The Recent and Continuing Assembly of Field Elliptical Galaxies by Red Mergers

We present a study of tidal debris associated with 126 nearby red galaxies, selected from the 1.2 deg2 Multiwavelength Survey by Yale-Chile and the 9.3 deg2 NOAO Deep Wide-Field Survey. In the full sample, 67 galaxies (53%) show morphological signatures of tidal interactions consisting of broad fans of stars, tails, and other asymmetries at very faint surface brightness levels. When restricting the sample to the 86 bulge-dominated early-type galaxies, the fraction of tidally disturbed galaxies rises to 71%, which implies that for every normal undisturbed elliptical there are two that show clear signs of interactions. The tidal features are red and smooth and often extend over >50 kpc. Of the tidally distorted galaxies, about two-thirds are remnants, and one-third are interacting with a companion galaxy. The companions are usually bright red galaxies as well; the median R-band luminosity ratio of the tidal pairs is 0.31, and the median color difference after correcting for the slope of the color-magnitude relation is -0.02 in B - R. If the ongoing mergers are representative for the progenitors of the remnants, ~35% of bulge-dominated galaxies experienced a merger with mass ratio >1 : 4 in the recent past. With further assumptions it is estimated that the present-day mass accretion rate of galaxies on the red sequence ΔM/M = 0.09 ± 0.04 Gyr-1. For a constant or increasing mass accretion rate with redshift, we find that red mergers may lead to an evolution of a factor of 2 in the stellar mass density in luminous red galaxies over the redshift range 0 < z < 1, consistent with recent studies of the evolution of the luminosity density. We conclude that the majority of today's most luminous field elliptical galaxies were assembled at low redshift through mergers of gas-poor, bulge-dominated systems. These dry mergers are consistent with the high central densities of ellipticals, their old stellar populations, and the strong correlations of their properties. It will be interesting to determine whether this mode of merging only plays an important role at low redshift or is relevant for galaxies at any redshift if they exceed a critical mass scale.