THE COSMOLOGICAL SIZE AND VELOCITY DISPERSION EVOLUTION OF MASSIVE EARLY-TYPE GALAXIES

Abstract

We analyze 40 cosmological re-simulations of individual massive galaxies with\npresent-day stellar masses of $M_{*} > 6.3 \\times 10^{10} M_{\\odot}$ in order\nto investigate the physical origin of the observed strong increase in galaxy\nsizes and the decrease of the stellar velocity dispersions since redshift $z\n\\approx 2$. At present 25 out of 40 galaxies are quiescent with structural\nparameters (sizes and velocity dispersions) in agreement with local early type\ngalaxies. At z=2 all simulated galaxies with $M_* \\gtrsim 10^{11}M_{\\odot}$ (11\nout of 40) at z=2 are compact with projected half-mass radii of $\\approx$ 0.77\n($\\pm$0.24) kpc and line-of-sight velocity dispersions within the projected\nhalf-mass radius of $\\approx$ 262 ($\\pm$28) kms$^{-1}$ (3 out of 11 are already\nquiescent). Similar to observed compact early-type galaxies at high redshift\nthe simulated galaxies are clearly offset from the local mass-size and\nmass-velocity dispersion relations. Towards redshift zero the sizes increase by\na factor of $\\sim 5-6$, following $R_{1/2} \\propto (1+z)^{\\alpha}$ with $\\alpha\n= -1.44$ for quiescent galaxies ($\\alpha = -1.12$ for all galaxies). The\nvelocity dispersions drop by about one-third since $z \\approx 2$, following\n$\\sigma_{1/2} \\propto (1+z)^{\\beta}$ with $\\beta = 0.44$ for the quiescent\ngalaxies ($\\beta = 0.37$ for all galaxies). The simulated size and dispersion\nevolution is in good agreement with observations and results from the\nsubsequent accretion and merging of stellar systems at $z\\lesssim 2$ which is a\nnatural consequence of the hierarchical structure formation. A significant\nnumber of the simulated massive galaxies (7 out of 40) experience no merger\nmore massive than 1:4 (usually considered as major mergers). On average, the\ndominant accretion mode is stellar minor mergers with a mass-weighted\nmass-ratio of 1:5. (abridged)\n