Why Are Rotating Elliptical Galaxies Less Elliptical at X‐Ray Frequencies?

Abstract

If mass and angular momentum are conserved in the cooling flows associated with luminous, slowly rotating elliptical galaxies, the flow cools onto extended, massive disks of rotationally supported cold gas in the equatorial plane. As the hot interstellar gas approaches the disks, its density and thermal X-ray emission increase, resulting in X-ray images that are considerably flattened toward the equatorial plane out to an optical effective radius or beyond. Remarkably, the flattening of X-ray images due to rotation is very small or absent at the spatial resolution currently available to X-ray observations. This is strong evidence that mass and angular momentum are not in fact conserved. In particular, if cooling flows are depleted by localized radiative cooling at numerous sites distributed throughout the flows, disks of cooled gas do not form and the X-ray images appear nearly circular. Even in this case, however, the spatial distribution of the cooled gas and any young stars that may have formed from this gas would be decidedly flatter than the old stellar population; if the young stars are optically luminous, the Balmer lines they contribute to the stellar spectra should be more elliptical than the total stellar light. In principle, X-ray images of galactic cooling flows can also be circularized by the turbulent diffusion of angular momentum away from the axis of rotation. But the effective turbulent viscosity of known processes—stellar mass loss, supernovae, cooling site evolution, etc.—is insufficient to circularize the X-ray images appreciably. Radial gradients in the interstellar iron abundance in elliptical galaxies similar to those observed are unaffected by the expected level of interstellar turbulence since these gradients are quickly reestablished by Type Ia supernovae.