Black Holes in Galaxy Mergers: The Formation of Red Elliptical Galaxies

We use hydrodynamical simulations to study the color transformations inducedby star formation and active galactic nuclei (AGN) during major mergers ofspiral galaxies. Our modeling accounts for radiative cooling, star formation,and supernova feedback. Moreover, we include a treatment of accretion ontosupermassive black holes embedded in the nuclei of the merging galaxies. Weassume that a small fraction of the bolometric luminosity of an accreting blackhole couples thermally to surrounding gas, providing a feedback mechanism thatregulates its growth. The encounter and coalescence of the galaxies triggersnuclear gas inflow which fuels both a powerful starburst and strong black holeaccretion. Comparing simulations with and without black holes, we show that AGNfeedback can quench star formation and accretion on a shorttimescale,particularly in large galaxies where the black holes can drivepowerful winds once they become sufficiently massive. The color evolution ofthe remnant differs markedly between mergers with and without central blackholes. Without AGN, gas-rich mergers lead to ellipticals which remain blueowing to residual star formation, even after more than 7 Gyrs have elapsed. Incontrast, mergers with black holes produce ellipticals that redden much faster,an effect that is more pronounced in massive remnants where a nearly completetermination of star formation occurs, allowing them to redden to u-r ~ 2.3 inless than one Gyr. AGN feedback may thus be required to explain the populationof extremely red massive early type-galaxies, and it appears to be an importantdriver in generating the observed bimodal color distribution of galaxies in theLocal Universe.Comment: 4 pages, 4 figures, submitted to ApJL (a few references added