Tiny-scale atomic structure (TSAS), on scales of tens of astronomical units, was first detected twenty years ago using HI VLBI techniques; it was no’t until fourteen years later that the discovery was confirmed. TSAS has now been detected by time-variable HI absorption towards pulsars, caused by the moving pulsar sweeping its line-of-sight over the structure. In fact TSAS has been seen against every source towards which it has been sought. It has also been detected with angle-variable optical absorption lines against a sample of closely-spaced stars. The TSAS is ubiquitous within the ordinary cold neutral medium (CNM). The phenomenon is puzzling because the implied pressures are some 44 times larger than the hydrostatic pressure on the galactic plane, and 300 times larger than the standard CNM thermal pressure. This is unacceptable because the TSAS is quiescent and ubiquitous. This component has been largely neglected in interpretative discussions of the ISM. However, its very existence is antithetical to the standard ISM picture in which the various components have rough pressure equality and, at the very least, pressures low enough to be confined by the Galaxy’'s gravitational field. Recently, the TSAS has been discussed in terms of very small curved filaments or sheets. These anisotropic geometrical structures reduce the required volume densities and comcomant pressures to acceptable levels. The CNM clouds are permeated by these dense structures and the interstices are occupied by less dense, warmer gas. This is a fundamental change in our concept of the structure of cold interstellar clouds.
The NRAO 140-ft telescope has been used to detect distinct HI clouds towards X-ray ‘shadows’, regions where ROSAT results indicate that soft X-rays appear to be absorbed by intervening interstellar matter. Results in the Eridanus region indicate that most of the absorption occurs at a distance of about 100 pc, which is consistent with the model in which most of the soft X-ray background originates in a hot bubble of gas surrounding the Sun.
The NRAO 140-ft telescope has been used in a sensitive search for high-velocity HI clouds near and along line-of-sight to QSOs. About 37% of the observed directions contained high-velocity gas at velocities greater than 100 km s.
In NRAO 12-m telescope observations probing the ISM, CO J=1-0 absorption has
been detected towards some compact extragalactic mm-wave continuum sources.
Similar observations have been made with the IRAM interferometer for HCO
J=1-0 absorption; the absorption was found to be surprisingly common, occurring
in 30% as often as HI absorption for the same directions.