Dark energy and the structure of the Coma cluster of galaxies

¹ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 21500 Piikkiö, Finland
e-mail: pekkatee@utu.fi
² Sternberg Astronomical Institute, Moscow University, 119899 Moscow, Russia
³ Space Research Institute, Russian Academy of Sciences, 117997 Moscow, Russia
⁴ University of Alabama, Tuscaloosa, AL 35487-0324, USA
⁵ Department of Physics, University of Rome “La Sapienza”, 00185 Rome, Italy

Received: 23 November 2012
Accepted: 18 March 2013

Abstract

Context. We consider the Coma cluster of galaxies as a gravitationally bound physical system embedded in the perfectly uniform static dark energy background as implied by ΛCDM cosmology.

Aims. We ask if the density of dark energy is high enough to affect the structure of a large and rich cluster of galaxies.

Methods. We base our work on recent observational data on the Coma cluster, and apply our theory of local dynamical effects of dark energy, including the zero-gravity radius R_ZG of the local force field as the key parameter.

Results. 1) Three masses are defined that characterize the structure of a regular cluster: the matter mass M_M, the dark-energy effective mass M_DE (<0), and the gravitating mass M_G (=M_M + M_DE). 2) A new matter-density profile is suggested that reproduces the observational data well for the Coma cluster in the radius range from 1.4 Mpc to 14 Mpc and takes the dark energy background into account. 3) Using this profile, we calculate upper limits for the total size of the Coma cluster, R ≤ R_ZG ≈ 20 Mpc, and its total matter mass, M_M ≲ M_M(R_ZG) = 6.2 × 10¹⁵ M_⊙.

Conclusions. The dark energy antigravity affects the structure of the Coma cluster strongly at large radii R ≳ 14 Mpc and should be considered when its total mass is derived.