Evidence for Galaxy Interactions/Mergers from Meduim-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<=24.3 mag, whereas in the MDS sample, fields have varying 95% completeness limits in the range I<=23.6--24.8 mag. In each field roughly 400 galaxies per 5 square arcminute field are detected. We exploit high resolution WFPC2 images to systematically determine morphological classifications of galaxies as disk- or bulge-dominated down to I<=23 mag (V<=24 mag), and to differentiate galaxies from stars one magnitude fainter. Down to I<=25 mag the number of galaxy pairs with separations less than 3 arcsec is consistent with a shortward extrapolation of the angular two-point correlation function w(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 w(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 <3 arcsec-pair sample. 
L. W. Neuschaefer, M. Im, K. U. Ratnatunga, R. E. Griffiths and S.   Casertano