Blake Ledger

McMaster University

Blake (he/him) is a Ph.D. student at McMaster University working in the research group of Prof. Christine Wilson. He completed his Hon. B.Sc. in Physics and Astrophysics at the University of Calgary in 2019 and has since moved to McMaster to pursue his graduate studies. Blake has been a member of the CASCA Graduate Student Committee for two years, serving as the McMaster student representative and now as the Vice-Chair. Blake is heavily involved in student life at McMaster as well, serving as the Chair of the McMaster Physics and Astronomy Student Association (MAPSA) and as an executive member of Promoting Inclusion in Physics and Astronomy (PIPA). Blake is committed to using his privilege and voice to promote positive, forward change in his Department and at the national level of astronomy in Canada. Blake recently took over as the William J. McCallion Planetarium manager at McMaster and is as passionate about community outreach as he is about his research.

Blake classifies himself as an observer. His research uses telescope observations from some of the most advanced ground-based observatories in the world, like the Atacama Large Millimeter Array (ALMA). In his work, Blake leads one of the first multi-line studies of HCN and CN in other galaxies. He has a complete set of ALMA observations of 6 transition lines for these two molecules, which he uses to study the properties of dense molecular gas. The objective of Blake’s Ph.D. thesis is to use his observations to determine how radiation fields produced by recent star formation and active galactic nuclei impact the physical and chemical properties of molecular gas. In particular, Blake is investigating the use of the cyanide radical, CN, as a new dense gas tracer that can be linked with molecular gas properties in regions irradiated by some external UV and/or X-ray field. The benefits of using CN in observations are that it can be traced to the densest molecular gas, the chemical networks which produce this molecule are activated by strong radiation fields, and CN can be observed with ALMA “for free”. The latter point here is a result of the CN (1-0) line often being simultaneously observed with the CO (1-0) line with ALMA, leaving a free CN observation in the wake of almost every CO observation. Additionally, CN is typically found to be optically thin. The hyperfine structure of the CN (1-0) line can be used to measure optical depth, which is often a key parameter that is needed to determine the properties of molecular gas.

The main takeaway from these two figures is that the intensity ratio between CN (1-0) and HCN (1-0) has subtle variations. These variations are found between individual galaxies and in localized regions within individual galaxies. In particular, regions with an enhanced UV radiation field, like a starburst, are found to have a higher CN (1-0)/HCN (1-0) intensity ratio. Both figures are from Ledger et al. (2021), MNRAS, 504, 5863. Left: A pixel-by-pixel comparison of the intensities of the CN (1-0) and HCN (1-0) transition lines in the three galaxies NGC 3256, NGC 7469, and IRAS 13120. The triangle symbols represent average measures in the nuclei of each galaxy (NGC 3256 has two nuclei, so the up and down triangles represent the northern and southern nuclei, respectively). The stars and pentagons represent the average measures globally and in the non-nuclear pixels, respectively. The dotted line represents a 1-to-1 ratio. Right: An example of the spatially resolved CN (1-0)/HCN (1-0) intensity ratio in NGC 3256, in units of K km/s. The scale bar is 500 pc, and the beam size is 2.2” round, which corresponds to ~470 pc at the distance of NGC 3256. The white apertures are the size of the beam and are centred on the northern and southern nuclei as identified by radio continuum data. The CN (1-0)/HCN (1-0) intensity ratio is higher in the northern nucleus of NGC 3256, which is known to be in a starburst.