Tiny Hi Clouds in the Local Ism

We report deep, high spectral resolution WSRT H I 21cm 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 I column densities of 0.4–8 ×1018 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 I structures and their distribution in the ISM is at present unknown. We measure small-scale opacity fluctuations two orders of magnitude larger than predicted from the structure function models of Deshpande (2000), suggesting significant injection of small-scale power into the ISM.