is only a property of equations describing a sector of matter at a given scale, an absolute frame (the "vacuum rest frame") may exist without contradicting the minkowskian structure felt by ordinary particles. Then c , the speed of light, will not necessarily be the only critical speed in vacuum and superluminal sectors of matter may equally exist feeling space-times with critical speeds larger than c . We present a discussion of possible cosmological implications of such a scenario, assuming that the superluminal sectors couple weakly to ordinary matter. The universality of the equivalence between inertial and gravitational mass will be lost. The Big Bang scenario will undergo important modifications, and the evolution of the Universe may be strongly influenced by superluminal particles.
Cosmological Parameters and Hyper-Parameters: The Hubble Constant from Boomerang and Maxima  [PDF]
Ofer Lahav
Physics , 2000,
Abstract: We generalise the procedure for joint estimation of cosmological parameters to allow freedom in the relative weights of various probes. This is done by including in the joint Likelihood function a set of 'Hyper-Parameters', which are dealt with using Bayesian considerations. The resulting algorithm is simple to implement. We illustrate the method by estimating the Hubble constant H_0 from the recent Cosmic Microwave Background experiments Boomerang and Maxima. For an assumed flat Lambda-CDM model with fixed parameters (n=1, Omega_m = 1-lambda = 0.3, Omega_b h^2 = 0.03, Qrms = 18 mu K) we solve for a single parameter, H_0= 79 +- 4 km/sec/Mpc (95 % CL, random errors only), slightly higher but still consistent with recent results from Cepheids. We discuss how the 'Hyper-Parameters' approach can be generalised for a combination of cosmic probes, and for other priors on the Hyper-Parameters.
The specific star formation rate and stellar mass fraction of low-mass central galaxies in cosmological simulations  [PDF]
V. Avila-Reese,P. Colín,A. González-Samaniego,O. Valenzuela,C. Firmani,H. Velázquez,D. Ceverino
Physics , 2011, DOI: 10.1088/0004-637X/736/2/134
Abstract: (Abridged) By means of high-resolution cosmological simulations in the context of the LCDM scenario, the specific star formation rate (SSFR=SFR/Ms, Ms is the stellar mass)--Ms and stellar mass fraction (Fs=Ms/Mh, Mh is the halo mass)--Ms relations of low-mass galaxies (2.5< Mh/10^10 Msun <50 at redshift z=0) at different epochs are predicted. The Hydrodynamics ART code was used and some variations of the sub-grid parameters were explored. Most of simulated galaxies, specially those with the highest resolutions, have significant disk components and their structural and dynamical properties are in reasonable agreement with observations of sub-M* field galaxies. However, the SSFRs are 5-10 times smaller than the averages of several (compiled and homogenized here) observational determinations for field blue/star-forming galaxies at z<0.3 (at low masses, most of observed field galaxies are actually blue/star-forming). This inconsistency seems to remain even at z~1.5 though less drastic. The Fs of simulated galaxies increases with Mh as semi-empirical inferences show, but in absolute values the former are ~5-10 times larger than the latter at z=0; this difference increases probably to larger factors at z~1-1.5. The inconsistencies reported here imply that simulated low-mass galaxies (0.2
Self-Consistent Solution of Cosmological Radiation-Hydrodynamics and Chemical Ionization  [PDF]
Daniel R. Reynolds,John C. Hayes,Pascal Paschos,Michael L. Norman
Physics , 2009, DOI: 10.1016/j.jcp.2009.06.006
Abstract: We consider a PDE system comprising compressible hydrodynamics, flux-limited diffusion radiation transport and chemical ionization kinetics in a cosmologically-expanding universe. Under an operator-split framework, the cosmological hydrodynamics equations are solved through the Piecewise Parabolic Method, as implemented in the Enzo community hydrodynamics code. The remainder of the model, including radiation transport, chemical ionization kinetics, and gas energy feedback, form a stiff coupled PDE system, which we solve using a fully-implicit inexact Newton approach, and which forms the crux of this paper. The inner linear Newton systems are solved using a Schur complement formulation, and employ a multigrid-preconditioned conjugate gradient solver for the inner Schur systems. We describe this approach and provide results on a suite of test problems, demonstrating its accuracy, robustness, and scalability to very large problems.
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