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Session 117 - Galactic ISM.
Oral session, Thursday, January 16
Harbour B,
A model is proposed which allows an estimation of the distance to infalling interstellar clouds in the galactic halo by measuring the cloud velocity and column density, and assuming a model for the vertical density distribution of the Galactic interstellar medium. It is shown that falling clouds with N(H I) ^<_\sim10^19 cm^-2 can be decelerated to a terminal velocity which increases with increasing height above the Galactic plane. This terminal velocity model correctly predicts the distance to high velocity cloud Complex M and several other interstellar structures of previously determined distance. It is demonstrated how interstellar absorption spectra alone may be used to predict the distances of the clouds producing the absorption. If the distance, velocities, and column densities of enough interstellar clouds are known, the procedure can be reversed, and the terminal velocity model may also be used to estimate the vertical density structure (both mean density and porosity) of the ISM.
The same model may be used to interpret quasar absorption line spectra. Since both the ionization level and terminal velocity of halo clouds increase with increasing distance from the central galaxy, velocity resolved profiles of highly ionized gas will have a greater width than low ionization gas. A line of sight passing through the center of a spherical, static halo (an idealized damped Ly \alpha system), will yield low ionization absorption at the velocity of the galaxy, flanked by high ionization on either side. Reasonable halo parameters yield total velocity extents for C IV of \Delta v_C IV=100-200 km s^-1. This qualitative and quantitative behavior agrees with the recent observations of Lu et al (1997). This work was supported by the Minnesota Supercomputer Institute.
