A Study of ISM of Dwarf Galaxies Using HI Power Spectrum Analysis

We estimate the power spectrum of HI intensity fluctuations for a sample of 8 galaxies (7 dwarf and one spiral). The power spectrum can be fitted to a power law P_HI(U) = A U^\alpha for 6 of these galaxies, indicating turbulence is operational. The estimated best fit value for the slope ranges from ~ -1.5 (AND IV, NGC 628, UGC 4459 and GR 8) to ~ -2.6 (DDO 210 and NGC 3741). We interpret this bi-modality as being due to having effectively 2D turbulence on length scales much larger than the scale height of the galaxy disk and 3D otherwise. This allows us to use the estimated slope to set bounds on the scale heights of the face-on galaxies in our sample. We also find that the power law slope remains constant as we increase the channel thickness for all these galaxies, suggesting that the fluctuations in HI intensity are due to density fluctuations and not velocity fluctuations, or that the slope of the velocity structure function is ~ 0. Finally, for the four galaxies with "2D turbulence" we find that the slope \alpha correlates with the star formation rate per unit area, with larger star formation rates leading to steeper power laws. Given our small sample size this result needs to be confirmed with a larger sample.