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Globular clusters

Bart Buelens, July 2015

I contributed to a project by the COIN group - the COsmostatistics INitiative, about the relation between physical properties of galaxies and the number of globular clusters found in their halos. See below for a link to the resulting article.
 
Globular clusters are gravitationally bound collections of stars typically found in the halo of galaxies. The size of a galaxy's globular cluster population is a prolonged puzzle in astronomy. We used a recently published catalog of galaxies (Harris et al. 2013) to study the connection between the number of globular clusters in a galaxy and various galaxy properties including the mass of the central black hole.

The main novelties of the approach followed in our article are the modeling of the count data using negative binomial distributions, and the use of Bayesian methods that naturally allow for taking into account uncertainties associated with measurements of galaxy properties.  

We find that the mass of a galaxy's central black hole is a good predictor of its number of globular clusters, see this graph:



The points are the galaxies in the catalog with elliptical galaxies as blue circles, spiral galaxies as red triangles and lenticular galaxies as orange asterisks. The horizontal and vertical lines associated with each galaxy are error bars indicating the precision of the measurements as given in the catalog. The black dashed line is obtained through our model and represents the expected value of the number of globular clusters (on the y-axis) given the mass of the central black hole expressed as the logarithm of its mass in units of solar masses (on the x-axis). The areas shaded in gray indicate our model's prediction intervals of 50% (darkest), 95% (lighter), and 99% (lightest).

The Milky Way is marked in the plot. Given the mass of its central black hole, its number of globular clusters is within the 95% prediction interval. Previously, the Milky Way was sometimes thought of as an outlier but using our approach we find that not to be the case.

Our article contains more than this brief synopsis. More galaxy properties are studied and more models are considered. We refer the interested reader to the full article. Astronomers - professional or amateur - interested in exploring the catalog for themselves are encouraged to do so. The data are publicly available, as are our analyses.

References & links:

The galaxy catalog is discussed in:
     Harris, W.E., Harris, G.L.H, and Alessi, M. 2013, ApJ 772, 82
and is publicly available at:

Our article is available on arxiv: 
    R.S. de Souza, J.M. Hilbe, B. Buelens, J.D. Riggs, E. Cameron, E.E.O. Ishida, A.L. Chies-Santos, M. Killedar, for the COIN collaboration, The Overlooked Potential of Generalized Linear Models in Astronomy-III: Bayesian Negative Binomial Regression and Globular Cluster Populations, http://arxiv.org/abs/1506.04792

The github repository with our code is here: https://github.com/COINtoolbox/NB_GCs

COsmostatistics INitiative: http://cointoolbox.github.io