HIabsorption data from the VGPS and SGPS were used to determine the Kinematic Distance Ambiguity Resolution for 119 radio continuum sources. Kinematic distances were calculated for 86 of these HII regions. These KDAR determinations are mostly in agreement with previous studies (<25% disagreement), providing strong evidence that theSchanalysis is an efficient KDAR method.
New RRL velocity surveys (i.e. GBTHRDS) will identify increasingly fainter HIIregions increasingly further from the Sun. In order to assess these new HIIregions, especially those beyond the Solar circle, a KDAR method that is applicable to faint sources will be required. The SchKDAR analysis used in this paper can provide that method. The sample of resolved kinematic distances appears to trace Galactic structure components, including spiral arms and possibly the end of the Bar. Existing KDAR methods are more unreliable for existing (low) resolution HI data, including the SGPS used thoughout this work (Urquhart et al. 2012). Therefore, in order to achieve as many KDARs as possible, a method that can be applied to both faint sources - likely to be found in the southern equivalent of the GBTHRDS - as well as exisiting data is paramount. The velocity channel summation KDAR method used in this work can be applied to faint sources in existing low resolution data (i.e. SGPS) and will be applicable to future KDAR works.
Several sources with HI absorption beyond the Solar Circle were identified and flagged for future RRL observations in order to constrain their location. Whole sky infrared image catalogs are now available that will aid the classification of these objects (HIIregion or PNe). These sources will trace the Outer Arm (McClure-Griffiths et al. 2004) and supplement the general understanding of spiral structure, particularly on the far side of the Milky Way.
Figure 3.9: SGPS or VGPS radio continuum (blue) and WISE 22µm (red) contours on
the Spitzer GLIMPSE 8µm image cutouts for sources which display significant absorption
beyond the Solar circle. Note the saturation in the top right panel, appearing as ghosting in the GLIMPSE image.
son, JR; McClure-Griffiths, NM; Anderson, LD and Bania, TM, Astrophysical Journal, 774, (2) Article 117. ISSN 0004-637X (2013); doi:10.1088/0004-637X/774/2/117. The published paper is referenced by this thesis as Jones et al. (2013).
The introductory and methodology sections have been adapted to avoid unnecessary duplica- tion - see Chapters 1 and 2. The individual source descriptions, an Appendix of the published work, have been included in this thesis in Appendix B. Figure Set 1, of the published work, has been included in this thesis as part of Appendix C.
Abstract
We make a comprehensive study of HI absorption toward HII regions located within |l| < 10o. Structures in the extreme inner Galaxy are traced using the longitude-velocity space distribution of this absorption. We find significant HI absorption associated with the Near and Far 3kpc Arms, the Connecting Arm, Bania’s Clump 1 and the HI Tilted Disk. We also constrain the line of sight distances to HII regions, by using HI absorption spectra together with the HII region velocities measured by radio recombination lines.
4.1 Introduction
The Extreme Inner Galaxy (EIG) has long been the subject of intense astrophysical study as it provides excellent opportunities to explore dynamics, phenomena (from stellar to galactic scales) and physical environments which do not exist in the large-scale Galactic disk.
Throughout this Chapter, we refer to the area inside of, and including, the 3kpc Arms as the EIG (i.e. RGal ≲4kpc). ‘Inner Galaxy’ is a term already used to describe the areas of the Milky Way inside the Solar Circle, likewise the term ‘Galactic Center’ (GC) usually refers to the relatively small area with a Galactocentric radius less than a few hundred parsecs.
Useful reviews of the EIG environment include Morris & Serabyn (1996) and Blitz et al. (1993), who discuss the interstellar medium (ISM) and structural components respectively.
Radio observations of the EIG region have been performed since the 1950s (using the Dwingeloo 26 m antenna, van Woerden et al. 1957). These early studies discovered large- scale HIfeatures with non-circular motions (Oort 1977), and concentrated on understanding these individual structures, or particular objects.
The EIG has been extensively observed in CO. Molecular tracers probe denser material than neutral hydrogen (HI) and CO is readily observed; therefore CO observations allow for analysis of regions in which the ISM is concentrated into structures such as arms and bars (Dame et al. 2001). In contrast, observations of atomic gas trace diffuse interstellar clouds.
While HIin the EIG has been well studied at low angular resolution, it is only recently that high-resolution HIdata which cover the entire EIG region have become available (i.e. ATCA HI Galactic Center Survey (HIGCS) McClure-Griffiths et al. 2012). These high-resolution HIdata allow an analysis of the beginnings of the spiral arms; the transition between orbits associated with the bar; a comparison to high-resolution molecular observations, dynamical models and molecular transitions; as well as investigations into the association of HI with the Galactic wind (McClure-Griffiths et al. 2012).
As a result of the variation in the temperature of interstellar hydrogen, HI emission and absorption spectra probe different phases of the ISM. In most emission spectra it is the warmer components that dominate. However, cool gas is readily observed in absorption against background continuum sources, where it may be disentangled from warmer material along the line of sight. One advantage to studying HI absorption in the EIG is that it probes this predominantly cool material, which tends to be more localised in space, and more closely confined to structural entities such as arms.
Previous HI absorption studies have been vital to our understanding of the structure, rotation and the nature of atomic gas in the EIG region. These include observations of absorption features associated with non-circular velocities, Radio Arc non-thermal filaments as well as particular objects including SgrA* (Lang et al. 2010, and references therein).
While high-resolution HIabsorption measurements have been made towards several bright, or otherwise interesting, EIG continuum sources (Uchida et al. 1992; Roy 2003; Lang et al. 2010, and references therein) a complete HIabsorption study of the EIG region has not been attempted. This present HI absorption survey constitutes the most complete study of HI absorption against the continuum emission from the entire sample of HIIregions known with
recombination line (RRL) velocities. This sub-sample is discussed in Section 4.2, and the method of HI absorption is described in Section 4.3.
We then summarise the known EIG structures (Section 4.4.1) and their locations in Longitude-Velocity (lv) space. We plot these structures on an ‘lv crayon diagram’, and use
the diagram to consider the EIG lv distribution of HIabsorption, in Section 4.5, and later for HIIregions (Section 4.6).
We combine the results from Sections 4.5 and 4.6 to explore the Galactic distribution of HII regions (Section 4.7) - through determining the lower limit of the line of sight distance to each HIIregion based on its HI absorption profile and systemic velocity.
Finally, a discussion of individual sources appears in Appendix B.