The Formation of Halos via Mergers. The Organized and Organizing Dynamics of Mergers

Abstract

N-body models of the growth of large and small scale structures in the Universe have proved to be valuable tools in the quest for the values of fundamental cosmological parameters. From the shapes of halo rotation curves (Quinn, Salmon and Zurek, 1986, QSZ) and the form of the two-point spatial correlation function (Davis et. al., 1985), it would appear that small and intermediate scale structures imply initial density perturbations with power spectra (P(k)) that decline as a function of wavenumber (k) faster than k⁻¹ (P=Akⁿ, n<−1). The mass perturbations associated with these spectra imply a spherical collapse time t_coll αM^(n+3)/4. For the case n >−3, small mass perturbations collapse before large ones and the evolution proceeds as a cascade of mergers from small to large scales. Protogalactic complexes destined to form galaxies similar to the Milky Way must have collapsed for the first time at redshifts less than 4, ie. when the Universe was one collapse time old for these systems (Fall, 1989). This onset of major merging and agglomeration of subsystems seems to have continued for a large number of galaxies up until redshifts of less than 0.5. New studies of Butcher-Oemler clusters by Lavery and Henry (1988) suggest that the source of the blue galaxies in these clusters is bursts of star formation triggered by galactic encounters. At even smaller redshifts the strong deceleration of colliding spirals and the transient nature of tidal tails pointed out by the Toomres (1972), the number of ellipticals will shells (Schweizer, 1982), the abundance of ellipticals with counterrotating cores (Balcells and Quinn, 1989) and the old thick disks of spirals (Hernquist and Quinn, 1989) all point to a long merger related history for a large fraction of currently isolated, bright galaxies. In this article we will examine the dynamics and effects of the merger process on the dark and luminous components of galaxies at high and low redshift. As we shall see, the same dynamical processes acting in cosmological and nearby mergers have imparted a detailed memory of formation to galaxies that will be useful in probing the initial conditions and evolution of protogalactic systems.