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

Abstract

We describe the evolution of hot interstellar gas in a family of low luminosity elliptical galaxies all having $M_B = -20$ but with different degrees of flattening (E0, E2, and E6) caused by rotation. The interstellar gas is furnished by stellar mass loss. The soft X-ray luminosity Lx of the hot interstellar gas after 15 Gyrs decreases dramatically with galactic rotation. The hot gas cools onto a large disk having a size that depends inversely on the assumed supernova rate. Although Lx of the hot, rotating interstellar gas may be difficult to observe, the photoionized part of the cooled gaseous disk gas can be observed in optical line emission. The mass of HII gas ($\sim 10^8$ $M_{\odot}$) may be much greater than previously realized since large regions of rotationally supported, low density HII are optically faint. We interpret the stellar disks that are common in low luminosity ellipticals as stars that have formed in the non-ionized part of the cold disk gas. The total mass and distribution of cold disk gas available for star formation are similar to those of stellar disks observed. The high stellar H$β$ 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.