LOW-TEMPERATURE GALACTIC FOUNTAINS

Observations show that neutral gas is present in the halo of the galaxy with a characteristic height of 1 kpc, NH ∼ 1019-1020 cm-2, and -100 km s-1 < v < 50 km s-1. To examine whether these observations can be reproduced by the galactic fountain model, we have performed analytic as well as one- and two-dimensional hydrodynamic calculations. The temperature and density at the base of the halo are the important free parameters. Fluid flow in the halo is calculated in a Cartesian frame with a constant gravitational force and optically thin radiative losses. When the cooling time (τcool) is less than the sound crossing time (τs) in the halo gas, a transonic fountain occurs in which cool clouds are produced in the upward flow. If τs < τcool < 10τs, a steady subsonic fountain occurs, also with a well-defined cloud formation layer. In these cases, the density at the base of the flow determines the velocity and mass flux, while the choice of temperature determines the height of the cloud formation layer. For halo pressures appropriate for the solar circle, transonic flow occurs for T ≳ 3 × 105 K, n ≳ 1.5 × 10-3 cm-3, and steady subsonic flow occurs when 2 × 105 K < T ≲ 106 K, 5 × 10-4 cm-3 ≲ n ≲ 1.5 × 10-3 cm-3. When τcool ≳ 10τs, which occurs when T ≳ 106 K, n ≲ 5 × 10-4 cm-3, convective or Rayleigh-Taylor instabilities disrupt the steady flow, broadening and complicating the region of cloud formation. The model that best reproduces the observations is near the transonic regime and has a temperature and density at the base of the fountain of 3 × 105 K and 1 × 10-3 cm-3. The cooling time for the hot gas is 3.3 × 107 yr, and the time for newly formed clouds to return to the disk is 4.7 × 107 yr, neglecting drag forces. If extended uniformly over the galaxy using a radius of 10 kpc, the mass flux rate onto one side of the disk is 0.2 M⊙ yr-1, and the energy radiated is 2.9 × 1039 ergs s-1, mostly in the extreme ultraviolet region.