Interacting Binary Galaxies. III. Observations of NGC 1587/1588 and NGC 7236/7237

The catalog of isolated galaxy pairs prepared by Karachentsev has been culled for its E-E constituents. Major disturbances have been identified in ~10% of all such pairs of ellipticals. CCD pictures and long-slit spectroscopic observations have been obtained at KPNO for the most photometrically distorted of these systems. Such pairs very likely represent strongly interacting, physically associated, binary elliptical galaxies. Radial variations of rotation velocity and velocity dispersion are extracted from the spectroscopic data for each of the two galaxies in a given pair. We describe here such observations for two Karachentsev pairs, Nos. 99 and 564. The observed disturbances in rotation velocity and luminosity distribution are discussed in terms of the gravitational interaction hypothesis. In light of this discussion, we believe that we have uncovered and identified observational evidence of tidal friction in action. The evidence includes U-shaped rotation velocity curves and the morphology of the observed luminosity asymmetries. Imaging and dynamical data such as presented here are essential to the numerical simulation of interacting binaries. A preceding companion paper (Paper II) describes a particular physical model and simulation technique, while the following companion paper (Paper IV) presents specific interaction models that successfully match all of the available data for K99 and K564. In NGC 1587, the brighter component of K99, we find one of the highest rotation rates known for an E2 galaxy of average luminosity. Its rotation rate (in units of the central velocity dispersion) is v_rot_/σ = 0.6. Because this rotation is in the same sense as the binary orbital motion, the net angular momentum in this isolated binary system is large, challenging simple tidal torque theories to identify the source of this momentum. It is possible that NGC 1587 is itself a merger remnant, with its spin being a relic of the orbital angular momentum in that previous binary interaction.