New Constraints on the Efficiencies of Ram-pressure Stripping and the Tidal Disruption of Satellite Galaxies

Using data from the Sloan Digital Sky Survey (SDSS) it has recently been shown that the red fraction of satellite galaxies increases with stellar mass. Semi-analytical models, however, predict red satellite fractions that are independent of stellar mass, and much higher than observed. It has been argued that this discrepancy owes to the fact that the models assume that satellite galaxies are instantaneously stripped of their hot gas reservoirs at the moment they are accreted into a bigger halo. In this letter we show that the fraction of red satellites can be brought in better agreement with the data by simply decreasing this stripping efficiency. However, this also results in a red fraction of massive centrals that is much too low. This owes to the fact that the massive centrals now accrete satellite galaxies that are bluer and more gas-rich. However, if a significant fraction of low mass satellite galaxies is tidally disrupted before being accreted by their central host galaxy, as suggested by recent studies, the red fractions of both centrals and satellites can be reproduced reasonably well. A problem remains with the red fraction of centrals of intermediate mass, which is likely to reflect an oversimplified treatment of AGN feedback. Subject headings: galaxies: general – galaxies: formation – intergalactic medium