Interacting Binary Galaxies. V. NGC 4782/4783 (3C 278): Unbound Colliders, Not a Supermassive Pair

The elliptical galaxies NGC 4782 and 4783 form a close pair with a very high relative velocity (658 km s^-1^ along the line of sight). This is indicative of either a very large system mass, a chance superposition, or an unbound encounter. From both the large relative velocity of the pair and the large central stellar velocity dispersions (392 km s^-1^ in NGC 4782; 297 km s^-1^ in NGC 4783), it would appear that one or both of these galaxies is very massive. Photometric distortions in the galaxies, whose shape and orientation are indicative of an intense tidal interaction, argue against superposition and in favor of a high-velocity encounter. In order to test these various hypotheses and to assist in the search for a viable interaction model for this system, CCD imaging data have been analyzed in conjunction with published spectroscopic measurements of this system. These data provide the input to our gravitational interaction model. Quantitative comparisons between the models and the data are made by using measurable indices that quantify the distortions seen in the galaxies. Strong constraints are imposed by the large relative velocity of the pair and by the degree to which the galaxies overlap. An excellent match between model and observation has resulted, permitting us to deproject the pair, to deduce the binary orbital parameters, and to determine the total system mass with a systematic error significantly smaller than that of standard mass estimators. Our best-fit model indicates that the binary orbit was hyperbolic before the encounter, but that energy loss during the collision has led to a loosely bound system. The pair will merge in 800 million years unless its binary orbit is disturbed by another member of the group of galaxies to which NGC 4782/4783 belongs. The total system mass in the region occupied by the luminous matter is found to be 1.4 x 10^12^ M_sun_ (H_0_ = 60 km s^-1^ Mpc^-1^), corresponding to a ratio of mass to blue luminosity of 10 (solar units). We are now observing the system just after closest approach, at an angle of ~25^deg^ from the orbital plane. Our simulations confirm earlier theoretical studies which found that the internal velocity dispersions of colliding galaxies tend to increase just past closest approach, which then explains the large values measured for the central stellar velocity dispersions in these galaxies.