Robust determination of the major merger fraction at z = 0.6 in Groth
  Strip
  (Abridged) We measure the fraction of galaxies undergoing disk-disk major
mergers (f_m) at intermediate redshifts (0.35 <= z < 0.85) by studying the
asymmetry index A of galaxy images. Results are provided for B- and Ks-band
absolute magnitude selected samples from the Groth strip in the GOYA
photometric survey. Three sources of systematic error are carefully addressed:
(i) we avoid morphological K-corrections, (ii) we measure asymmetries in
artificially redshifted to z_d = 0.75 galaxies to lead with loss of
morphological information with redshift, and (iii) we take into account the
observational errors in z and A, that tend to overestimate the merger fraction,
by maximum likelihood techniques. We find: (i) our data allow for a robust
merger fraction to be provided for a single redshift bin centered at z=0.6.
(ii) Merger fractions have low values: f_m = 0.045 for M_B <= -20 galaxies, and
f_m = 0.031 for M_Ks <= -23.5 galaxies. And, (iii) failure to address the
effects of the observational errors leads to overestimating f_m by factors of
10%-60%. Combining our results with those on literature, and parameterizing the
merger fraction evolution as f_m(z) = f_m(0)(1+z)^m, we obtain that m = 2.9 +-
0.8, and f_m(0) = 0.012 +- 0.004$. Assuming a Ks-band mass-to-light ratio not
varying with luminosity, we infer that the merger rate of galaxies with stellar
mass M >= 3.5x10^10 M_Sun is R_m = 1.6x10^-4 Mpc^-3 Gyr^-1. When we compare
with previous studies at similar redshifts, we find that the merger rate
decreases when mass increases.
