The structure and 3D kinematics within 4 kpc of the Galactic Centre

Understanding the structure and associated dynamics of the inner 4-kpc region of the Milky Way is very challenging because of high obscuration at optical wavelengths and its impact on our ability to accurately measure the distance and 3D motions of sources at these distances. One way to resolve this challenge is by measuring parallaxes to high mass star forming regions in both the first and fourth quadrants of the Galaxy, especially in the fourth quadrant, which is only visible from the southern hemisphere. So far there have been 22 trigonometric parallax measurements reported for sources within 4 kpc of the Galactic Centre. Almost all of these sources are in the first quadrant of the Galaxy and were measured by either the BeSSeL or VERA surveys. To date, there has been a lack of parallax measurements in the fourth quadrant of the Galaxy, and obtaining such information is key to refining our understanding of the dynamics of this important region accurately.
With this in mind, a large maser astrometry project called the ’Southern Hemisphere Parallax Interferometric Radio Astronomy Legacy Survey (S𝜋RALS )’ was started in 2020. This survey aims to measure trigonometric parallaxes to High-Mass Star Forming Regions (HMSFRs) from the southern hemisphere. S𝜋RALS uses two 12m AuScope radio telescopes and two 30m radio telescopes, situated around Australia for observations. Because of its technical limitations, compared to more sophisticated arrays in the northern hemisphere, new observation methods and calibration techniques were developed as part of this project to be able to attain a measurement accuracy similar to northern arrays.
The aims and work undertaken for this thesis are as follows:
In Chapter 3, to improve our understanding of the early stages of high-mass star formation, I studied a flaring event in the star-forming region ’NGC6334I’. The star-forming region NGC6334I is associated with many different maser transitions from a range of molecules. I analysed the 6.7 GHz class II methanol maser very long baseline interferometry (VLBI) data from 4 different. Previous studies of this region have reported a dramatic flaring event in this transition. My observations provide the highest angular resolution information on the new maser features which emerged during the flare. Comparison of the spatial distribution of the 6.7 GHz maser emission before, during and after the flaring event has been used to investigate this rare flaring event and provide critical new information on the early stages of high-mass star formation.
In Chapter 4, to measure the trigonometric parallax to three 22 GHz water masers, I analysed the data for several sources observed with the Very Long Baseline Array (VLBA) as part of the BeSSeL survey. All three sources are located in the first quadrant of the Galaxy and the parallax distance measurements obtained show that they are within 4 kpc of the Galactic Centre. I have successfully been able to measure the trigonometric parallax, proper motion and peculiar motions of these sources. I have also been able to measure the inner motion of the maser features for each of the maser sources. I have used these results to determine the location of the sources within the Milky Way accounting for both parallax and their position in longitude-velocity (𝑙 − 𝑣) diagrams.
In Chapter 5, I analysed VLBI astrometric data for two 6.7 GHz class II methanol masers observed as part of the S𝜋RALS survey. The observations employed the inverse-MultiView (iMV) calibration technique developed by Hyland et al. (2022a). I have been successfully able to measure the trigonometric parallaxes and proper motions to both maser sources and determined the location of these sources within the Milky Way accounting for both their parallax and their position in l-v diagrams. From these results, I have been successfully able to show that iMV outperforms traditional VLBI phase referencing. One of the 2 methanol maser sources had previously had a trigonometric parallax measurement from observations with the Long Baseline Array (Krishnan et al., 2015). My results are consistent with the early measurement but provide significantly higher accuracy (19 compared to 80 by Krishnan et al. (2015)).
Finally, in Chapter 6, I studied the dynamics and 3D structure of the Galaxy within 4 kpc of the Galactic Centre. For my analysis, I used all parallax measurements for sources in this region obtained from the BeSSeL and VERA surveys. These data were used to develop a model for the orbits of the associated high-mass star formation regions around the Galactic Centre. The model for the orbits uses only the measurements obtained from the maser observations and has no prior assumptions. It shows they are orbiting in an orbit about the Galactic Long Bar around the Galactic Centre which with a semi-major axis of 4±0.1 kpc, semi-minor axis of 1.9±0.2 kpc and a Galactocentric azimuth of 34±5 degrees. These results provide the first direct measurements of the dynamics and 3D motions of the sources in orbit about the Galactic Long Bar. Here I propose a new model of the inner regions of the Milky Way in which the “Expanding 3-kpc arms” are neither expanding, nor at 3 kiloparsecs, nor spiral arms, but in fact, are features of my best-fit elliptical orbit about the Galactic Long Bar.