The Evolution of Central Group Galaxies in Hydrodynamical Simulations

We trace the evolution of central galaxies in three ∼ 10 M⊙ 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 resembling either elliptical or S0 galaxies, even without including energy feedback from active galactic nuclei (AGNs). Their z ∼ 2−2.5 main progenitors are massive (M∗ ∼ 3−10×10 10 M⊙), star forming (20−60M⊙/yr) galaxies which host substantial reservoirs of cold gas (∼ 5 × 10 M⊙) in extended gas disks. Our simulations thus show that star forming galaxies observed at z ∼ 2 are likely the main progenitors of central galaxies in galaxy groups at z = 0. At z ∼ 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 ∼ 2 kpc. Consequently the density within the effective radius decreases by up to two orders of magnitude. Both major and minor mergers contribute to most (70 −15%) 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 % and 16 +6 −11%, respectively. Overall, the simulated galaxies grow by a factor ∼ 4− 5 in mass and size since redshift z ∼ 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 galaxy mergers restore the early-type character of the central galaxy in the cooling flow group on a timescale of ∼ 1 − 2 Gyr. Although the average properties of central galaxies may be set by their halo masses, our simulations demonstrate that, in galaxy groups, 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. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: evolution — galaxies: structure