Statistical properties of the emission in mixed-morphology (E+S) pairs. I.Optical results

We present a statistical analysis of the optical emission properties for a mixed-morphology (E + S) sample of galaxy pairs. The sample is large and di verse enough (similar to 150 pairs) to permit discrimination of pairs by mo rphological types. It samples a large enough volume of space to justify der ivation of the optical luminosity function. We find an average factor of 1. 6 enhancement in the optical luminosity of the late-type pair components re lative to an isolated galaxy control sample. This is interpreted as the opt ical signature of the interaction-star formation connection. We find a comp lete absence of dwarf pairs (M-Zw greater than or equal to -17.0) that is p robably related to a corresponding lack of isolated E/S0 dwarf galaxies in the control sample. The spirals in (E + S) and (S + S) pairs show similar l evels of optical enhancement but fail to show a trend toward higher luminos ity with decreasing companion separation. A first-order comparison of galax y type distributions between our pair and isolated samples suggests that (1 ) pairs show an excess of elliptical and early-type spiral (Sa-b) component s and (2) corresponding deficit of S0s and late-type spirals in the "E" and "S" pair components, respectively. We consider both possible bias and dyna mical explanations for (1) the observed (E + S) spiral-component luminosity excess and (2) the morphology excesses/deficits.