X-Ray Constraints on Galaxy-Gas-Jet Interactions in the Dumbbell Galaxies NGC 4782 and NGC 4783 in the LGG 316 Galaxy Group

Abstract

We present results from a 49.3 ks Chandra X-ray observation of the strongly interacting dumbbell galaxies NGC 4782 (3C 278) and NGC 4783 that constrain the kinematics of the interaction and models for bending the radio jets associated with NGC 4782. The galaxies are embedded in an approximately spherical distribution of group gas, centered on NGC 4782, that, away from the interaction region, is nearly isothermal with mean temperature kT~1.4+/-0.4 keV. The X-ray morphology suggests that NGC 4783 is infalling into a single, massive galaxy group (LGG 316) with NGC 4782 nearly at rest at the center of the group potential. NGC 4783 shows a sharp X-ray surface brightness edge (cold front) to the east and a ~15 kpc ram pressure-stripped tail to the west. Analysis of this cold front indicates that NGC 4783 is moving east with a total velocity 870⁺²⁷⁰_-400 km s^-1 (Mach 1.4^+0.5_-0.7) at an inclination angle 46° (>33°) toward us with respect to the plane of the sky. A ~45 Myr old X-ray cavity, with enthalpy of 4.4×10⁵⁷ ergs, coincides with the eastern radio lobe of 3C 278. X-ray knots are found on both the radio jet and counterjet, coincident with peaks in the radio emission. If we assume a light, mildly relativistic jet in 3C 278, then ram pressure velocities of 100-200 km s^-1 impacting the eastern jet and ~170 km s^-1 acting on the western radio lobe are sufficient to produce their observed bending. These velocities may be caused by bulk motions of the surrounding gas induced by the high-velocity interaction between the galaxies, by the motion of the host galaxy NGC 4782 relative to the IGM, or by a combination of these processes.