A slow merger history of field galaxies since z ∼ 1

Using deep infrared observations conducted with the CISCO imager on the Subaru Telescope, we investigate the field-corrected pair fraction and the implied merger rate of galaxies in redshift survey fields with Hubble Space Telescope (HST) imaging. In the redshift interval, 0.5 < z < 1.5, the fraction of infrared-selected pairs increases only modestly with redshift to 7% ± 6% at z ≈ 1. This is nearly a factor of 3 less than the fraction, 22% ± 8%, determined using the same technique on HST optical images and as measured in a previous similar study. Tests support the hypothesis that optical pair fractions at z ∼ 1 are inflated by bright star-forming regions that are unlikely to be representative of the underlying mass distribution. By determining stellar masses for the companions, we estimate the mass accretion rate associated with merging galaxies. At z ∼ 1, we estimate this to be 2 × 10^9±0.2 M_odot; galaxy^-1 Gyr ^-1. Although uncertainties remain, our results suggest that the growth of galaxies via the accretion of preexisting fragments remains as significant a phenomenon in the redshift range studied as that estimated from ongoing star formation in independent surveys.