KITP: structure of globulars

In the discussion of these talks there were some interesting ideas from theorists about the data analysis of extragalactic globular cluster observations. Perhaps the most important point that may have been missed was that sufficiently deep HST observations of extragalactic globular clusters enable the determination of both the half-light radius and a radial profile shape of extragalactic globular clusters. With the assumption that these old clusters follow King models, this means that the data can constrain both the half-light radius and the concentration. Extensive simulations demonstrating this and delineating the signal-to-noise requirements were published by Carlson & Holtzman in 2001, among others. The significant dynamical range that allows this to be done can also be seen in the slides in the Waters talk.
Not all HST datasets meet these S/N requirements, but the very deep observations of M87 do. Thus the analysis of these data by Waters provides large number statistics to investigate questions about the correlations among globular cluster properties such as luminosity and parameters that follow from King models such as core, half-light, and tidal radii, along with other phenomena such as the presence of a bright X-ray binary. The measurements of the King model parameters have uncertainties, but the values are constrained by the data.