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The Formation of a Giant Elliptical Galaxy

Within 3 Gyr of the start of the simulation (by z=0.8), the four most massive galaxies merge to form a central object resembling an elliptical galaxy and as we describe below its measured surface brightness and velocity dispersion profile are very similar to real giant ellipticals. Three of the galaxies fall down a line which can be identified with the primordial filament apparent in the early formation of the dark matter cluster. The fourth galaxy comes from a different direction, but the infall of material generally follows the line of the filament (Fig. 3).


Figure 3: Snapshots of the evolution of the cluster and bright central galaxy. Each strip is 1 Mpc wide. The top strip shows the view perpendicular to the chain of 3 galaxies which fall together to make the BCG. The bottom strip show the view looking approximately down the filament. The following mpeg animations illustrate the evolution and structure of the forming giant elliptical galaxy.

320x240 - xy projection

320x240 - xz projection

320x240 - yz projection

320x240 - z=0.35 rotation

320x240 - z=0.0 rotation

Over the next 5 Gyr (ending around z=0.4), 9 more galaxies are accreted (some as merged products themselves) and of the total of 13 merging to form the elliptical, 7 are galaxies with circular velocities greater than 200 km/s while the remaining 6 are smaller galaxies with circular velocities tex2html_wrap_inline809 km/s. The epoch around z=0.4 is marked by a period of intense activity in which many of the galaxies are merging and experiencing strong tidal perturbations resulting in tidal tails from close passes with the cluster center (Fig 4) and is an illustration of the galaxy harassment process (Moore et al. 1996).


Figure 4: Close-up of the central region of the cluster at z=0.35, during a very active phase of the collapse. The giant elliptical is undergoing a major merger while various disk galaxies are throwing off tidal tails resulting from strong tidal interactions with the cluster centre.

From z=0.4 to the present, there are no more large mergers with the BCG. At the end of the simulation, only 59 galaxies can be identified orbiting in the cluster. Of the 41 ``missing'' galaxies, 13 have merged to form the central massive elliptical galaxy. The remaining 28 have been incorporated into other galaxies through mergers. The group simulated here is too small to detect the density-morphology effect (Dressler 1984)[19], the cluster centre), however, the few ellipticals created are close to the cluster centre (R < 500 kpc) and on eccentric orbits that are measureably decaying by dynamical friction.

next up previous
Next: Analysis of the Merger Up: Results Previous: Results

John Dubinski
Tue Feb 17 16:03:05 EST 1998