Robust determination of the major merger fraction at z = 0.6 in Groth Strip
Abstract
(Abridged) We measure the fraction of galaxies undergoing disk-disk major mergers (fm) 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 zd = 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: fm = 0.045 for MB <= -20 galaxies, and fm = 0.031 for MKs <= -23.5 galaxies. And, (iii) failure to address the effects of the observational errors leads to overestimating fm by factors of 10%-60%. Combining our results with those on literature, and parameterizing the merger fraction evolution as fm(z) = fm(0)(1+z)m, we obtain that m = 2.9 +- 0.8, and fm(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 >= 3.5x1010 MSun is Rm = 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.