Ja n 20 06 Massive and Red Objects predicted by the semianalytical model of galaxy formation

We study whether hierarchical galaxy formation in a concordance ΛCDM universe can produce enough massive and red galaxies compared with the observations. We implement a semi-analytical model (SAM) in which the central black holes gain their mass during major mergers of galaxies and the energy feedback from active galaxy nuclei (AGN) suppresses the gas cooling in their host halos. The energy feedback from AGN acts effectively only in massive galaxies when supermassive black holes have been formed in the central bulges. Compared with previous models without black hole formation, our model predicts more massive and luminous galaxies at high redshift, agreeing with the observations of K20 up to z ∼ 3. Also the predicted stellar mass density in massive galaxies agrees with the observations of GDDS. Because of the energy feedback from AGN, the formation of new stars is stopped in massive galaxies with the termination of gas cooling and these galaxies soon become red with color R − K >5, comparable with the observations of Extremely Red Objects (EROs) at redshift z ∼1-2. Though the predicted number density of EROs is lower than observed at z ∼ 2, those EROs which are dust-enshrouded star-forming galaxies are not properly modelled in our work because of the lack of a reliable dust extinction model for high redshift galaxies.