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 thegalaxy interaction/merger rate with redshift using the unique star-formingcharacteristics of collisional ring galaxies. We have identified 25 distantcollisional ring galaxy candidates (CRGCs) in a total of 162 deep Hubble SpaceTelescope Wide Field/Planetary Camera-2 images obtained from the HST Archives.Based on measured and estimated redshifts, these 25 CRGCs all lie in theredshift interval of 0.1 < z < 1. Using the local collisional ring galaxyvolume density and the new ``standard'' cosmology, we find that in order toaccount for the number of identified CRGCs in our surveyed fields, the galaxyinteraction/merger rate, parameterized as (1 + z)^m, must increase steeply withredshift.We determine a minimum value of m = 5.2 $\pm$ 0.7, though m could beas high as 7 or 8. We can rule out a non-evolving (m = 0) and weakly evolving(m = 1-2) galaxy interaction/merger rate at greater than the 4 sigma level ofconfidence.