Very Small Scale Clustering and Merger Rate of Luminous Red Galaxies

Abstract

We present the small-scale (0.01 Mpc < r < 8 h-1 Mpc) projected correlation function wp(rp) and real-space correlation function ξ(r) of 24,520 luminous early-type galaxies from the Sloan Digital Sky Survey (SDSS) Luminous Red Galaxy (LRG) sample (0.16 < z < 0.36). "Fiber collision" incompleteness of the SDSS spectroscopic sample at scales smaller than 55'' prevents measurements of the correlation function for LRGs on scales smaller than ~0.3 Mpc by the usual methods. In this work, we cross-correlate the spectroscopic sample with the imaging sample, with a weighting scheme to account for the collisions, extensively tested against mock catalogs. We correct for photometric biases in the SDSS imaging of close galaxy pairs. We find that the correlation function ξ(r) is surprisingly close to a r-2 power law over more than 4 orders of magnitude in separation r. This result is too steep at small scales to be explained in current versions of the halo model for galaxy clustering. We infer an LRG-LRG merger rate of ≲0.6 × 104 Gyr-1 Gpc-3 for this sample. This result suggests that the LRG-LRG mergers are not the main mode of mass growth for LRGs at z < 0.36.