We have performed N-body simulations of the dynamical evolution of groups of galaxies with a variety of initial conditions and with all the mass initially bound to galaxies. Simulations with a common halo encompassing the group will be discussed in a forthcoming article. Some of the simulations correspond to free collapses with and without spherical symmetry and the rest to initially virialised systems. Our results are relevant to large groups or poor clusters, subcondensations within larger clusters, during times when the influence of the surrounding cluster can be neglected, or to subunits that have come together during cluster formation time to form a large cluster.
The dynamics of all but one of these systems is driven by the merging instability (Carnevali et al. 1981). We find that the condition for this instability to be operative in virialised systems is that there must be a central concentration of matter that drives the orbits of the galaxies to the center of the system. When this condition is fulfilled, both collapsing and virialised groups form a giant central object in the center. In this paper we have studied its properties as a function of the initial conditions of the simulations.
These central objects grow in time using two different mechanisms. The first is known as "galactic cannibalism" and consists in the merging of galaxy satellites that pass near the giant galaxies. The second one is the accretion of galactic material that is stripped from the satellite galaxies by tidal forces. The prevalence of one or another mechanism depends on the initial conditions of the group. In the more extended and virialised systems the rate of mass increase due to tidal stripping is comparable to the mass growth due to merging. In the rest of the simulations that form a central galaxy, the mass growth by merging is more important.
The objects formed are oblate or mildly triaxial in nature, especially in the cases of collapsing groups with aspherical initial conditions. In such cases the orientation of the central object correlates well with that of the initial group. In general the triaxiality is stronger in the outer parts of the central object, in good agreement with observations (Porter, Schneider & Hoessel 1991; Mackie, Visvanathan & Carter 1990). Collapsing systems are supported by anisotropic velocity dispersion tensors. The tightly bound and virialised groups form nearly spherical central galaxies. In most cases these systems can be described as a stratification of ellipsoids with the same axial ratios. These galaxies have isotropic velocity dispersion tensors. In these cases the volumetric density is well fitted by an ellipsoidal Hernquist profile.
Projecting the density distribution, in order to compare them with the properties of ellipticals and brightest cluster members, we obtain surface density profiles that can be compared with the surface brightness profiles of real galaxies. We obtain three types of profiles that are representative of the profiles of the galaxies in the centers of clusters. Tightly bound virialised clusters and spherical collapses give objects with profiles well fitted by an law. These results are in agreement with the simulations of van Albada (1982) and May & van Albada (1984). The collapsing simulations from aspherical initial conditions give galaxies whose surface density profiles are well fitted by an law only in the main body of the object. The surface density in the external parts falls below the law. This kind of profile is also observed in real brightest cluster members. As the galaxies formed in these simulations are non-spherical objects, we suggest that this could be a signature of the triaxiality in the surface density profiles. The most interesting case corresponds to the central galaxy formed in the extended virialised clusters, where stripping is more important. The surface density profile of this object is also well fitted by an law in the main body of the galaxy, but in the external parts the surface density profile is systematically above this reference law, as is the case for cD galaxies in clusters. The central objects in our simulations have projected velocity dispersion profiles that are comparable to the profiles of real galaxies. They seem to have a greater luminosity than the luminosity corresponding to the central velocity dispersions given by the Faber-Jackson relation and thus again reproduce a property of brighter cluster members.
© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998