Turbulent power spectrum in warm and cold neutral medium using the Galactic HI 21 cm emission (1812.02184v1)

Abstract: Small-scale fluctuations of different tracers of the interstellar the medium can be used to study the nature of turbulence in astrophysical scales. Of these, the `continuum' emission traces the fluctuations integrated along the line of sight whereas, the spectral line tracers give the information along different velocity channels as well. Recently, Miville-Desch^enes et al. (2016) have measured the intensity fluctuation power spectrum of the continuum dust emission, and found a power law behaviour with a power law index of $-2.9 \pm 0.1$ for a region of our Galaxy. Here, we study the same region using high-velocity resolution 21-cm emission from the diffuse neutral medium, and estimate the power spectrum at different spectral channels. The measured 21-cm power spectrum also follows a power law, however, we see a significant variation in the power law index with velocity. The value of the power-law index estimated from the integrated map for different components are quite different which is indicative of the different nature of turbulence depending on temperature, density and ionization fraction. We also measure the power spectra after smoothing the 21 cm emission to velocity resolution ranging from $1.03$ to $13.39~{\rm km~s^{-1}}$, but the power spectrum remains unchanged within the error bar. This indicates that the observed fluctuations are dominantly due to density fluctuations, and we can only constrain the power-law index of velocity structure function of $0.0 \pm 1.1$ which is consistent with the predicted Kolmogorov turbulence $(\gamma=2/3)$ and also with a shock-dominated medium $(\gamma=1.0)$.