Are the nearby groups of galaxies gravitationally bound objects?
Version of Record online: 16 NOV 2007
Monthly Notices of the Royal Astronomical Society
Volume 382, Issue 4, pages 1864–1876, December 2007
How to Cite
Niemi, S.-M., Nurmi, P., Heinämäki, P. and Valtonen, M. (2007), Are the nearby groups of galaxies gravitationally bound objects?. Monthly Notices of the Royal Astronomical Society, 382: 1864–1876. doi: 10.1111/j.1365-2966.2007.12498.x
- Issue online: 26 NOV 2007
- Version of Record online: 16 NOV 2007
- Accepted 2007 September 19. Received 2007 September 19; in original form 2007 April 2
- methods: numerical;
- galaxies: clusters: general;
- galaxies: haloes;
- dark matter;
- large-scale structure of Universe
We have compared numerical simulations to observations for the nearby (<40 Mpc) groups of galaxies. The group identification is carried out using a group-finding algorithm developed by Huchra & Geller. Using cosmological N-body simulation code with the Λcold dark matter (ΛCDM) cosmology, we show that the dynamical properties of groups of galaxies identified from the simulation data are, in general, in a moderate, within 2σ, agreement with the observational catalogues of groups of galaxies. As simulations offer more dynamical information than observations, we used the N-body simulation data to calculate whether the nearby groups of galaxies are gravitationally bound objects by using their virial ratio. We show that in a ΛCDM cosmology about 20 per cent of nearby groups of galaxies, identified by the same algorithm as in the case of observations, are not bound, but merely groups in a visual sense. This is quite significant, specifically because estimations of group masses in observations are often based on an assumption that groups of galaxies found by the friends-of-friends algorithm are gravitationally bound objects. Simulations with different resolutions show the same results. We also show how the fraction of gravitationally unbound groups varies when the apparent magnitude limit of the sample and the value of the cosmological constant Λ is changed. In general, a larger value of the ΩΛ generates slightly more unbound groups.