Minor mergers and the size evolution of elliptical galaxies

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  • 11 March 2009
Abstract
Using a high resolution hydrodynamical cosmological simulation of the formation of an individual massive spheroidal galaxy we show that elliptical galaxies can be very compact and massive at high redshift in good agreement with recent observations. Accretion of stripped in-falling stellar material increases the size of the system with time and the central concentration is reduced by dynamical friction of the surviving cores. In a specific case of a spheroidal galaxy with final stellar mass of 1.5 x 10^{11} M_{\odot} we find that the effective radius r_e increases from 0.7 kpc at z = 3 to r_e = 2.4 kpc at z = 0 with a concomitant decrease in peak central density of one order of magnitude and a decrease of the central velocity dispersion of approximately 20% over this time interval. A simple argument based on the virial theorem shows that during the accretion of weakly bound material (minor mergers) the radius can increase as the square of the mass in contrast to the usual linear rate of increase for major mergers. By undergoing minor mergers compact high redshift spheroids can evolve into present day systems with sizes and concentrations similar to observed local ellipticals. This indicates that minor mergers may be the main driver for the late evolution of size and densities of early-type galaxies.

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