H i 21 cm opacity fluctuations power spectra towards Cassiopeia A

Abstract

The angular power spectrum of HI 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 HI 21cm opacity fluctuation power spectrum towards the supernova remnant Cas A from interferometric data with spacial resolution of 5arcsec 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 Pτ(U) =Uα with a power-law index of α∼−2.86 ± 0.10 (3σ error) over the scales of 0.07–2.3 pc for the gas in the Perseus spiral arm and 0.002–0.07 pc (480–15730 au) for that in the local arm. This estimated power-law index is consistent with earlier observational results based on both HI 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).