# JohannesBuchner / LightRayRider

Ray tracing of hydrodynamic simulations to compute column densities

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# LightRayRider

A fast library for calculating intersections of a line with many spheres or inhomogeneous material.

## Introduction

Technically speaking, the library performs three-dimensional line integrals through various geometric bodies, such as millions of spheres of various sizes, three-dimensional uniform grids, and arbitrary density fields approximated by their point densities and voronoi tesselation.

In astrophysics, a point source can be obscured by a gas distribution along the line-of-sight. For hydrodynamic simulations which produce such a gas distribution, this code can compute the total density along a arbitrary ray. The output is a column density, also known as N_H if hydrogen gas is irradiated.

## Line/Sphere cutting

Input:

• Points in space representing sphere centres.
• One or more arbitrary lines from the origin.

Output:

• This computes the total length / column density cut.
• From distance 0 or another chosen minimal distance (or multiple).

Method:

## Voronoi cutting

Input:

• Points in space.
• Densities.
• One or more arbitrary lines from the origin

Output:

• This computes the total length along the line, where every point on the line is assigned the density from the nearest point (Voronoi segmentation).
• From distance 0 or another chosen minimal distance (or multiple).

Method:

• Segmentation of the line where points become equi-distant. Performs approximately linearly with number of points.

## Grid cutting

Input:

• 3D Grid with densities at each location
• One or more arbitrary lines from a point

Output:

• This computes the total length along the line, where every point on the line is assigned the density from the grid cell it passes through.
• From distance 0 or another chosen minimal distance (or multiple).

Method:

• Finding intersections of the cell borders (planes) with the lines, and checking which cell to consider next.

## Usage

Compile the c library first with:

```\$ make
```

To use from Python, use raytrace.py:

```from raytrace import *
```

You can find the declaration of how to call these functions in raytrace.py. Basically, you pass the coordinates of your gas particles, the associated densities and the starting point and direction of your raytracing.

Example usage is demonstrated in irradiate.py. This was used for Illustris and EAGLE particle in the associated paper. There are additional unit test examples in test/.

## Parallel processing

LightRayRider supports multiple processors through OpenMP. Set the variable OMP_NUM_THREADS to the number of processors you want to use, and the parallel library ray-parallel.so will be loaded.

If you use this code, please cite "Galaxy gas as obscurer: II. Separating the galaxy-scale and nuclear obscurers of Active Galactic Nuclei", by Buchner & Bauer (2017), https://arxiv.org/abs/1610.09380

Bibcode:

```@ARTICLE{2017MNRAS.465.4348B,
author = {{Buchner}, J. and {Bauer}, F.~E.},
title = "{Galaxy gas as obscurer - II. Separating the galaxy-scale and nuclear obscurers of active galactic nuclei}",
journal = {\mnras},
archivePrefix = "arXiv",
eprint = {1610.09380},
primaryClass = "astro-ph.HE",
keywords = {dust, extinction, ISM: general, galaxies: active, galaxies: general, galaxies: ISM, X-rays: ISM},
year = 2017,
month = mar,
volume = 465,
pages = {4348-4362},
doi = {10.1093/mnras/stw2955},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}
```

Ray tracing of hydrodynamic simulations to compute column densities