A study of interstellar medium of dwarf galaxies using HI power spectrum analysis

Abstract

We estimate the power spectrum of HI intensity fluctuations for a sample of eight galaxies (seven dwarf and one spiral). The power spectrum can be fitted to a power-law P_Hi(U) for six of these galaxies, indicating turbulence is operational. The estimated best-fitting 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 disc 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 H i 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 α correlates with the star formation rate (SFR) per unit area, with larger SFRs leading to steeper power laws. Given our small sample size, this result needs to be confirmed with a larger sample.