HI 21 cm opacity fluctuations power spectra towards Cassiopeia A

Abstract

The angular power spectrum of H I 21 cm opacity fluctuations is a useful statistic for quantifying the observed opacity fluctuations as well as for comparing these with theoretical models. We present here the H I 21 cm opacity fluctuation power spectrum towards the supernova remnant Cas A from interferometric data with spacial resolution of 5 arcsec and spectral resolution of 0.4 km s⁻¹. The power spectrum has been estimated using a simple but robust visibility-based technique. We find that the power spectrum is well fitted by a power law P_τ(U) =Uα with a power-law index of α^~;−2.86 ± 0.10 (3s error) over the scales of 0.07-2.3 pc for the gas in the Perseus spiral arm and 0.002-0.07 pc (480-15 730 au) for that in the local arm. This estimated power-law index is consistent with earlier observational results based on both H I emission over larger scales and absorption studies over a similar range of scales. We do not detect any statistically significant change in the power-law index with the velocity width of the frequency channels. This constrains the power-law index of the velocity structure function to be β= 0.2 ± 0.6 (3σ error).