We measure the fraction of galaxies undergoing disk-disk major mergers ( f _ { m } ^ { mph } ) at intermediate redshifts ( 0.35 \leq z < 0.85 ) by studying the asymmetry index A of galaxy images .
Results are provided for B - and K _ { s } -band absolute magnitude selected samples from the Groth strip in the GOYA photometric survey .
Three sources of systematic error are carefully addressed and quantified .
The effects of the large errors in the photometric redshifts and asymmetry indices are corrected with maximum likelihood techniques .
Biases linked to the redshift degradation of the morphological information in the images are treated by measuring asymmetries on images artificially redshifted to a reference redshift of z _ { d } = 0.75 .
Morphological K-corrections are further constrained by staying within redshifts where the images sample redward of 4000Ã .
We find : ( i ) our data allow for a robust merger fraction to be provided for a single redshift bin , which we center at z = 0.6 .
( ii ) Merger fractions at that z have lower values than previous determinations : f _ { m } ^ { mph } = 0.045 ^ { +0.014 } _ { -0.011 } for M _ { B } \leq - 20 galaxies , and f _ { m } ^ { mph } = 0.031 ^ { +0.013 } _ { -0.009 } for M _ { K _ { s } } \leq - 23.5 galaxies .
And , ( iii ) failure to address the effects of the large observational errors leads to overestimating f _ { m } ^ { mph } by factors of 10 % -60 % .

Combining our results with those on other B -band selected samples , and parameterizing the merger fraction evolution as f _ { m } ^ { mph } ( z ) = f _ { m } ^ { mph } ( 0 ) ( 1 + z ) ^ { m } , we obtain that m = 2.9 \pm 0.8 , and f _ { m } ^ { mph } ( 0 ) = 0.012 \pm 0.004 .
For an assumed merger time-scale between 0.35-0.6 Gyr , these values imply that only 20 % -35 % of present day M _ { B } \leq - 20 galaxies have undergone a disk-disk major merger since z \sim 1

Assuming a K _ { s } -band mass-to-light ratio not varying with luminosity , we infer that the merger rate of galaxies with stellar mass M _ { \star } \gtrsim 3.5 \times 10 ^ { 10 } M _ { \odot } is \Re _ { m } = 1.6 ^ { +0.9 } _ { -0.6 } \times 10 ^ { -4 } { Mpc ^ { -3 } } { Gyr ^ { -1 } } at z = 0.6 .
When we compare with previous studies at similar redshifts , we find that the merger rate decreases when mass increases .