Multivariate analysis of the globular clusters in M87

TL;DR

This study uses multivariate analysis to classify 147 globular clusters in M87, revealing insights into the galaxy's formation history and chemical properties, including evidence for helium enrichment in metal-rich clusters.

Contribution

It introduces a combined use of independent component analysis and K-means clustering to classify globular clusters and infer galaxy formation mechanisms.

Findings

Four distinct globular cluster groups identified

Evidence for a two-phase galaxy formation process

Chemical differences between M87 clusters and those in the Milky Way and M31

Abstract

An objective classification of 147 globular clusters in the inner region of the giant elliptical galaxy M87 is carried out with the help of two methods of multivariate analysis. First independent component analysis is used to determine a set of independent variables that are linear combinations of various observed parameters (mostly Lick indices) of the globular clusters. Next K-means cluster analysis is applied on the independent components, to find the optimum number of homogeneous groups having an underlying structure. The properties of the four groups of globular clusters thus uncovered are used to explain the formation mechanism of the host galaxy. It is suggested that M87 formed in two successive phases. First a monolithic collapse, which gave rise to an inner group of metal-rich clusters with little systematic rotation and an outer group of metal-poor clusters in eccentric orbits. In a second phase, the galaxy accreted low-mass satellites in a dissipationless fashion, from the gas of which the two other groups of globular clusters formed. Evidence is given {\bf for a blue stellar population in the more metal rich clusters, which we interpret by Helium enrichment.} Finally, it is found that the clusters of M87 differ in some of their chemical properties (NaD, TiO1, light element abundances) from globular clusters in our Galaxy and M31.