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Monthly Notices of the Royal Astronomical Society

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 one, two, and three 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.

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Enzo: An Adaptive Mesh Refinement Code for Astrophysics

In the current paradigm there is a non-trivial bias expected in the process of galaxy formation. Thus, the observed statistical properties of the galaxy distribution do not necessarily extend to the underlying matter distribution. Gravitational evolution of initially Gaussian seed fluctuations predicts that the connected moments of the matter fluctuations exhibit a hierarchical structure, at least in the limit of small dispersion. This same hierarchical structure has been found in the galaxy distribution, but it is not clear to what extent it reflects properties of the matter distribution or properties of a galaxy formation bias. In this paper we consider the consequences of an arbitrary, effectively local biasing transformation of a hierarchical underlying matter distribution. We show that a general form of such a transformation preserves the hierarchical properties and the shape of the dispersion in the limit of small fluctuations, i.e. on large scales, although the values of the

Biasing and Hierarchical Statistics in Large-scale Structure

If gravitational clustering is a hierarchical process, the present large-scale structure of the galaxy distribution implies that structures on smaller scales must have formed at high redshift. We simulate the formation of small-scale structure (average cell mass: $\Delta \bar m_b=10^{4.2}$M$_\odot$) and the evolution of photoionized gas, in the specific case of a CDM model with a cosmological constant. The photoionized gas has a natural minimal scale of collapse, the Jeans scale ($m_{b,J}\simeq 10^{9}$M$_\odot$). We find that low column density ($\nhi \le 10^{14}\cm^{-2}$) lines originate in regions resembling Zel'dovich pancakes, where gas with overdensities in the range $3 - 30$ is enclosed by two shocks but is typically re-expanding at approximately the Hubble velocity. However, higher column density ($\nhi \ge 10^{15}\cm^{-2}$) lines stem from more overdense regions where the shocked gas is cooling. We show that this model can probably account for the observed number of lines, their distribution in column density and b-parameters, as well as the cloud physical sizes as observed in gravitationally lensed quasars. We find a redshift evolution that is

Gravitational Collapse of Small-Scale Structure as the Origin of the Lyman Alpha Forest

Estimates of the metagalactic hydrogen ionization rate from the Lyman-alpha forest opacity in QSO absorption spectra depend on the complex density distribution of neutral hydrogen along the line-of-sight. We use a large suite of high resolution hydrodynamical simulations to investigate in detail the dependence of such estimates on physical and numerical parameters in the context of Lambda-CDM models. Adopting fiducial values for cosmological parameters together with published values of the temperature of the IGM and the effective optical depth, the metagalactic ionization rates which reproduce the Lyman-alpha effective optical depth at z=[2,3,4] are Gamma_HI=[1.3\pm^0.8_0.5, 0.9\pm0.3, 1.0\pm^0.5_0.3] \times 10^-12 s^-1, respectively. The errors include estimates of uncertainties in the relevant physical parameters and the numerical accuracy of the simulations. We find the errors are dominated by the uncertainty in the temperature of the low-density IGM. The estimated metagalactic hydrogen ionization rate for the neutral hydrogen distribution in the current concordance Lambda-CDM model is more than four times the value inferred for that in an Einstein-de Sitter model of the same r.m.s. density fluctuation amplitude sigma_8. The estimated ionization rate is also more than double that expected from updated estimates of the emissivity of observed QSOs alone. A substantial contribution from galaxies appears to be required at all redshifts.

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The Lyman-alpha forest opacity and the metagalactic hydrogen ionization rate at z~2-4

We present redshift space two-point ($\xi$), three-point ($\zeta$) and reduced three-point (Q) correlation of Ly$\alpha$ absorbers (i.e Voigt profile components having HI column density $N_{HI}>10^{13.5}$cm$^{-2}$) over three redshift bins spanning $1.7<z<3.5$ using high resolution spectra of 292 quasars. We detect positive $\xi$ up to 8$h^{-1}$cMpc in all three redshift bins. The strongest detection of $\zeta$ is seen in $z=1.7-2.3$ redshift bin at $1-2h^{-1}$cMpc with an amplitude of $1.81\pm0.59$ ($\sim3.1\sigma$ level). The corresponding Q is found to be $0.68\pm0.23$. The measured $\xi$ and $\zeta$ values show an increasing trend with $N_{HI}$, while Q remains relatively independent of $N_{HI}$. We find $\xi$ and $\zeta$ to evolve strongly with $z$ over the redshift range studied. Using hydrodynamical simulations, we find that the $\xi$ and especially $\zeta$ seen in real space may be strongly amplified by peculiar velocities in redshift space. Simulations also suggest that while feedback, thermal and pressure smoothing effects affect the clustering of Ly$\alpha$ absorbers at small scales, i.e $<0.5h^{-1}$cMpc, the HI photo-ionization rates ($\Gamma_{HI}$) strongly influence the correlation amplitudes at all scales. We find that the strong redshift evolution shown by $\xi$ and $\zeta$ is primarily sourced by the redshift evolution of the relationship between $N_{HI}$ and baryon overdensity ($\Delta$). Our simulations that uses available best fitted $\Gamma_{HI}(z)$ measurements produce consistent clustering signals with observations at $z\sim2$ but under-predict the clustering at higher redshifts. One possible remedy is to have higher values of $\Gamma_{HI}$ at higher redshifts compared to the existing measurements. Alternatively the discrepancy could be related to non-equilibrium and inhomogeneous conditions prevailing during HeII reionization not captured by our simulations.

Measurement of redshift space two- and three-point correlation of Lyα absorbers at 1.7 < z < 3.5: Implications on evolution of the physical properties of IGM

Prakash Gaikwad

Papers

Measurement of redshift space two- and three-point correlation of Lyα absorbers at 1.7 < z < 3.5: Implications on evolution of the physical properties of IGM