Tuesday, 19 July 2011

A Bayesian Approach to Locating the Red Giant Branch Tip Magnitude (Part I)

PhD student, Anthony Conn from Macquarie University, has been working with me on measuring distances to dwarf galaxies and substructure in our nearest cosmological companion, the Andromeda Galaxy. I'm very pleased to say that Anthony's first paper, using Bayesian methods to measure these distances, has now been accepted for publication in the Astrophysical Journal. Excellent result, Anthony!!

A Bayesian Approach to Locating the Red Giant Branch Tip Magnitude (Part I)

A. R. Conn, G. F. Lewis, R. A. Ibata, Q. A. Parker, D. B. Zucker, A. W. McConnachie, N. F. Martin, M. J. Irwin, N. Tanvir, M. A. Fardal, A. M. N. Ferguson

 We present a new approach for identifying the Tip of the Red Giant Branch (TRGB) which, as we show, works robustly even on sparsely populated targets. Moreover, the approach is highly adaptable to the available data for the stellar population under study, with prior information readily incorporable into the algorithm. The uncertainty in the derived distances is also made tangible and easily calculable from posterior probability distributions. We provide an outline of the development of the algorithm and present the results of tests designed to characterize its capabilities and limitations. We then apply the new algorithm to three M31 satellites: Andromeda I, Andromeda II and the fainter Andromeda XXIII, using data from the Pan-Andromeda Archaeological Survey (PAndAS), and derive their distances as $731^{(+ 5) + 18}_{(- 4) - 17}$ kpc, $634^{(+ 2) + 15}_{(- 2) - 14}$ kpc and $733^{(+ 13)+ 23}_{(- 11) - 22}$ kpc respectively, where the errors appearing in parentheses are the components intrinsic to the method, while the larger values give the errors after accounting for additional sources of error. These results agree well with the best distance determinations in the literature and provide the smallest uncertainties to date. This paper is an introduction to the workings and capabilities of our new approach in its basic form, while a follow-up paper shall make full use of the method's ability to incorporate priors and use the resulting algorithm to systematically obtain distances to all of M31's satellites identifiable in the PAndAS survey area.

Just as a passing note, this is another cool astro-ph date, with another Geraint on there.  Have a read of;

An MCMC approach to extracting the global 21-cm signal during the cosmic dawn from sky-averaged radio observations

by Geraint Harker. I wonder if he is related to the famous Jonathan Harker (vampire slasher)? At some point, I'll write about the mathematical modelling of vampire outbreaks!! One thing I know, is that a vampire outbreak will not look like this!!

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