Structure and Evolution of Interstellar Gas in Flattened, Rotating Elliptical Galaxies

We study the time-dependent evolution of interstellar gas in a family of elliptical galaxies having identical masses and central densities but various ellipticities and total angular momenta. Dark halos are assumed to be flattened in the same manner as the stars. Normal mass loss from evolving galactic stars is sufficient to account for the amount of hot interstellar gas observed. Gas ejected from stars shares the random motions of the stars and the bulk stellar velocity relative to the local interstellar medium; the ejected gas thermalizes to a temperature similar to the virial temperature of the stellar system. The random stellar motions and galactic rotation are found by solving Jeans's equations in cylindrical geometry. For a sequence of galaxies differing only in degree of flattening-E0, E2, and E4-we find that the X-ray images and luminosities are not very different. As the hot interstellar gas loses energy by radiation, it cools to the very center of these nonrotating galaxies regardless of flattening. The X-ray surface brightness is generally slightly steeper than the optical surface brightness of starlight. However, when a small but typical galactic rotation is introduced, the evolution of the interstellar medium is radically altered. The average X- ray surface brightness {SIGMA}_X_ is lower in the galactic center compared to nonrotating galaxies. This lower {SIGMA}_X_ can be achieved without invoking an ad hoc mass dropout from the hot gas. As the gas cools in rotating galaxies, it is deposited in a large disk comparable in size to the effective radius. Alter evolving for several gigayears, most of the new gas in the cooling flow is constrained by angular momentum conservation to arrive at the outer edge of the disk, Causing a local enhancement in the X-ray surface brightness. This results in flattened inner X-ray surface brightness contours that have peanut shapes when viewed nearly perpendicular to the axis of galactic rotation. As gas approaches the outer edge of the cold disk, it spins up to the local circular velocity, ~500 km s^-1^ for the galaxy studied here. This rotation can be observed with AXAF. The properties of this interesting region may be influenced by magnetic stresses that are expected to be amplified in the shearing flow. The toroidal enhancement of the X-ray images in our rotating models is large enough (~60"-100" at Virgo) to have been resolved by the Einstein satellite, but such flattening of the inner X-ray isophotes is evidently rare. The galactic X-ray isophotes can become rounder if one or more additional assumptions are made: the galactic rotation is lower beyond ~5-7 effective radii, gas is lost from the outer galaxy by ram pressure stripping or supernova winds, magnetic stresses reduce the X-ray emissivity as gas cools toward a disk, a major component of elliptical galaxies is youthful (a relatively recent merger may introduce stars and gas having relatively low specific angular momentum), etc. For galaxies that satisfy the assumptions we have made, globally rotating ellipticals with an old stellar population, the size of the toroidal X-ray cooling region increases steadily with galactic age. If this region can be observed in X-ray images, it should be possible to estimate the age of ellipticals from the size of the disk and from the observed galactic flattening and rotation. Also, if the disk is circular, the inclination of the galaxy should be immediately apparent from the X- ray image. Quite generally we predict that X-ray observations will provide important new information about the global rotation properties of elliptical galaxies at large radii, far beyond those accessible to optical observations.