Neutral Hydrogen (HI) 21 cm as a probe: Investigating Spatial Variations in Interstellar Turbulent Properties

Abstract: Interstellar turbulence shapes the HI distribution in the Milky Way (MW). How this affects large-scale statistical properties of HI column density across the MW remains largely unconstrained. We use approx 13,000 square-degree GALFA-HI survey to map statistical fluctuations of HI over the 40 km s-1 velocity range. We calculate the spatial power spectrum (SPS) of HI column density image by running a 3-degree kernel and measuring SPS slope over a range of angular scales from 16 arcmin to 20 degree. Due to GALFA complex observing and calibration strategy, we construct detailed estimates of the noise contribution and account for GALFA beam effects on SPS. This allows us to systematically analyze HI images that trace a wide range of interstellar environments. We find that SPS slope varies between -2.6 at high Galactic latitudes, and -3.2 close to Galactic plane. The range of SPS slope values becomes tighter when we consider HI optical depth and line-of-sight length caused by the plane-parallel geometry of HI disk. This relatively uniform, large-scale distribution of SPS slope is suggestive of large-scale turbulent driving being a dominant mechanism for shaping HI structures in the MW and/or the stellar feedback turbulence being efficiently dissipated within dense molecular clouds. Only at latitudes above 60 degrees we find evidence for HI SPS slope being consistently more shallow. Those directions are largely within the Local Bubble, suggesting the recent history of this cavity, shaped by multiple supernovae explosions, has modified the turbulent state of HI and/or fractions of HI phases.