Interaction rate at z~1

We found 25 galaxies with probable tidal tails in the Hubble Deep Fields North and South at z=0.5-1.5. General characteristics of the selected tidal features are very close to characteristics of tidal tails of local interacting galaxies. Using objects with z=0.5-1.0, we found that volume density of galaxies with tidal tails changes with z as (1+z)^4\pm1. Therefore, we estimated the rate of close encounters between the galaxies of comparable mass leading to the formation of extended tidal structures. If this rate reflects the merger rate, our data support a steeply increasing merger rate at z~1 and consistent with zero curvature universe.