Statistical Shape Analysis of Galactic Hii Regions

Hii regions are diffuse nebulae of ionised hydrogen, excited by the extreme ultraviolet emission from massive stars. Due to the embedded nature of massive star formation, there are many observational difficulties involved when investigating such stars. Hii regions, however, are readily observed via their infrared and radio emission. As such, they highlight the location of their massive star sources. Furthermore, Hii region properties are directly resultant of their progenitors and environment. The overall aim of the work presented herein, is to determine whether statistical shape analysis of observational and numerically modelled Hii region data can be used to probe the associated astrophysical properties.

Radio continuum and computer simulated synthetic images of Hii regions were analysed using the shape extraction and statistical comparison methods constructed in this work. For the radio data, six morphological groups were identified. Visual inspection and quantitative ordinance techniques confirmed that the shape analysis and grouping procedure were working as intended. It was found that in the first Galactic quadrant, location is mostly independent of group, with a small preference for regions of similar Galactic longitudes to share common morphologies. The shapes are homogeneously distributed across Galactocentric distance and latitude. One group contained regions that are all younger than 0.5 Myr and ionised by relatively low- to intermediate-mass sources. Those in another group are all driven by intermediate- to high-mass sources. One group was distinctly separated from the other five and contained regions at the surface brightness detection limit for the survey. The hierarchical procedure employed was most sensitive to the spatial sampling resolution used, which is determined for each region from its heliocentric distance.

The numerical Hii region data was the result of photoionisation and feedback of a 34 M⊙ star, in a 1000 M⊙ cloud. Synthetic observations (SOs) were provided, comprising four evolutionary snapshots (0.1, 0.2, 0.4 and 0.6Myr), and multiple viewing projection angles. The shape analysis results provided conclusive evidence of the efficacy of the numerical simulations. When comparing the shapes of the synthetic regions to their observational counterparts, the SOs were grouped in amongst the Galactic Hii regions by the hierarchical procedure. There was also an association between the evolutionary distribution of regions of the respective samples. This suggested that this method could be further developed for classification of the observational regions by using the synthetic data, with its well defined parameters.