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Technical Report CMS-TR-20140501


Title The structure function of Galactic H I opacity fluctuations on au scales based on MERLIN, VLA and VLBA data
Author/s Prasun Dutta, Jayaram N. Chengalur,
National Centre for Radio Astrophysics, Post Bag 3, Ganeshkhind, Pune 411 007, India

Nirupam Roy
Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, D-53121 Bonn, Germany

W. M. Goss
National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA


Mihir Arjunwadkar
National Centre for Radio Astrophysics, Post Bag 3, Ganeshkhind, Pune 411 007, India
Centre for Modeling and Simulation, Savitribai Phule Pune University, Pune 411 007, India


Anthony H. Minter
National Radio Astronomy Observatory, PO Box 2, Green Bank, WV 24944, USA

Crystal L. Brogan
National Radio Astronomy Observatory, PO Box 2, Green Bank, WV 24944, USA


and

T. J. W. Lazio
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Abstract We use MERLIN, VLA and VLBA observations of Galactic H I absorption towards 3C138 to estimate the structure function of the H I opacity fluctuations at au scales. Using Monte Carlo simulations, we show that there is likely to be a significant bias in the estimated structure function at signal-to-noise ratios characteristic of our observations, if the structure function is constructed in the manner most commonly used in the literature. We develop a new estimator that is free from this bias and use it to estimate the true underlying structure function slope on length-scales ranging from 5 to 40 au. From a power-law fit to the structure function, we derive a slope of 0.81+0.14−0.13, i.e. similar to the value observed at parsec scales. The estimated upper limit for the amplitude of the structure function is also consistent with the measurements carried out at parsec scales. Our measurements are hence consistent with the H I opacity fluctuation in the Galaxy being characterized by a power-law structure function over length-scales that span six orders of magnitude. This result implies that the dissipation scale has to be smaller than a few au if the fluctuations are produced by turbulence. This inferred smaller dissipation scale implies that the dissipation occurs either in (i) regions with densities >~ 103 cm-3 (i.e. similar to that inferred for ‘tiny scale’ atomic clouds or (ii) regions with a mix of ionized and atomic gas (i.e. the observed structure in the atomic gas has a magnetohydrodynamic origin).
Keywords Turbulence; ISM: atoms; ISM: general; ISM: structure; radio lines: ISM
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Citing This Document Prasun Dutta, Jayaram N. Chengalur, Nirupam Roy, W. M. Goss, Mihir Arjunwadkar, Anthony H. Minter, Crystal L. Brogan, and T. J. W. Lazio , The structure function of Galactic H I opacity fluctuations on au scales based on MERLIN, VLA and VLBA data . Technical Report CMS-TR-20140501 of the Centre for Modeling and Simulation, Savitribai Phule Pune University, Pune 411007, India (2014); available at http://cms.unipune.ac.in/reports/.
Notes, Published Reference, Etc. Published as MNRAS 442, 647–655 (2014)
Contact prasun AT ncra.tifr.res.in
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