AAS 197, January 2001
Session 7. Gas in the Galactic ISM
Display, Monday, January 8, 2001, 9:30am-7:00pm, Exhibit Hall

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[7.13] The Structure of the Cold Neutral ISM on 10-100 Astronomical Unit Scales

M. D. Faison (Northwestern University/Adler Planetarium/NRAO), W. M. Goss (NRAO)

We have used the Very Long Baseline Array (VLBA) and the Very Large Array (VLA) to image Galactic neutral hydrogen in absorption towards four compact extragalactic radio sources with 10 milliarcsecond resolution. Previous VLBA data by Faison et al (1998) have shown the existence of prominent structures in the direction of the extragalactic source 3C~138 with scale sizes of 10-20 AU with changes in HI optical depth in excess of 0.8 ± 0.1. In this paper we confirm the small scale HI optical depth variations toward 3C~147 suggested earlier at a level up to 20 % ± 5% . The sources 3C~119, 2352+495 and 0831+557 show no significant change in HI optical depth across the sources with one sigma limits of 30%, 50%, and 100%. Of the seven sources recently investigated with the VLBA and VLA , only 3C~138 and 3C~147 show statistically significant variations in HI opacities.

Deshpande (2000) have attempted to explain the observed small-scale structure as an extension of the observed power spectrum of structure on parsec size scales. The predictions of Deshpande (2000) are consistent with the VLBA HI data observed in the directions of a number of sources, including 3C~147, but are not consistent with our previous observations of the HI opacity structure toward 3C~138.

The VLBA and VLA are operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc.

The author(s) of this abstract have provided an email address for comments about the abstract: m-faison@nwu.edu

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