On the Spatial Structure and Properties of Cold Neutral Hydrogen Gas In the Milky Way

Abstract

Despite its long period of study, there still remains no consensus on the dominant processes that control the rate of star formation. Neutral hydrogen (HI), being the most abundant element in the Universe, provides an important tracer, both for large scale galactic morphology and as the fundamental building block of stars. Improving our knowledge of the physical properties of HI and its transition between warm and cold phases is thus key to understanding the cycle of star formation and chemical enrichment in the Milky Way. Despite ongoing efforts, there still remains great uncertainty on the physical properties of cold and warm HI due to observational constraints of previous surveys. Presented here in this thesis are attempts to address questions surrounding the phase distribution and spatial structure of HI through detailed analysis of HI absorption and emission of the Milky Way from the Galactic Australian Square Kilometre Array Pathfinder HI Survey (GASKAP-HI).

The first component of my thesis makes use of the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope, using the ~500 sightlines where HI 21cm absorption was detected in the Milky Way at high Galactic latitudes, and the ~2000 sightlines with no detections. The signal-to-noise is reduced through stacking sightlines to detect warmer gas in absorption. Gas was detected with a temperature of ~1300 K gas ubiquitous throughout the region, along with a lack of temperature variation spatially across the plane-of-sky. The work also provides a test for the efficacy of stacking HI spectra to detect higher temperature gas for future studies, finding that stacking, while producing lower limits on optical depth measurements, still allows for accurate measurements of temperature for components previously hidden under noise.

Next, I have analysed the spatial variation of neutral hydrogen cold gas density across the plane-of-sky through computing structure functions of the detected GASKAP-HI absorption sightlines. The warm and cold gas follow different spatial power-law slopes (-2.6 vs -2.3 respectively), with both slopes being shallower than Kolmogorov turbulence (-3.6). The cold gas has a shallower slope due to more power existing on smaller spatial scales, where the cold gas is formed through condensation and shocks. I have also, for the first time, calculated the structure function of temperature of cold HI gas across the plane-of-sky. The structure function slope of the spin temperatures was flat, implying the temperature of cold gas is random across the plane-of-sky, not tied to any specific physical HI structures. Comparing with MHD simulations, the flatness of the structure function may indicate potential uncharacterised errors in the calculation of spin temperatures, and the need for a review of the currently accepted method for estimating temperatures for potential improvements.

Finally, I present work focussed on characterising instabilities found in the HI emission data of a Galactic filament. Through Gaussian decomposition tools such as ROHSA, I was able to separate out spatially coherent maps of the individual neutral HI phases from emission data to study their interactions. The cold gas is concentrated within small scale structures undergoing Rayleigh-Taylor instabilities, with the warmer, more diffuse phases of HI gas enveloping the structure. Using the correlation between HI and dust to estimate physical distances to the structure, the wavelength of the Rayleigh-Taylor instabilities is estimated to be approximately 1 parsec in size. The work also investigates the effects of resolution on estimating cold gas mass fractions, since extra-galactic observations lack the spatial resolution of Milky Way surveys. Spatial resolution appears to have no effect on the mean and median cold neutral medium mass fraction, but the maximum value decreases with decreasing resolution. Show more