THE EVOLUTION OF CENTRAL GROUP GALAXIES IN HYDRODYNAMICAL SIMULATIONS

We trace the evolution of central galaxies in three similar to 10(13) M-circle dot galaxy groups simulated at high resolution in cosmological hydrodynamical simulations. In all three cases, the evolution in the group potential leads, at z = 0, to central galaxies that are massive, gas-poor early-type systems supported by stellar velocity dispersion and which resemble either elliptical or S0 galaxies. Their z similar to 2-2.5 main progenitors are massive (M-* similar to (3-10) x10(10) M-circle dot), star-forming (20-60 M-circle dot yr(-1)) galaxies which host substantial reservoirs of cold gas (similar to 5 x 10(9) M-circle dot) in extended gas disks. Our simulations thus show that star-forming galaxies observed at z similar to 2 are likely the main progenitors of central galaxies in galaxy groups at z = 0. At z similar to 2 the stellar component of all galaxies is compact, with a half-mass radius < 1 kpc. The central stellar density stays approximatively constant from such early epochs down to z = 0. Instead, the galaxies grow inside out, by acquiring a stellar envelope outside the innermost similar to 2 kpc. Consequently the density within the effective radius decreases by up to 2 orders of magnitude. Both major and minor mergers contribute to most (70(-15)(+20)%) of the mass accreted outside the effective radius and thus drive an episodical evolution of the half-mass radii, particularly below z = 1. In situ star formation and secular evolution processes contribute to 14(-9)(+18)% and 16(-11)(+6)%, respectively. Overall, the simulated galaxies grow by a factor similar to 4-5 in mass and size since redshift z similar to 2. The short cooling time in the center of groups can foster a "hot accretion" mode. In one of the three simulated groups this leads to a dramatic rejuvenation of the central group galaxy at z < 1, affecting its morphology, kinematics, and colors. This episode is eventually terminated by a group-group merger. Mergers also appear to be responsible for the suppression of cooling flows in the other two groups. Passive stellar evolution and minor galaxy mergers gradually restore the early-type character of the central galaxy in the cooling flow group on a timescale of similar to 1-2 Gyr. Although the average properties of central galaxies may be set by their halo masses, our simulations demonstrate that the interplay between halo mass assembly, galaxy merging, and gas accretion has a substantial influence on the star formation histories and z = 0 morphologies of central galaxies in galaxy groups.