Evolution of Interstellar Gas in Rapidly Rotating Elliptical Galaxies: Formation of Disks

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Abstract
We describe the evolution of interstellar gas in a family of low-luminosity elliptical galaxies all having MB = -20 but with different degrees of flattening (E0, E2, and E6) and two current supernova rates, SNu = 0.01 and 0.04. The galaxies are composed of 90% dark matter, are rotationally flattened, and have isotropic stellar velocity dispersions. The soft X-ray luminosity of the hot interstellar gas after evolving for 15 Gyr decreases dramatically with increasing galactic rotation. As the rotating hot interstellar gas loses energy in the galactic potential, it cools onto a large disk. The outer radius of the disk can be much reduced by increasing the supernova rate, which drives a gentle galactic wind transporting high angular momentum gas out of the galaxy. The total mass of cooled disk gas is less sensitive to the supernova rate. Although the hot interstellar gas may be difficult to observe in rotating low-luminosity ellipticals, the cooled disk gas can be observed (1) in optical line emission, since part of the cooled disk gas is photoionized by stellar UV, and (2) in the optical continuum, assuming the colder disk gas forms into luminous stars. The mass of H II gas (~108 M☉) may be much greater than previously realized, since rotationally supported, low-density H II contributes little to the global optical line emission. We interpret the stellar disks that are common (or ubiquitous) in low-luminosity ellipticals as stars that have formed in the cold disk gas. The total mass of cold disk gas available for star formation is similar to the masses of stellar disks observed. The high stellar Hβ photometric index observed in disky ellipticals can be understood by combining the light of young disk stellar populations with that of the old bulge population.
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