News and Updates

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DR10 void catalog and VIDE coming soon

posted Apr 23, 2014, 11:31 AM by Paul M. Sutter

We plan to release a DR10 void catalog soon, and this summer we will release VIDE (Void IDentificaiton and Examination), which is our extension of ZOBOV as well as a void analysis tool suite. Stay tuned!

SDSS DR9 void catalog released!

posted Oct 28, 2013, 7:31 PM by Paul M. Sutter

We are happy to announce the release of an updated catalog which includes voids identified in SDSS DR9 as well as several mock galaxy populations. Download the latest catalog here and be sure to read our submitted journal article.

A response to Nadathur & Hotchkiss (2013)

posted Oct 23, 2013, 11:06 AM by Paul M. Sutter

Recently, Nadathur & Hotchkiss (2013) submitted a paper discussing a new cosmic void catalog. This paper includes claims about the void catalog described in Sutter et al. (2012). We have posted a note in which we respond to those claims, clarify some discrepancies between the text of Sutter et al. (2012) and the most recent version of the catalog, and provide some comments on the differences between our catalog and that of Nadathur & Hotchkiss (2013).

a guide to filtering the catalog

posted Oct 14, 2013, 3:44 AM by Paul M. Sutter   [ updated Oct 14, 2013, 4:06 AM ]

We have developed a philosophy about this void catalog in which we attempt to report as many voids as possible and let you, the user, apply cuts and filters as you see appropriate. We have a set of filters that we apply for our own analysis, and for historical reasons this has become the "default" catalog. We also provide the full unfiltered catalog (except for a hard minimum size cut, to be discussed below) so you can create your own version of the catalog. 

Some cuts you may consider applying:

- Size: We only provide voids with effective radii greater than the mean particle separation. However, the smallest voids are likely highly unreliable. Thus, you may want to only take voids with two times the minimum radius. You can find the radius in the "centers_" files.

- Minimum density: The watershed algorithm reports all basins, even if they have densities higher than you may like. You can, for example, only take voids with minimum Voronoi cell density < 0.2 or 0.3 times the mean density (as in Nadathur et al. 2013). Since minimum density scales inversely with void size, this also effectively removes most small voids. However, this quantity is very noisy, so your cuts may be more/less aggressive than you think. The minimum density can be found in the "voidDesc_" files. Since this is a raw ZOBOV output, it is expressed in units normalized to the total number of particles, including boundary particles. So you can either a) correct for the presence of boundary particles by rescaling by a factor of (n_g/n_tot)^3, or b) just look at the distribution of minimum densities and make a cut as you see fit. With the current version of the catalog n_g (number of galaxies) and n_tot (number of galaxies + boundary particles) are a bit difficult (but not impossible) to extract. We will provide these numbers more clearly in the next catalog release.

- Central density: This is similar in spirit to the minimum density above, but is typically computed in spheres about the barycenter. For the default catalog, we take spheres of 1/4 the void effective radius. This tends to affect larger voids, and becomes less effective for smaller voids (since we start to probe below the mean particle separation). If you want to calculate this on your own, then you have to use the dumpVoidParticles tool to extract the member particles.

- Density contrast: This is the ratio between the density on the void ridgeline and the minimum density. You can compare this to, say, the probability of getting that density ratio in a Poisson distribution (Neyrinck 2008), but this procedure is not necessarily appropriate for sparse, biased populations. You can find this quantity in both the voidDesc_ and centers_ files.

- Tree level: There is a full hierarchy of voids, subvoids, sub-subvoids, etc.. You may wish to take only top-level "parent" voids, or only leaf-level "children" voids. The default catalog contains only parent voids. You can find the tree level in the centers_ files.

Our default catalog contains only top-level parent voids with central densities < 0.2 times the mean density. If you are happy with these cuts, then you can just use the un-prefixed catalog.

If you want to make your own cuts, then start with the "untrimmed_" and help yourself. 

In addition, we also have the "all" and "central" catalog division. 

We would like to thank Seshadri Nadathur; our conversations with him prompted the creation of this post.

The text above will be added to the catalog README with the next release.

a note about density thresholds

posted Oct 9, 2013, 8:02 AM by Paul M. Sutter   [ updated Oct 10, 2013, 5:47 AM ]

We would like to thank Mark Neyrinck, Seshadri Nadathur, and Shaun Hotchkiss for pointing out an inconsistency regarding density thresholds between the text of our Sutter et al. (2012) paper and the actual implementation. In the text, we state that we impose an overall density threshold to limit voids to less than 0.2 the mean density (rho_mean).

What is actually implemented is a 0.2*rho_mean criterion for merging zones (here, a "zone" is a group of Voronoi cells sharing a common basin) together in the process of creating larger parent voids, as described in Neryrinck (2008). Zones are only added to a void if the minimum density in that zone is < 0.2*rho_mean. This corresponds to a value of 0.2 (suitably adjusted to account for the presence of boundary particles) as the last argument to the jovoz code. Please see the ZOBOV manual page for more details.

Thus, voids presented in our catalog will be overdense on average, since the watershed algorithm naturally includes high-density edge galaxies in the void definition. Also, even the minimum density of a void can be greater than 0.2*rho_mean, since voids can (and do) consist of only a single zone, and this criterion is not applied until the zone-merging step. This, again, is discussed in Neyrinck (2008). However, as we discuss in Sutter et al. (2012), the centers of our identified voids are still underdense. 

This becomes much more severe as the void size approaches the mean particle separation, since small voids tend to be surrounded by very large overcompensated regions. The philosophy of this catalog is to produce as many voids as possible and allow users to make their own cuts as they see fit. As we have before, we strongly urge caution in the use of the smallest reported voids!

We apologize for this misunderstanding, and look forward to further improvements of our catalog.

Galaxy samples posted

posted Aug 12, 2013, 2:15 PM by Paul M. Sutter

After numerous requests, we have posted the volume-limited galaxy samples used to construct the public void catalog. See the "Public Catalogs" tab for the link.

Coming soon: new public release

posted Jun 5, 2013, 3:45 AM by Paul M. Sutter

This summer, we will make publicly available our void catalogs from SDSS DR9 as well as many simulations. Stay tuned! 

Updated private catalog

posted Apr 24, 2013, 7:59 PM by Paul M. Sutter

We have updated the private catalog to include all the latest bug fixes, improvements, and additions available in the public catalog.

new catalog - now with ellipticities!

posted Feb 21, 2013, 1:09 PM by Paul M. Sutter

We have released a new catalog with a bug fix and several new additions:
  • The central density cut was not applied to the catalogs due to a bug. Thus the "centers_" and "centers_nocut_" files were the same. We have corrected this.
  • We have used a higher resolution HEALpix map for calculating the area of the survey mask and hence have more precise estimates of the volume and mean galaxy separation of the sample. The mean separation for all Main Sample volume-limited samples is now lower, so there are correspondingly more voids. But please remember to use the smallest voids with caution and at your own risk.
  • We now include highly preliminary ellipticity information - the eigenvalues and eigenvectors of the moment of inertia tensor for each void is in the "shapes_" file. 

catalog reconciliation

posted Feb 4, 2013, 7:08 AM by Paul M. Sutter   [ updated Feb 4, 2013, 7:13 AM ]

There have been a few questions raised recently about apparent inconsistencies among the three (July, August, and November) releases. We would like to take this opportunity to explain the differences in more detail. 

If you are not already doing so, please use the November release!

The catalog inconsistencies can be explained by:

1) A lower radius threshold for including voids in the November release. Previously, it was two times the mean galaxy separation. We now include voids all the way down to the mean galaxy separation. Also note that we miscalculated the mean galaxy separation for the dim1 sample; this has been corrected. Use the smallest voids at your own risk; you may wish to impose an arbitrary cutoff (e.g., 2*r_min or 4*r_min) for your analysis.

2) Different handling of the high-redshift cap of each subsample. In July and August, we were placing mock particles along the cap and treating voids there just like voids at the mask boundaries (i.e., these would appear as "edge" voids). However, this procedure was giving us difficulty at higher redshift, so we switched to a simpler approach: we no longer place mock particles at the redshift cap and remove all voids that might intersect the upper redshift boundary of the subsample.
 
3) A bug fix in the central density calculation. Previously, all voids were using a fixed radius to determine the central density. This is not what we intended. We fixed this to calculate central densities using 1/4 of the effective radius, as we state in the paper. So some voids that were excluded are now included and vice-versa.

4) The August release (but not the July release) contained a bug in the barycenter calculation. Centers were still inside the voids but slightly offset from the true barycenter. This has been fixed for November. 

5) There was a bug in the catalog release pipeline script which switched the "central" voids with the "edge" voids. This has been corrected for November: "central" voids are now actually central voids!

Changes 1) and 2) generally explain the different void populations in the "all" sample. The redshift cap handling especially affected the lrgbright sample. If you were using the "all" sample, then change 5) didn't affect you. However, if you are using the "central" sample, then you will see a completely different catalog in November.

Thank you for your understanding!

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