Tiny scale opacity fluctuations from VLBA, MERLIN and VLA observations of HI absorption toward 3C 138

TL;DR

This study uses high-resolution radio observations to analyze tiny scale structures in neutral hydrogen around 3C 138, revealing more small-scale structures than previously thought, which impacts understanding of interstellar turbulence.

Contribution

First detailed measurement of HI opacity fluctuations at 10-200 milliarcsec scales using combined VLBA, MERLIN, and VLA data, revealing a shallower power spectrum slope.

Findings

Opacity fluctuation power spectrum index ~ 0.33

Amplitude of small-scale structures is 20-60 times higher than extrapolated from larger scales

AU scale structures are more abundant than earlier pc scale observations

Abstract

The structure function of opacity fluctuations is a useful statistical tool to study tiny scale structures of neutral hydrogen. Here we present high resolution observation of HI absorption towards 3C 138, and estimate the structure function of opacity fluctuations from the combined VLA, MERLIN and VLBA data. The angular scales probed in this work are ~ 10-200 milliarcsec (about 5-100 AU). The structure function in this range is found to be well represented by a power law S_tau(x) ~ x^{beta} with index beta ~ 0.33 +/- 0.07 corresponding to a power spectrum P_tau(U) ~ U^{-2.33}. This is slightly shallower than the earlier reported power law index of ~ 2.5-3.0 at ~ 1000 AU to few pc scales. The amplitude of the derived structure function is a factor of ~ 20-60 times higher than the extrapolated amplitude from observation of Cas A at larger scales. On the other hand, extrapolating the AU scale structure function for 3C 138 predicts the observed structure function for Cas A at the pc scale correctly. These results clearly establish that the atomic gas has significantly more structures in AU scales than expected from earlier pc scale observations. Some plausible reasons are identified and discussed here to explain these results. The observational evidence of a shallower slope and the presence of rich small scale structures may have implications for the current understanding of the interstellar turbulence.