Michael Faison ( Yale ), Miller Goss ( NRAO) , and Tom Muxlow ( Jodrell Bank )

1. “Intermediate” Scale Structure in Cold, Galactic HI Detected with the MERLIN Array Distance ~400 pc This optical depth greyscale image shows an unusual “hole” of low opacity across part of the continuum peak. The contour lines show the 21 cm continuum emission at 1 to 5% of the peak. This crosscut shows 1s error bars in optical depth. At 400 pc, 1” = 400 AU. Distance ~1900 pc This low-opacity velocity channel shows essentially uniform HI opacity. At 1900 pc, 1” = 1900 AU. The line of sight to 3C 123 shows strong local HI absorption and a weak component at about -20 km/s. Distance < 500 pc The local HI optical depth image shows a very strong gradient in opacity from the southeast to the northwest of this lobe of 21 cm continuum emission. The absorption line goes from completely opaque to nearly zero in about 1 arcsecond. Michael Faison (Yale), Miller Goss (NRAO), and Tom Muxlow (Jodrell Bank) In this case, “intermediate” scales are angular scales of 130 to 2000 arcseconds (corresponding to physical scales of ~100 to 4000 AU) in the gap between the angular resolutions of the VLA and the VLBA at 21 cm. We’ve used the MERLIN array to image Galactic HI in absorption toward two moderately resolved extragalactic sources, and we’ve detected significant optical depth variations in the local HI over these scales. The line of sight to 3C 161 through Quadrant III shows large local HI opacity and absorption components out to a kinematic distance of ~3.6 kpc. This position-velocity spectrum along the main axis of 3C 161 shows the optical depth variations in adjacent velocity channels as well as the typical signal-to-noise ratio in low continuum channels. Some features may show gradients in velocity, such as the optically thick feature labeled “A.” A position-velocity spectrum along the main axis of 3C 123 shows the strong optical depth gradient in the local HI. The absorption component at -20 km/s is essentially uniform. Again, some gradient in velocity may be seen in the feature labeled “B.” Conclusions: • 140 mas angular scale variations detected in cold, Galactic HI on two new sight-lines. • The local gas toward 3C 161 shows a “hole” with a 50% drop in optical depth over a projected scale of ~ 200 AU. • The local gas toward 3C 123 shows a strong gradient from opaque to clear over a projected scale less than ~ 500 AU. • More sensitivity on these angular scales is needed to bridge the connected-element interferometer and VLBI probes of Galactic HI structure.