H I absorption in the fourth galactic quadrant
Creators: name: Brown, C
Publisher: name: University Of Tasmania
Year: 2023

description: The position of the Sun, within the Milky Way, prohibits us from observing our Galactic system as a whole. Our knowledge of the structure of our Galaxy has increased dramatically; however, there is still no consensus on many seemingly simple questions. Primarily, How many spiral arms does the Milky Way have?, Where are they? And, What's on the far side of the Galactic center? Galactic H II regions are the formation sites of massive OB stars. Because the main sequence lifetimes of OB stars are short - only ‚Äöv†¬¿tens of millions of years - they thus reveal the locations of current star formation sites. They are therefore found only in locations of active star formation, primarily the spiral arms; H II regions are therefore the archetypal tracers of Galactic spiral structure. The systemic velocity of each H II region, from an observed radio recombination line (RRL), can be applied to a Galactic rotation model in order to calculate the kinematic distance to the region. In the inner Galaxy, line of sight distances are bi-valued such that a single systemic velocity corresponds to two line of sight distances equidistant to the subcentral (tangent) point. Kinematic Distance Ambiguity Resolutions (KDARs) can be made via an analysis of neutral hydrogen (H I) absorption towards each region. We determine a modification of the terminal-velocity KDAR method which investigates H I absorption in velocity channel intervals, rather than in single channels. In addition, we use H I absorption associated with known Galactic structures (in longitude-velocity space) to provide lower-limits on line of sight distances. In this manner we are able to constrain the line of sight distance to over 200 H II regions; including regions located in the extreme inner Galaxy. We also compile a complete census of HII regions with known RRL velocities, taken from the literature - and the H I emission/absorption spectrum pairs toward them ‚Äö- within the bounds of the Southern Galactic Plane Survey; a total of 252 regions. As the result of this work, the list of known fourth quadrant H II regions for which a KDAR analysis has either been prepared for, or attempted, is now exhausted ‚Äö- comparable to the northern hemisphere at the conclusion of Anderson &amp; Bania (2009). This exhaustion of candidates in the northern hemisphere was overcome through the undertaking of large scale HII region discovery surveys. The results of which more than doubled the number of known HII regions within the surveyed longitude range ‚Äö- reinvigorating HII region kinematic distance studies in the northern hemisphere. Following the candidate selection strategies of the northern surveys, we perform a pilot HII region discovery survey in the southern hemisphere. Using the Australia Telescope Compact Array we confirm 28 H II region candidates as genuine regions for the first time. As a result of these analyses, we have made the largest study of H I absorption towards H II regions in the fourth Galactic quadrant ever performed ‚Äö- as well as providing datasets for future works.