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Session 46 - Interstellar Scattering and Scintillation as Tools in Radio Astronomy.
Topical, Oral session, Tuesday, June 09
Sierra/Padre,
We have used the Very Long Baseline Array (VLBA) in addition to the phased Very Large Array (VLA) with angular resolution 10 - 20 milliarcseconds (mas) to image the Galactic HI in absorption in the direction of three bright extragalactic objects 3C138, CJ1 2255+416, and CJ1 0404+768. The angular resolution of these images is 20 milliarcseconds in the case of 3C138 and 10 milliarcseconds for 2255+416 and 0404+768. HI optical depth variations up to a factor of two are seen in the directions of 3C138 and 2255+416 on angular scales down to the resolution of the images. 0404+768 shows much smaller variations of only about 15% over 10 milliarcseconds, and is consistent within the errors of showing no significant optical depth variations. The small-scale opacity structure seen towards 3C138 and 2255+416 could be due to density variations, spin temperature variations, velocity turbulence in the atomic gas, or a combination of these effects. If the variations are due to density fluctuations, this implies high density structures of order 10^6 cm^-3 in the cold neutral medium on physical scales of 10 AU or less. This implies thermal pressures in the small-scale structure several orders of magnitude greater than the that observed in the general diffuse ISM. However, if the large column density variations are due to elongated structures such as filaments or sheets seen edge on, the densities (and pressures) can be reduced by envoking an elongation factor along the line of sight.
