Evidence for galaxy interactions/mergers from medium deep survey WFPC2 data

We examine the morphological and statistical properties of close galaxy pairs from two sets of 28 WFPC2 fields, acquired for the Medium Deep Survey (MDS) and the Groth-Westphal Survey (GWS) in F606W (V) and F814W (I) passbands. In the GWS sample all fields have uniform 95% completeness down to I less than or similar to 24.3 mag, whereas in the MDS sample, fields have varying 95% completeness limits in the range I less than or similar to 23.6-24.8 mag. In each field similar to 400 galaxies per similar or equal to 5 square arcmin field are detected. We exploit high-resolution WFPC2 images to systematically determine morphological classifications of galaxies as disk or bulge dominated down to I less than or equal to 23 mag (V less than or similar to 24 mag) and to differentiate galaxies from stars 1 magnitude fainter. Down to I less than or equal to 25 mag the number of galaxy pairs with separations theta less than or equal to 3.'' 0 is consistent with a shortward extrapolation of the angular two-point correlation function omega(theta)proportional to theta(-0.8) observed from the same data; the fraction of such pairs showing morphological evidence for physical association accounts for a third of the total numbers suggested by a shortward extrapolation of omega(theta). The latter result may not be too surprising given the low surface brightness of the tidal tails resulting from galaxy interactions; i.e., much of the evidence for interactions may fall below our detection limit. Moreover, we find no trend between apparent physical association (on the basis of morphology) and (V-I) color or I-magnitude difference between pair members of the theta less than or similar to 3.'' 0 pair sample. We use recent galaxy redshift surveys to estimate the rate of galaxy merging occurring in the MDS and GWS galaxy pair samples. From this work we find that merging has a moderate dependence on redshift: we derive an estimate for the galaxy pair fraction P-f proportional to (1+z)(m), with m=1.2+/-0.4 for galaxies with I less than or equal to 25 mag (z(med)less than or similar to 1-2). Two scenarios are consistent both with this low value of m and for the low correlation amplitude: (a) a low-density universe with strong clustering evolution parameterized by a clustering exponent e similar or equal to 1.0 such that galaxy cluster-scale structures shrink relative to the proper coordinate frame; and/or (b) a weakly clustered galaxy population, the majority of which fade or dissipate below z(med)less than or equal to 0.5 (I less than or similar to 20 mag), thus mimicking the apparently strong evolution in the correlation amplitude, A(omega). Although not directly observed using our data, the possible flattening of the slope of omega(theta) with increasing survey depth can explain the strong decline in A(omega) and allow for greater pair fraction evolution limited to m less than or equal to 1.6.