Probing the evolution of the galaxy interaction/merger rate using collisional ring galaxies

We present the results from our program to determine the evolution of the galaxy interaction/merger rate with redshift using the unique star-forming characteristics of collisional ring galaxies. We have identified 25 distant collisional ring galaxy candidates (CRGCs) in a total of 162 deep Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images obtained from the HST archives. Based on measured and estimated redshifts, these 25 CRGCs all lie in the redshift interval of 0.1less than or equal tozless than or equal to1. Using the local collisional ring galaxy volume density and the new "standard'' cosmology, we find that in order to account for the number of identified CRGCs in our surveyed fields, the galaxy interaction/merger rate, parameterized as (1+z)(m), must increase steeply with redshift. We determine a minimum value of m=5.2+/-0.7, although m could be as high as 7 or 8. We can rule out a nonevolving (m=0) and weakly evolving (m=1-2) galaxy interaction/merger rate at greater than the 4 sigma level of confidence.