Galaxy growth by merging in the nearby universe

We measure the mass growth rate by merging for a wide range of galaxy types. We present the small-scale (0.014 h ^-1 ₇₀ Mpc < r < 11 h ₇₀ ^-1 Mpc) projected cross-correlation functions w(r _p) of galaxy subsamples from the spectroscopic sample of the NYU Value-Added Galaxy Catalog (5 × 10⁵ galaxies of redshifts 0.03 < z < 0.15) with galaxy subsamples from the Sloan Digital Sky Survey imaging (4 × 10⁷ galaxies). We use smooth fits to de-project the two-dimensional functions w(r _p) to obtain smooth three-dimensional real-space cross-correlation functions ξ(r) for each of several spectroscopic subsamples with each of several imaging subsamples. Because close pairs are expected to merge, the three-space functions and dynamical evolution time estimates provide galaxy accretion rates. We find that the accretion onto massive blue galaxies and onto red galaxies is dominated by red companions, and that onto small-mass blue galaxies, red and blue galaxies make comparable contributions. We integrate over all types of companions and find that at fixed stellar mass, the total fractional accretion rates onto red galaxies (∼3h ₇₀ percent per Gyr) are greater than that onto blue galaxies (∼1h ₇₀ percent per Gyr). These rates are almost certainly overestimates because we have assumed that all close pairs merge as quickly as the merger time that we used. One conclusion of this work is that if the total growth of red galaxies from z = 1 to z = 0 is mainly due to merging, the merger rates must have been higher in the past.