NCRA Colloquium

Speaker : Avinash Deshpande (Raman Research Institute) 
Title : The Small-scale Structure in Interstellar HI: Truth Or Consequences. 

Abstract: 
During the past decades, measurements of Galactic HI absorption against extra-galactic sources, as well as multi-epoch observations in pulsar directions, have detected opacity variations on small transverse scales corresponding to tens of AU at the distance of the absorbing matter. Hitherto these measurements have been interpreted as small-scale structure in the HI distribution with densities n_HI ∼ 10^ − 10^5 per cc, orders of magnitude greater than those of the parsec-scale structure. 
Naturally it is difficult to imagine how such structures could exist in equilibrium with other components of the ISM. 

The talk will highlight some of the turning points in the HI studies that lead to excitement about the so-called small-scale structure, particularly being viewed as due to a "new population" of dense cloudlets, similar to the Fiddler blobs in the context of extreme scattering events. We will reassess the expected magnitude of small-scale variations based on distribution of HI opacities at larger scales, and question some of the naive assumptions made in early interpretations. For both, the HI and the ionized components, we will see how there is (a) no compelling evidence for wide-spread over-dense, over-pressured structures, and (b) no surprise in these apparent density enhancements on small transverse scales. For completeness, we will briefly discuss other indications that suggest existence of a small component of ISM which may be out of pressure equilibrium with the rest of the medium.