In the two decades which have passed since Shapiro and Field (1976,
ApJ, 205,
762) first proposed the Galactic Fountain model for the galactic halo of
interstellar gas and Shapiro and Moore (1976, ApJ, 207, 460)
calculated the
time-dependent, nonequilibrium ionization and radiative cooling of a hot,
optically thin cosmic gas, including the emergent radiation spectrum, the
observational support for the Galactic Fountain model has grown considerably.
One of the longstanding puzzles which made the Fountain explanation of halo
gas difficult, however, was only resolved in the last several years, by
a renewed attack on the theory of the Galactic Fountain by Shapiro and his
Ph.D. student Benjamin, which addressed the fact that UV absorption line
strengths of highly ionized species C IV, Si IV, and N V due to halo gas
were inconsistent with both the collisionally-ionized gas calculations of
Shapiro and Moore and others and with the alternative picture of a halo of
photoionized gas. In an attempt to reconcile the model with these and other
observations of the galactic halo of the Milky Way and other galaxies,
radiative transfer was included
for the first time, in new calculations of the
nonequilibrium thermal and ionization history of gas cooling radiatively
from T~10^6K to T~10^4K
in a 1D, planar, steady flow model of the galactic
fountain
(
Shapiro and Benjamin 1993; Benjamin & Shapiro 2000, submitted;
Benjamin Ph.D. thesis).
These results showed that such a
flow is capable of matching the UV absorption and emission lines observed from
highly ionized species in our Galactic halo, provided that "self-ionization"
by "self-illumination" - the photoionization feedback of radiation
emitted by
the flow on the flow itself - is included, and that gas stops compressing
isobarically at some point as it cools. A transverse magnetic field
in the cooling flow would naturally produce this required non-isobaric
cooling. To demonstrate this explicitly, an MHD radiative shock code was also
written which incorporated all of the nonequilibrium ionization rate equations,
cooling, and radiative transfer described above, and the results confirmed
that a radiative shock of a few hundred kilometers per second in gas with
microgauss strength magnetic field would reproduce the results of the
cooling flow calculations described above, as required to explain the halo
observations
(Benjamin & Shapiro 2000, submitted;
Benjamin Ph.D. thesis).
This is but one plausible example of
how the necessary non-isobaricity of the Fountain flow might be achieved.
The same basic Fountain model was then applied to explain the absorption lines
due to metals in Lyman Limit System quasar absorption line gas at high
redshift as due to cooling gas in galactic halos and a successful match to
observed line strengths was obtained, including the prediction that the same
gas should show other highly ionized species like O VI, which has since been
detected
(
Shapiro and Benjamin 1993; Benjamin & Shapiro 2000, submitted;
Benjamin Ph.D. thesis).