Interaction rate at z similar to 1

We found 25 galaxies with probable tidal tails in the Hubble Deep Fields No rth and South at z=0.5-1.5. General characteristics of the selected tidal f eatures are very close to characteristics of tidal tails of local interacti ng 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+/-1). Therefore, we e stimated the rate of close encounters between the galaxies of comparable ma ss leading to the formation of extended tidal structures. if this rate refl ects the merger rate, our data support a steeply increasing merger rate at z similar to 1 and consistent with zero curvature universe.