Tiny HI Clouds in the Local ISM

We report deep, high spectral resolution WSRT H  21 cm observations of four high latitude compact radio sources, that have revealed a new population of tiny, discrete clouds in the diffuse ISM, with peak optical depths τ ∼ 0.1−2%, H  column densities of 0.4-8 × 10 18 cm −2 and core temperatures of 20-80 K. Imaging detections confirm these low column densities and imply linear core dimensions of a few thousand AU, assuming a distance of 100 pc. The physical origin of these tiny H  structures and their distribution in the ISM is at present unknown. Further observations will be required to determine whether they are a ubiquitous component of the ISM.