Letters to the Editor

Cosmic distance duality relation and the shape of galaxy clusters

¹ Departamento de Astronomia, Instituto Astronômico e Geofísico, Universidade de São Paulo – USP, São Paulo, Brazil
e-mail: holanda@astro.iag.usp.br; limajas@astro.iag.usp.br
² Instituto de Física, Universidade Federal do Rio de Janeiro – UFRJ, Rio de Janeiro, Brazil
e-mail: mbr@if.ufrj.br

Received: 8 August 2010
Accepted: 24 January 2011

Abstract

Context. Observations in the cosmological domain are heavily dependent on the validity of the cosmic distance-duality (DD) relation, η = D_L(z)(1 + z)²/D_A(z) = 1, an exact result required by the Etherington reciprocity theorem where D_L(z) and D_A(z) are, respectively, the luminosity and angular diameter distances. In the limit of very small redshifts D_A(z) = D_L(z) and this ratio is trivially satisfied. Measurements of Sunyaev-Zeldovich effect (SZE) and X-rays combined with the DD relation have been used to determine D_A(z) from galaxy clusters. This combination offers the possibility of testing the validity of the DD relation, as well as determining which physical processes occur in galaxy clusters via their shapes.

Aims. We use WMAP (7 years) results by fixing the conventional ΛCDM model to verify the consistence between the validity of DD relation and different assumptions about galaxy cluster geometries usually adopted in the literature.

Methods. We assume that η is a function of the redshift parametrized by two different relations: η(z) = 1 + η₀z, and η(z) = 1 + η₀z/(1 + z), where η₀ is a constant parameter quantifying the possible departure from the strict validity of the DD relation. In order to determine the probability density function (PDF) of η₀, we consider the angular diameter distances from galaxy clusters recently studied by two different groups by assuming elliptical (isothermal) and spherical (non-isothermal) β models. The strict validity of the DD relation will occur only if the maximum value of η₀ PDF is centered on η₀ = 0.

Results. It was found that the elliptical β model is in good agreement with the data, showing no violation of the DD relation (PDF peaked close to η₀ = 0 at 1σ), while the spherical (non-isothermal) one is only marginally compatible at 3σ.

Conclusions. The present results derived by combining the SZE and X-ray surface brightness data from galaxy clusters with the latest WMAP results (7-years) favors the elliptical geometry for galaxy clusters. It is remarkable that a local property like the geometry of galaxy clusters might be constrained by a global argument provided by the cosmic DD relation.