Observations of the structure of galactic H I absorption on small angular scales

Double-component extragalactic radio sources were observed using 21-cm spectral-line interferometry to measure the variation of galactic H I opacity on angular scales of 20 arcsec to 4 arcmin. The interstellar hydrogen optical depth is highly correlated over such small angles, at least for galactic latitudes of at least about 5 deg (+ and -); in a few cases the optical depth variation is less than 0.1. The result constrains models for the internal structure of diffuse clouds; in the 'cloudlet ensemble' model, individual elements must have diameters smaller than a few arc seconds. This result supports the validity of H I spin temperatures derived from Arecibo emission-absorption observations by ruling out errors due to beam blending of small-scale opacity structure. Toward the strong source Cyg A, low-level optical depth variations are detected at low velocity and at high negative velocities.