Deep, Large Scale Surveys of Star Forming Regions throughout the Milky Way

Abstract

Star formation and the processes involved are not only important to the Milky Way as an astronomical object, but are also crucial to understand our own origin. For instance, only lower-mass stars like our own Sun have supposedly a long enough lifetime to allow for the development of life, whereas high-mass stars are the major source of the heavy elements beyond iron, that are needed to form life as we know it. Therefore, understanding the influence of the Galactic environment on low- to high-mass star<br /> formation is crucial to understand our own place in the Galaxy.<br /> In order to obtain the most complete view on star formation throughout the Milky Way to date we use several different dust-continuum surveys (ATLASGAL, Hi-GAL, MSX and WISE) to obtain photometric data, and consecutively model the spectral<br /> energy distributions of the dense molecular clumps in which stars and star clusters form. From these we derive the physical properties such as dust temperatures, integrated fluxes and H₂ peak column densities. We derive distances from line-of-sight<br /> velocities obtained from CO observations for the majority of the sources, allowing us to calculate clump masses and bolometric luminosities and analyse the properties of the sources in the context of their large scale environment.<br /> We started with a small subsample of the ATLASGAL survey, the ATLASGAL Top100, investigating the ~100 brightest and most massive clumps in 4 distinct evolutionary stages in the inner Galaxy (König et al. 2017). The methods developed for this sample were then applied to the full ATLASGAL compact source catalogue, investigating a complete sample of ~ 8000 sources located mostly in the inner part of the Milky Way (Urquhart et al. 2018). Finally, the analysis is extended to a sample of sources located in the southern outer Galaxy, unrevealing its structure and investigating star formation properties out to the edge of the Milky Way. With samples for the inner and outer Galaxy at hand, we investigate the star formation properties of the sources with respect to their evolutionary phase, their dependence on the distance to the Galactic centre and the influence of large-scale structures like the spiral arms or the second largest expanding supershell of the Galaxy.<br /> We established an evolutionary sequence based on the dust spectral energy distributions. Using dust continuum emission we are able to assign an evolutionary phase to individual clumps, and statistically analyse the physical properties, finding the dust temperature, bolometric luminosity and luminosity-to-mass ratio to increase over time. Using this classification scheme for the ATLASGAL sample we were furthermore able to calculate statistical lifetimes, finding that the quiescent stage is indeed very short (< 1x10⁴ years) for the most massive (>10,000Msun) clumps.<br /> We find the clumps' physical properties to vary significantly with Galactocentric distance. The dust temperature increases towards the outer Galaxy, whereas the average clump masses, bolometric luminosities and peak column densities significantly<br /> drop by almost an order of magnitude within just a few kiloparsec around the solar circle. As also the gas-to-dust ratio increases and the metallicity decreases in the outer Galaxy, we attribute the increased temperatures to a combination of less effective cooling and lower shielding against the interstellar radiation field due to the lower column densities.<br /> In contrast, we find the star formation activity as indicated by the luminosity-to-mass ratio to stay constant on kiloparsec scales throughout the Milky Way. Furthermore, we find the clump masses to be independent of the evolutionary stage indicated<br /> by the dust temperature, showing that once a clump begins to collapse, the evolution is largely independent of its large-scale environment. This is further supported by the fact that we find no influence of the spiral arms on the physical parameters and star<br /> formation activity of the dust clumps; they only seem to be responsible for organizing the interstellar material into clumps.