This page contains information about the cosmic void catalogs constructed from SDSS III data. The catalogs were constructed by Qingqing Mao at Vanderbilt University, using the ZOBOV void finding algorithm. The current void catalogs use the Baryon Oscillation Spectroscopic Survey (BOSS) DR12. All the catalog files can be downloaded here. Read below for a general description of the algorithm and the file formats for the catalogs. The catalogs are also described in detail in Mao et al. (2017). If you have any questions, please contact the authors.
We construct our void catalog using the ZOBOV void finding algorithm developed by Mark Neyrinck. The detailed description of the algorithm itself can be found in Neyrinck (2008), as well as on the ZOBOV website.
In short, ZOBOV uses Voronoi tessellation to estimate densities for all galaxies, and then uses the 'watershed' concept to connect low density regions in order to find voids and sub-voids.
We assume a flat ΛCDM universe with Ωm=0.3 to convert redshifts into comoving distances for all galaxies. We then convert (RA, Dec, R) into Cartesian coordinates (x, y, z) and run ZOBOV on that.
To deal with the survey boundaries, we first make a buffer of randomly distributed points to fill all the "edges" and "holes" in the survey data. Using a buffer allows the Voronoi tessellation to work properly. After the tessellation process, the densities of the buffer are set to be artificially high, so that the buffer points are never part of the voids. ZOBOV can distinguish between the buffer and the real data properly. Using a buffer also helps to distinguish whether a void is fully embedded in the survey volume or is truncated by the survey geometry.
After the tessellation and before the void finding process, all densities (i.e. Voronoi volumes) are weighted using galaxy weights. Weights account for close pair fiber collisions, redshift failures, systematic weights, as well as sector completeness.
Though ZOBOV can be parameter free, there is an optional density threshold parameter, which can limit the growth of voids into high-density regions. (See the ZOBOV help page for more information.) We provide catalogs with the density threshold parameter set to 0.5. If other parameters are preferred, please contact the authors.
We parse the ZOBOV output and construct the catalog. For each void, we define the center of the void as the Voronoi volume-weighted average position of galaxies that belong to the void.
ZOBOV identifies all the local minima, which means that the result includes "false voids" in high density regions. A "probability" cut is necessary to ensure the significant detection of voids (refer to the ZOBOV paper for details). We then only keep voids that have higher than 2-sigma significance, and also have a core density lower than 0.5 times the mean density, where the core density is defined as the density of the lowest density galaxy in the void. Note that the choice of (0.5*mean density) is arbitrary, and stricter cuts can be applied easily. We also provide an uncut version of the catalogs that does not include these two quality cuts.
Raw ZOBOV outputs can be provided upon request.
We construct our void catalog in four survey regions separately: CMASS North/South and LOWZ North/South. For each region there are several result files. Files from different regions can easily be identified by the file names. For each survey region, there is a set of *.dat files that contain the quality cuts described above, as well as a set of *_uncut.dat files that contain no quality cuts. Here we describe the format of the available files.
These files contain a list of voids, where each row includes the properties of one void. The columns are:
A file listing the galaxies that belong to each void. Each row contains the Void ID, Number of galaxies in the void, and a list of galaxy IDs. These IDs can be used to match the galaxies in the volume files "*.volumes.dat", which are all in orders starting from 0. Notice that each line has a different number of columns. You can first read the number of members in column (2) to know how many to expect. Columns are:
Void ID, Number of member galaxies, galaxy 1 ID, galaxy 2 ID, .....
A file listing the Voronoi volumes for individual galaxies, as well as galaxy positions. The Voronoi volumes have been weighted by the galaxy weights. Galaxy IDs are the IDs in the original galaxy catalog fits files. If the Voronoi volume is zero, it means that this galaxy is next to the edge of the survey geometry and its Voronoi volume is unreliable. Columns are:
Galaxy ID, RA, Dec, Distance, Voronoi volume