MALT-45 : a 7mm survey of the southern galaxy
Creators: name: Jordan, CH
Publisher: name: University Of Tasmania
Year: 2023

description: The last decade has seen vast improvement in the knowledge of star formation within our Galaxy, largely owing to improvements in instrumentation, allowing astronomers to compile more data. However, despite the advances of technology, the quest for understanding high- mass star formation (HMSF) continues. As we go on, breakthroughs have occurred; a prime example is the discovery of the class II methanol maser, which exclusively signposts on-going sites of HMSF, but still lacks the detail necessary to identify HMSF in all forms. Once we have understood where, why and how HMSF can occur, we will be able to diagnose Galactic structure and evolution. Untargeted, large area surveys of molecular gas are ideal for identifying HMSF regions across a broad range of evolutionary phases. For example, searches for molecular species with a high critical density can highlight dense gases, which can then be used to probe Galactic structure and star formation. Because HMSF occurs in regions of dense molecular gas, mapping high-density tracers serves well to identify regions for study. The (1,1), (2,2) and (3,3) inversion transitions of ammonia (NH3) have been successfully mapped by the H2O Southern Galactic Plane Survey (HOPS), identifying previously unknown sites of star formation, as well as probing the structure of the Milky Way's spiral arms. Fortunately, HMSF can be identified by bright spectral lines in maser emission; HOPS also mapped the Galactic plane for water (H2O) masers and, perhaps more importantly, the Methanol MultiBeam survey identities class II methanol (CH3OH) masers, which are exclusively associated with HMSF. While class II CH3OH masers always signpost HMSF, they appear only in a specific evolu- tionary stage, and therefore other species (such as H2O masers) are required to identify other stages. Another, even higher density gas tracer useful for detecting HMSF and mapping the structure of our Galaxy is carbon monosulfide (CS). The ground state transition J = 1-0 for CS lies within the 7mm waveband, which also contains the poorly understood class I CH3OH maser. Unlike the class II variant, class I masers are not exclusively associated with HMSF, but do appear in star-forming regions across a wide range of evolutionary stages. A large problem for class I CH3OH maser studies is the bias in the targeted searches which have been used to find them; they have only been identified towards other masing regions (such as class II CH3OH), and therefore the properties of these masers are somewhat unclear. In this thesis, results focus on the MALT-45 survey using the Australia Telescope Compact Array (ATCA) in auto-correlation ('single-dish' mode). To date, MALT-45 has mapped the Galactic plane within 330‚Äöv=¬¿ &lt; l &lt; 335‚Äöv=¬¿, jbj&lt; 0:5‚Äöv=¬¿, which contains several known star-forming regions, including the G333 giant molecular cloud. MALT-45 surveys 12 spectral lines, but primarily CS (1-0), class I CH3OH masers and SiO (1-0) v = 0; 1; 2; 3. Bright, extended CS emission is detected across the survey region, and highlight two distinct velocities, due to different spiral arms of the Galaxy. In addition to the previously known 19 class I CH3OH masers, 58 new masers were detected. SiO masers were detected towards 47 regions, in various combinations of vibrational mode v = 1; 2; 3, all towards evolved infrared stars. Thermal SiO v = 0 emission is also detected across the survey region. Major science results from MALT-45 include: (i) A CS to NH3 comparison, which highlights cold, dense clumps as well as hot, evolved clumps. The cold and dense clumps appear to have self-absorption of CS emission in their centres and a relative over-abundance of NH3, while evolved clumps appear to have very little NH3 emission, despite being a dense gas tracer; (ii) Almost all (94 per cent) of ATLASGAL 870 um dust emission point sources are associated with at least a 3˜ìvâ peak of CS emission; (iii) By comparing with peak CS velocities, class I CH3OH masers are good indicators of the systemic velocities of clouds; (iv) More than half (55 per cent) of the detected class I CH3OH masers are not associated with any other kind of maser; (v) Class II CH3OH, H2O and hydroxyl (OH) masers associate well with class I CH3OH masers, confirming that class I CH3OH masers occur towards a wide range of evolutionary stages in HMSF; (vi) Class I CH3OH masers appear to have no correlation in intensity or luminosity with other maser species; (vii) Class I CH3OH masers have typical projected linear distances from other masers associated with star formation, peaks of CS and 870 ˜í¬¿m point sources within 0.5 pc; (viii) Class I CH3OH masers are spread over a larger area when also associated with class II CH3OH or OH masers, perhaps due to their more evolved state; (ix) Almost all (95 per cent) of class I CH3OH masers are associated with an ATLASGAL source; (x) Using ATLASGAL source parameters, a clump mass is calculated. The population of class I CH3OH masers has a broad range of associated masses (101:5 to 104:5M'), but peaks between 103:0 and 103:5M'. Higher masses tend to be associated with evolved regions of star formation, while lower masses tend to be non-evolved regions; (xi) SiO masers typically decrease in intensity with vibrational mode (v = 1; 2; 3), but eleven cases of stronger v = 2 than v = 1 emission were found, and two regions of only v = 2 emission were found; (xii) The relatively rare v = 3 vibrational mode of SiO (1-0) was detected towards three evolved infrared stars.