Toward a Robust Estimate of the Merger Rate Evolution Using Near-IR Photometry

Abstract

We use a combination of deep, high angular resolution imaging data from the CDFS (HST/ACS GOODS survey) and ground-based near-IR K_s images to derive the evolution of the galaxy major merger rate in the redshift range 0.2 <= z<= 1.2. We select galaxies solely on the basis of their J-band rest-frame absolute magnitude, which is a good tracer of the stellar mass. We find steep evolution with redshift, with the merger rate vprop(1 + z)^{3.43 +/- 0.49} for optically selected pairs and vprop(1 + z)^{2.18 +/- 0.18} for pairs selected in the near-IR. Our result is unlikely to be affected by luminosity evolution that is relatively modest when using rest-frame J-band selection. The apparently more rapid evolution that we find in the visible is likely caused by biases relating to incompleteness and spatial resolution affecting the ground-based near-IR photometry, underestimating pair counts at higher redshifts in the near-IR. The major merger rate was ~5.6 times higher at z ~ 1.2 than at the current epoch. Overall, 41% × (0.5 Gyr/τ) of all galaxies with M_J <= - 19.5 have undergone a major merger in the last ~8 Gyr, where τ is the merger timescale. Interestingly, we find no effect on the derived major merger rate due to the presence of the large-scale structure at z = 0.735 in the CDFS.