The cluster-galaxy cross spectrum

An additional probe of cosmological and halo parameters

¹ Department of Physics and Astronomy, University College London, London WC1E 6BT, UK e-mail: [ghutsi;lahav]@star.ucl.ac.uk
² Tartu Observatory, Tõravere 61602, Estonia

Received: 23 May 2008
Accepted: 16 September 2008

Abstract

Context. There are several wide field galaxy and cluster surveys planned for the near future, e.g. BOSS, WFMOS, ADEPT, Hetdex, SPT, eROSITA. In the simplest approach, one would analyze these independently, thus neglecting the extra information provided by the cluster-galaxy cross pairs.

Aims. In this paper we have focused on the possible synergy between these surveys by investigating the amount of information encoded in the cross pairs.

Methods. We present a model for the cluster-galaxy cross spectrum within the halo model framework. To assess the gain in performance due to inclusion of the cluster-galaxy cross pairs, we carry out a Fisher matrix analysis for a BOSS-like galaxy redshift survey targeting luminous red galaxies and a hypothetical mass-limited cluster redshift survey with a lower mass threshold of 1.7 10¹⁴ h^-1 over the same volume.

Results. On small scales, a cluster-galaxy cross spectrum directly probes the density profile of the halos, instead of the density profile convolved with itself, as is the case for the galaxy power spectrum. Due to this different behavior, adding information from the cross pairs helps to tighten constraints on the halo occupation distribution (e.g. a factor of ~2 compression of the error ellipses on the  plane) and offers an alternative mechanism compared with techniques that directly fit halo density profiles. By inclusion of the cross pairs, a factor of ~2 stronger constraints are obtained for , while the improvement for the dark energy figure-of-merit is somewhat weaker: an increase by a factor of 1.4.
We have also written down the formalism for the case when only photometric redshifts are available for both the clusters and the galaxies. For the analysis of the photometric surveys the inclusion of the cluster-galaxy cross pairs might be very beneficial since the photo-z errors for the clusters are usually significantly smaller than for the typical galaxies.