Towards an optimal sampling of peculiar velocity surveys for Wiener Filter reconstructions

Abstract

The Wiener Filter (WF) technique enables the reconstruction of density and velocity fields from observed radial peculiar velocities. This paper aims at identifying the optimal design of peculiar velocity surveys within the WF framework. The prime goal is to test the dependence of the quality of the reconstruction on the distribution and nature of data points. Mock datasets, extending to 250 h −1 Mpc, are drawn from a constrained simulation that mimics the local Universe to produce realistic mock catalogs. Reconstructed fields obtained with these mocks are compared to the reference simulation. Comparisons, including residual distributions, cell-to-cell and bulk velocities, imply that the presence of field data points is essential to properly measure the flows. The fields reconstructed from mocks that consist only of galaxy cluster data points exhibit poor quality bulk velocities. In addition, the quality of the reconstruction depends strongly on the grouping of individual data points into single points to suppress virial motions in high density regions. Conversely, the presence of a Zone of Avoidance hardly affects the reconstruction. For a given number of data points, a uniform sample does not score any better than a sample with decreasing number of data points with the distance. The best reconstructions are obtained with a grouped survey containing field galaxies: Assuming no error, they differ from the simulated field by less than 100 km s −1 up to the extreme edge of the catalogs or up to a distance of three times the mean distance of data points for non-uniform catalogs. The overall conclusions hold when errors are added.

How does the grouping scheme affect the Wiener Filter reconstruction of the local Universe?

“…Tests we made varying the default linking length (0.25 h −1 Mpc at redshift zero changed to 0.20 or 0.30 h −1 Mpc) in Tempel Grouping scheme and applying the WF technique to the resulting grouped catalogs show that indeed a large linking length permits increasing the stability but an excessive grouping (no more field galaxies) leads to wrong dipole values. This is in agreement with Sorce et al (2017) that show that galaxies in the fields are an absolute necessity. Additionally H 0 has to be chosen with more care: minimizing in absolute value the mean of the velocity distribution seems a reasonable approach to choose the value of H 0 .…”

“…It is worth noticing that again, the dipole at large radii is proven to be very stable whatever choices is made to built the WF reconstruction providing that the grouping is properly done and that the catalog contains both clusters/groups and galaxies in the field (e.g. Sorce et al 2017). Regarding the grouping scheme, there is a clear need for a balance between grouping to remove non linear motions to preserve the quality of the WF reconstruction (Sorce et al 2017) and its stability with respect to H 0 choice, and keeping some constraints to contrast the high overdensity regions with respect to other regions.…”

“…Using H 0 =75.2 (=100h) km s −1 Mpc −1 (the value given by Tully et al, 2013), it extends up to about 250 h −1 Mpc and about 50% of the data are within 70 h −1 Mpc and 90% within 160 h −1 Mpc. In a companion paper (Sorce et al 2017), we have shown that, in absence of a complete catalog and provided that it is properly grouped, the sampling of this catalog is optimal for Wiener Filter reconstructions with respect to uniformly distributed catalogs or catalogs of sole clusters. The goal is then to track the impact of the grouping technique on the resulting reconstructions.…”

Cosmic Dawn II (CoDa II): a new radiation-hydrodynamics simulation of the self-consistent coupling of galaxy formation and reionization

“…Galaxies in the radial peculiar velocity catalogue were then grouped (e.g. Tully 2015a, b) to produce a data set that traces the coherent large-scale velocity field, with non-linear virial motions removed that would affect the reconstruction obtained with the linear method as shown by Sorce, Hoffman & Gottlöber (2017) and Sorce & Tempel (2017).…”

The Hestia project: simulations of the Local Group

“…Grouping: The CF2 catalog includes a considerable number of galaxies that are members of groups and clusters, the velocities of which are affected by small-scale non-linear virial motions. Grouping these galaxies -namely collapsing data points identified as belonging to the same group or cluster into a single data point, provides an effective remedy to the problem (Sorce et al 2017) as it filters the non-linear component of the large scale velocity field. Here we have used the original grouping proposed for CF2 (Brent Tully, private communication).…”