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We use a set of four deep Cycle 1+2 fields with the HST Wide-Field Camera to constrain the rate of galaxy merging between the current epoch and approximately $z=0.7$. These fields were selected around weak radio sources not in rich or poor clusters so as to not bias these studies. Since most mergers occur between members of bound pairs, the merger rate is given by (half) the rate of disappearance of galaxy pairs. Using an objective criterion for pair membership, we find that more than 34\% of galaxies in the magnitude range $I=18-22$ mag belong to pairs, while careful study of nearby comparison samples shows that only 7\% of local galaxies belong to pairs. Hence, about 13\% of the galaxy population has disappeared to merging in the cosmic epoch corresponding to this magnitude interval (or 0.1$\le z\le$0.7). This pair fraction is a lower limit, since correction for pairs in which one member falls below our detection threshold would raise the fraction of pair members with I=18---22 mag to about 50\%. (we do not include physical system of higher multiplicity in these values). Hence, the number of galaxy pairs has dropped significantly between z$\sim$0.7 and the current epoch.
When using the best available I-band field galaxy redshift distributions, the HST pair-fraction grows with redshift as $\propto (1+z)^{3.0-3.5}$, quite consistent with the expected evolution in the merger-rate from the decrease in comoving volume ($\propto (1+z)^{3}$). This result has very significant implications for the interpretation of the ground-based galaxy counts (it explains the disappearance of faint blue galaxies), the cosmological evolution of faint radio sources and quasars (explains why these should indeed evolve as $\propto (1+z)^{3}$), the statistics of QSO companions, the galaxy content in distant clusters, and the merging history of a ``typical" galaxy.
This work was supported by STScI grants GO-2405.*-87A and GO-3545.*-91A (to WCK and RAW) and in part through EPSCoR grant EHR-9108761 (to WCK).
