MINOR MERGERS AND THE SIZE EVOLUTION OF ELLIPTICAL GALAXIES

Abstract

Using a high resolution hydrodynamical cosmological simulation of the\nformation of a massive spheroidal galaxy we show that elliptical galaxies can\nbe very compact and massive at high redshift in agreement with recent\nobservations. Accretion of stripped in-falling stellar material increases the\nsize of the system with time and the central concentration is reduced by\ndynamical friction of the surviving stellar cores. In a specific case of a\nspheroidal galaxy with a final stellar mass of $1.5 \\times 10^{11} M_{\\odot}$\nwe find that the effective radius $r_e$ increases from $0.7 \\pm 0.2 \\rm kpc$ at\nz = 3 to $r_e = 2.4 \\pm 0.4 \\rm kpc$ at z = 0 with a concomitant decrease in\nthe effective density of an order of magnitude and a decrease of the central\nvelocity dispersion by approximately 20% over this time interval. A simple\nargument based on the virial theorem shows that during the accretion of weakly\nbound material (minor mergers) the radius can increase as the square of the\nmass in contrast to the usual linear rate of increase for major mergers. By\nundergoing minor mergers compact high redshift spheroids can evolve into\npresent-day systems with sizes and concentrations similar to observed local\nellipticals. This indicates that minor mergers may be the main driver for the\nlate evolution of sizes and densities of early-type galaxies.\n