Probing the Evolution of the Galaxy Interaction/Merger Rate Using Collisional Ring Galaxies

Abstract

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 0.1<=z<=1. 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 σ level of confidence.