Angular structure in HI

Structure in Galactic HI is found on all scales accessible to observation. A good example of this range of structure may be found in the study of the M13 region by Shaw et al. (1996). Features are seen with the Lovell Telescope on scales of 12 arcmin and greater. Observations at 2 arcmin resolution with the DRAO Synthesis Telescope show structure on that scale while optical evidence from NaI and CaII spectra, and by implication HI, indicates significant differences in the line of sight to stars arcsec apart. This interstellar material is believed to be at a distance of pc where 1 arcmin corresponds to 0.03 pc.

Radio interferometer data on HI clouds seen in absorption against extragalactic radio sources show structure down to 0.1 arcsec. Davis, Diamond & Goss (1996) find 10-30 percent changes in optical depth over 0.1 arcsec in galactic HI lying in front of the sources 3C138 and 3C147.

б© Copyright Astronomical Society of Australia 1997