Warps Caused by Jet-Induced Pressure Gradients in Radio Galaxies

Abstract

In radio galaxies such as M84 dust features tend to be nearly perpendicular to radio jets yet are not aligned with the galaxy isophotes. The timescale for precession in the galaxy is short suggesting that an alternative mechanism causes the gas disk to be misaligned with the galaxy. In M84 we estimate the pressure on the disk required to overcome the torque from the galaxy and find that it is small compared to the thermal pressure in the hot ambient ISM estimated from the X-ray emission. We therefore propose that pressure gradients in a jet associated hot interstellar medium exert a torque on the gas disk in M84 causing it to be misaligned with the galaxy isophotal major axis. This mechanism could explain the connection between gas disk angular momentum and jet axes in nearby radio galaxies. By integrating the light of the galaxy through a warped gas and dust disk we find that the geometry of gas disk in M84 is likely to differ that predicted from a simple precession model. We find that the morphology of the gas disk in M84 is consistent with a warped geometry where precession is caused by a combination of a galactic torque and a larger torque due to pressure gradients in the ambient X-ray emitting gas. Precession occurs at an axis between the jet and galaxy major axis, but nearer to the jet axis implying that the pressure torque is 2-4 times larger than the galactic torque. A better model to the morphology of the disk is achieved when precession takes place about an elliptical rather than circular path. This suggests that the isobars in the hot medium are strongly dependent on angle from the jet axis.