Dynamically Close Galaxy Pairs and Merger Rate Evolution in the CNOC2 Redshift Survey

Abstract

We investigate redshift evolution in the galaxy merger and accretion rates, using a well-defined sample of 4184 galaxies with 0.12 ≤ z ≤ 0.55 and R C ≤ 21.5. We identify 88 galaxies in close (5 ≤ r p ≤ 20 h -1 kpc) dynamical (Δ v ≤ 500 km s -1 ) pairs. These galaxies are used to compute global pair statistics, after accounting for selection effects resulting from the flux limit, k -corrections, luminosity evolution, and spectroscopic incompleteness. We find that the number of companions per galaxy (for -21 ≤ M ≤ -18) is N c = 0.0321 ± 0.0077 at z = 0.3. The luminosity in companions, per galaxy, is L c = 0.0294 ± 0.0084 × 10 10 h 2 L ☉ . We assume that N c is proportional to the galaxy merger rate, while L c is directly related to the mass accretion rate. After increasing the maximum pair separation to 50 h -1 kpc and comparing with the low-redshift SSRS2 pair sample, we infer evolution in the galaxy merger and accretion rates of (1 + z ) 2.3±0.7 and (1 + z ) 2.3±0.9 , respectively. These are the first such estimates to be made using only confirmed dynamical pairs. When combined with several additional assumptions, this implies that approximately 15% of present epoch galaxies with -21 ≤ M B ≤ -18 have undergone a major merger since z = 1.