Galactic Drips and How to Stop Them!

Abstract

The temperature of hot interstellar gas at large radii in elliptical galaxies can be lower than the mean galactic virial temperature. If so, a nonlinear cooling wave can form in the hot interstellar gas and propagate slowly toward the galactic core. If the cooling wave survives hydrodynamic instabilities, it can intermittently deposit cold gas within about 15 effective radii. For a bright elliptical the total mass deposited in this manner can approach 10^10 solar masses. The cold gas that drips out at large galactic radii may account for the young stellar populations and extended gas at $\sim 10^4$ K observed in many ellipticals, features that are often attributed to galactic mergers. Galactic drips are expected in relatively isolated (field) ellipticals provided (i) the galactic stellar velocity ellipsoids are radially oriented at large galactic radii and (ii) the current Type Ia supernova rate is sufficiently small to be consistent with interstellar iron abundances found in recent X-ray studies. Galactic drips are surpressed in ellipticals located within clusters of galaxies; when the pressure in the ambient cluster gas exceeds that in the outer parts of the galactic interstellar medium, some cluster gas flows into the galaxy which surpresses the drips.