Globular clusters and supermassive black holes in galaxies: further analysis and a larger sample

We explore several correlations between various large-scale galaxy properties, particularly total globular cluster population (NGC), the central black hole mass (M•), velocity dispersion (nominally σe) and bulge mass (Mdyn). Our data sample of 49 galaxies, for which both NGC and M• are known, is larger than used in previous discussions of these two parameters and we employ the same sample to explore all pairs of correlations. Further, within this galaxy sample, we investigate the scatter in each quantity, with emphasis on the range of published values for σe and effective radius (Re) for any one galaxy. We find that these two quantities in particular are difficult to measure consistently and caution that precise intercomparison of galaxy properties involving Re and σe is particularly difficult. Using both conventional χ2-minimization and Monte Carlo Markov Chain fitting techniques, we show that quoted observational uncertainties for all parameters are too small to represent the true scatter in the data. We find that the correlation between Mdyn and NGC is stronger than either the M•–σe or the M•–NGC relations. We suggest that this is because both the galaxy bulge population and NGC were fundamentally established at an early epoch during the same series of star-forming events. By contrast, although the seed for M• was likely formed at a similar epoch, its growth over time is less similar from galaxy to galaxy and thus less predictable.