The origin of redshift asymmetries: how ΛCDM explains anomalous redshift

¹ University of Turku, Department of Physics and Astronomy, Tuorla Observatory, Väisäläntie , Piikkiö, Finland e-mail: saniem@utu.fi
² Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain

Received: 13 August 2008
Accepted: 13 November 2008

Abstract

Aims. Several authors have found a statistically significant excess of galaxies with higher redshifts relative to the group centre, so-called discordant redshifts, in particular in groups where the brightest galaxy, identified in apparent magnitudes, is a spiral. Our aim is to explain the observed redshift excess.

Methods. We use a semi-analytical galaxy catalogue constructed from the Millennium Simulation to study redshift asymmetries in spiral-dominated groups in the Λcold dark matter (ΛCDM) cosmology. We create two mock catalogues of galaxy groups with the Friends-of-Friends percolation algorithm to carry out this study.

Results. We show that discordant redshifts in small galaxy groups arise when these groups are gravitationally unbound and the dominant galaxy of the group is misidentified. About one quarter of all groups in our mock catalogues belong to this category. The redshift excess is especially significant when the apparently brightest galaxy can be identified as a spiral, in full agreement with observations. On the other hand, the groups that are gravitationally bound do not show a significant redshift asymmetry. When the dominant members of groups in mock catalogues are identified by using the absolute B-band magnitudes, our results show a small blueshift excess. This result is due to the magnitude limited observations that miss the faint background galaxies in groups.

Conclusions. When the group centre is not correctly identified it may cause the major part of the observed redshift excess. If the group is also gravitationally unbound, the level of the redshift excess becomes as high as in observations. There is no need to introduce any “anomalous” redshift mechanism to explain the observed redshift excess. Further, as the Friends-of-Friends percolation algorithm picks out the expanding parts of groups, in addition to the gravitationally bound group cores, group catalogues constructed in this way cannot be used as if the groups are purely bound systems.