# FULLY COUPLED SIMULATION OF COSMIC REIONIZATION. I. NUMERICAL METHODS AND TESTS

## Abstract

We describe an extension of the Enzo code to enable fully coupled radiation hydrodynamical simulation of inhomogeneous reionization in large ∼(100 Mpc){sup 3} cosmological volumes with thousands to millions of point sources. We solve all dynamical, radiative transfer, thermal, and ionization processes self-consistently on the same mesh, as opposed to a postprocessing approach which coarse-grains the radiative transfer. We do, however, employ a simple subgrid model for star formation which we calibrate to observations. The numerical method presented is a modification of an earlier method presented in Reynolds et al. differing principally in the operator splitting algorithm we use to advance the system of equations. Radiation transport is done in the gray flux-limited diffusion (FLD) approximation, which is solved by implicit time integration split off from the gas energy and ionization equations, which are solved separately. This results in a faster and more robust scheme for cosmological applications compared to the earlier method. The FLD equation is solved using the hypre optimally scalable geometric multigrid solver from LLNL. By treating the ionizing radiation as a grid field as opposed to rays, our method is scalable with respect to the number of ionizing sources, limited only by the parallel scaling properties ofmore »

- Authors:

- CASS, University of California, San Diego, 9500 Gilman Drive La Jolla, CA 92093-0424 (United States)
- Southern Methodist University, 6425 Boaz Lane, Dallas, TX 75205 (United States)
- SDSC, University of California, San Diego, 9500 Gilman Drive La Jolla, CA 92093-0505 (United States)
- Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street, Atlanta, GA 30332 (United States)

- Publication Date:

- OSTI Identifier:
- 22340107

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: Astrophysical Journal, Supplement Series; Journal Volume: 216; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCURACY; COSMOLOGY; DIFFUSION; EQUATIONS; GEOMETRY; HYDRODYNAMICS; IONIZATION; LAWRENCE LIVERMORE NATIONAL LABORATORY; MASS; NONLUMINOUS MATTER; POINT SOURCES; RADIANT HEAT TRANSFER; RADIATION TRANSPORT; REYNOLDS NUMBER; SIMULATION; STARS; VALIDATION

### Citation Formats

```
Norman, Michael L., So, Geoffrey C., Reynolds, Daniel R., Harkness, Robert P., and Wise, John H.
```*FULLY COUPLED SIMULATION OF COSMIC REIONIZATION. I. NUMERICAL METHODS AND TESTS*. United States: N. p., 2015.
Web. doi:10.1088/0067-0049/216/1/16.

```
Norman, Michael L., So, Geoffrey C., Reynolds, Daniel R., Harkness, Robert P., & Wise, John H.
```*FULLY COUPLED SIMULATION OF COSMIC REIONIZATION. I. NUMERICAL METHODS AND TESTS*. United States. doi:10.1088/0067-0049/216/1/16.

```
Norman, Michael L., So, Geoffrey C., Reynolds, Daniel R., Harkness, Robert P., and Wise, John H. Thu .
"FULLY COUPLED SIMULATION OF COSMIC REIONIZATION. I. NUMERICAL METHODS AND TESTS". United States.
doi:10.1088/0067-0049/216/1/16.
```

```
@article{osti_22340107,
```

title = {FULLY COUPLED SIMULATION OF COSMIC REIONIZATION. I. NUMERICAL METHODS AND TESTS},

author = {Norman, Michael L. and So, Geoffrey C. and Reynolds, Daniel R. and Harkness, Robert P. and Wise, John H.},

abstractNote = {We describe an extension of the Enzo code to enable fully coupled radiation hydrodynamical simulation of inhomogeneous reionization in large ∼(100 Mpc){sup 3} cosmological volumes with thousands to millions of point sources. We solve all dynamical, radiative transfer, thermal, and ionization processes self-consistently on the same mesh, as opposed to a postprocessing approach which coarse-grains the radiative transfer. We do, however, employ a simple subgrid model for star formation which we calibrate to observations. The numerical method presented is a modification of an earlier method presented in Reynolds et al. differing principally in the operator splitting algorithm we use to advance the system of equations. Radiation transport is done in the gray flux-limited diffusion (FLD) approximation, which is solved by implicit time integration split off from the gas energy and ionization equations, which are solved separately. This results in a faster and more robust scheme for cosmological applications compared to the earlier method. The FLD equation is solved using the hypre optimally scalable geometric multigrid solver from LLNL. By treating the ionizing radiation as a grid field as opposed to rays, our method is scalable with respect to the number of ionizing sources, limited only by the parallel scaling properties of the radiation solver. We test the speed and accuracy of our approach on a number of standard verification and validation tests. We show by direct comparison with Enzo's adaptive ray tracing method Moray that the well-known inability of FLD to cast a shadow behind opaque clouds has a minor effect on the evolution of ionized volume and mass fractions in a reionization simulation validation test. We illustrate an application of our method to the problem of inhomogeneous reionization in a 80 Mpc comoving box resolved with 3200{sup 3} Eulerian grid cells and dark matter particles.},

doi = {10.1088/0067-0049/216/1/16},

journal = {Astrophysical Journal, Supplement Series},

number = 1,

volume = 216,

place = {United States},

year = {Thu Jan 01 00:00:00 EST 2015},

month = {Thu Jan 01 00:00:00 EST 2015}

}