Measuring Q 0 from the Distortion of Voids in Redshift Space
Abstract
Because the transformation from distance to redshift is nonlinear, maps in redshift space become increasingly distorted as the redshift z becomes greater. As noted by Alcock & Paczyn'ski, observed redshift distortions can be used to estimate the deceleration parameter q_{0}. If q_{0} is greater than 1, voids in redshift space will be elongated along the line of sight. In addition, distant voids will have a greater volume in redshift space than nearby voids. Accurate measurement of the volume and the axis ratio of voids, as a function of their central redshift will provide an estimate of q_{0}.
To test this method of estimating q_{0}, I create a twodimensional toy universe, free of peculiar velocities, in which the galaxies are located near the walls of Voronoi cells. The galaxies are then mapped into redshift space by adopting different values of q_{0}. In redshift space, I estimate the area and the axis ratio of the voids by fitting ellipses within the voids, and by using an algorithm which maximizes the area of the empty ellipses and ensures that ellipses do not overlap. The accuracy of the estimated values of q_{0} is limited by the intrinsic scatter in the size and shape of the voids. In the toy universe, distinguishing between a q_{0} = 1 universe and a q_{0} = ½ universe requires a survey which goes to a depth z ≳ 0.1 in redshift space. Peculiar velocities will create an additional source of uncertainty for the values of q_{0} measured in Nbody simulations and in the real universe.
 Publication:

The Astrophysical Journal
 Pub Date:
 October 1995
 DOI:
 10.1086/176277
 arXiv:
 arXiv:astroph/9506028
 Bibcode:
 1995ApJ...452...25R
 Keywords:

 COSMOLOGY: LARGESCALE STRUCTURE OF UNIVERSE;
 COSMOLOGY: THEORY;
 GALAXIES: DISTANCES AND REDSHIFTS;
 Astrophysics
 EPrint:
 to appear in the ApJ. 14 pages uuencoded compressed postscript Seven figures available as a compressed tar file via ftp at ftp://bessel.mps.ohiostate.edu/pub/ryden/figs.tar.Z