Cooling Gas Outflows from Galaxies

We study steady, radial gas outflows from galaxies in an effort to understand the way tenuous and hot gas is transported to large distances away from galaxies. In particular, we obtain solutions for outflow problems and study the outflow topology, the effects of the galaxy potential (and thus the mass), the size of outflow regions, the efficiency of radiative cooling, and the fate of the cooled gas. Under general power-law forms for the cooling function and the gravitational field of galaxies, we show that the outflow solutions are determined by the two-parameter initial conditions. In an analogy with stellar wind or accretion problems, we demonstrate that there exist no transonic flows, but either subsonic or supersonic flows are obtainable. Solutions of the supersonic outflows are studied in detail as they are most likely to carry gas to large distances away from galaxies. We find that, if gravity is weak, the outflow is characterized by the ratio of the radiative cooling time to the flow time, t_c_/t_f_. If initially t_c_/t_f_ <~ 1 the gas cools as soon as it leaves the galaxy, whereas if t_c_/t_f_ >~ 1 the gas first cools adiabatically and then radiatively. However, if initially t_c_/t_f_ <~ 1 radiation cannot become important; in this case, depending on whether the initial gas temperature is above the escape temperature, the outflow results in either a galactic wind or a hot corona. The importance of the galactic gravitational field is characterized by the fractional energy lost radiatively within the flow time in outflows with velocity equal to the circular velocity of the galaxy; if the fraction is small, gravity is relatively strong, and it stops the outflow before the gas has a chance to cool radiatively, resulting in a hot corona. In case the gas does cool radiatively, the cooled gas is most likely to form clouds via various instabilities. The clouds coast farther away from the galaxy because of the finite kinetic energy they inherit. Depending on the initial energies, the clouds can either leave the galaxy or fall back ballistically. We apply the calculations to galaxies in general. We find that the hot gas in dwarf galaxies can either flow out as galactic winds, or cool radiatively to form clouds. In the latter case, the clouds escape the galaxies. In contrast, massive galaxies like our own tend to confine the gas. The gas released into the halo can either cool radiatively or result in a galactic corona. We present the surface brightness in various X-ray energy bands for some representative cooling outflows from dwarf and normal galaxies, and we find that the extent of the resultant X-ray emission is generally much smaller than the region of the outflows. In particular, the mean temperature averaged over entire outflow regions is shown to be a factor of 2-6 smaller than the base temperature. We also estimate the mean surface brightness of the O VI emission lines, and the predicted surface brightness is within the reach of current UV experiments. We briefly discuss the implications of cold clouds at large distances from dwarf galaxies for recent observations of QSO absorption- line systems.