The Properties of B045-G108

On May 27 Swift/BAT detected a source in M31. The associated GCN circular is here. Swift/XRT and UVOT localized the source, which is coincident with a well-studied globular cluster (GC) in M31. This GC is generally referred to as B045 or B045-G108 (the "B" referring to the Bologna catalog of M31 GCs) though the ID given in the GCN is FMZ2005-7. This page summarizes the properties of B045-G108 itself.

In general terms, B045-G108 is an old intermediate-metallicity massive globular cluster. In detail:

Age: Caldwell et al (2011) find a formal age of 14 Gyr, which they use to denote objects that "fall off" the model grids. It is fair to assume the actual age is in the range ~ 9-13 Gyr typical of old GCs.

Metallicity: The metallicity of B045-G108 has been recently measured by several methods: low-resolution spectroscopy in Caldwell et al (2011) ([Fe/H] = -0.9 +/- 0.1), high-resolution spectroscopy from Colucci et al (2009) ([Fe/H] = -0.94 +/- 0.03; here), and the slope of the RGB from an HST CMD ([Fe/H] ~ -0.8; Rich et 2005, here). [Fe/H] = -0.9 seems a reasonable value to adopt.

Photometry: Optical and near-IR photometry is collected in the Bologna catalog, which has many sources. UBVRIJHK + GALEX NUV is here, and SDSS (originally from Peacock et al 2010) is here. Benchmark values NOT reddening corrected: V=15.71+/-0.10 and K=12.92 +/- 0.03. Note that Caldwell et al (2011) infer a foreground reddening of E(B-V) = 0.18 on the basis of the fluxed spectrum, which is significantly higher than that in the SFD98 maps (0.06).

Structure: Barmby et al (2007) derive structural properties via King model fits to HST images of M31 GCs. For B045-G108 they find a half-light radius of 2.86 pc, a core radius of 1.07 pc, and a tidal radius of 30.2 pc. These are all very typical for old GCs. The inferred central density is 2.9e4 M_sun/pc^3, though this is probably an overestime given the dynamical constraints below.

Dynamics: Strader et al (2011) discuss dynamical masses for M31 GCs. B045-G108 has a central velocity dispersion of 10.6 +/- 0.6 km/s and a virial mass of 5.0e5 M_sun. This implies a mass-to-light ratio of M/L_V = 1.17 +/- 0.17, which is smaller than usually assumed for GCs (and why the central density listed above is probably overestimated), but consistent with the observed trend for more metal-rich GCs to have lower M/L. This mass is larger than the "typical" mass of old GCs, but it is not extraordinary. B045-G108 is not one of the most massive GCs in M31.

Distance: Rich et al (2005) note that B045-G108 is "possibly" located on the far side of M31 on the basis of its resolved CMD, using the brightness of the horizontal branch. However, it's possible this is mostly because the extinction was underestimated.

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