Shining light on the dark Milky Way: probing our galaxy's hidden gas

The evolution of galaxies is driven by the gas-to-stars cycle of matter in the interstellar medium (ISM). Throughout this process, the transition of hydrogen gas from its atomic (HI) to molecular (H2) form is at the heart of the story. This transition is strongly coupled with the evolution of interstellar dust, as dust grains play a key role in the formation of molecules. However, despite decades of effort, there still remains much uncertainty on the constraints for the physical properties of both the gas and dust in the ISM, due to the limits of our observational capabilities. Presented here in this thesis are attempts to address some of these outstanding issues via observations of atomic and molecular media and detailed data analyses of thermal dust emission and reddening. To begin with, I present a large-scale survey of H I emission and absorption using the Arecibo radio telescope, which explores the physical properties of the cold and warm atomic gas in the vicinity of five giant molecular clouds (GMCs): Taurus, California, Rosette, Mon OB1, NGC 2264. Strong HI absorption was detected towards all 79 background continuum sources in the 60 × 20 square degree area of the sky. Both Gaussian and pseudo-Voigt spectral decompositions were performed to derive the temperatures, optical depths and column densities of the cold and warm atomic gas. The properties of the cold gas in these regions of interest appear to be universal with an excitation (or spin) temperature of ∼ 50 K, consistent with existing estimates. The fraction of warm gas is about 60%, and nearly 40% of this warm gas resides in thermally unstable regime 500-5000 K. There is more cold gas around GMCs than in diffuse regions. The optically thin assumption, on average, underestimates HI column density by ∼ 24%. Comparing the total HI column densities with those derived under the optically thin assumption also allowed me to assess the impact of opacity effects, and determine the most suitable method for opacity correction - that is to apply region-dependent uniform spin temperatures for available HI emission data. Next, I dived into several data sets including opacity-corrected HI column densities from the Arecibo Millennium Survey and 21-SPONGE, together with thermal dust emission data from the Planck satellite, new dust reddening maps from Pan-STARRS 1 and 2MASS, newly published Millennium Survey OH data, and follow-up CO emission observations. In combining all of these datasets, I confirmed linear correlations between dust optical depth τ 353 , dust reddening E ( B − V ) and total gas column density N H in the range (1-30) × 10 20 cm − 2 , along atomic sightlines with no molecular gas detections. I found a 60% higher N H / E ( B − V ) ratio than the current "standard" value, and was able to disentangle the effects of dust grain evolution and so-called "dark gas" (the gas component not detected by H I and CO) on the variation of dust opacity σ 353 = τ 353 / N H . I also derived an OH/H 2 abundance ratio of X OH ∼ 1 × 10 − 7 , finding no evidence for systematic trends in X OH with molecular column density N H 2 in the range N H 2 ∼ ( 1 - 100 ) × 10 20 cm − 2 . These results suggest that the OH molecule may be used as a reliable tracer for the molecular ISM this range, which includes sightlines with both CO-dark and CO-bright gas. Finally, I was involved as co-author in a third project on OH emission-absorption measurements and dark molecular gas. We used OH absorption measurements from the Arecibo Millennium Survey and our own follow-up CO survey. The derived excitation temperatures of the two OH main lines at 1665 and 1667 MHz peak at ∼ 3.5 K, close to the background continuum temperature (CMB + synchrotron), providing an explanation for why OH is often difficult to detect in emission. The OH main lines are optically thin with optical depths always below 0.25; and in general these lines are not in local thermodynamic equilibrium, with a difference in excitation temperatures of T ex ( 1667 ) − Tex (1665) < 2K. About half of all detected OH absorption components do not have corresponding CO emission detections. This implies that the OH ground-state main lines trace more effectively molecular gas than traditional CO emission. In this work, I per- formed all the OH analysis, which is the main part of this study. However, I did not contribute to the discussions on CO analysis and the dark molecular gas.