Saturday, July 2, 2016

The Present Distribution of Globular Clusters

Figure 1: Artist’s impression of a planet in a globular cluster.

Galaxies with larger luminosities tend to host big populations of globular clusters. At the same time, galaxies with larger luminosities are also less common than galaxies with lower luminosities. Dwarf galaxies are low luminosity galaxies that individually have very few globular clusters, but they are very common in the universe. In contrast, giant elliptical galaxies are very luminous galaxies with huge populations of globular clusters, but these galaxies are extremely rare.

It is worth asking how globular clusters are distributed amongst galaxies and what kind of galaxies account for most of the globular clusters in the universe. A plot of the distribution of globular clusters show that half the population of blue (metal-poor) globular clusters are found in galaxies with less than ~13 billion times the Sun's luminosity, while half the population of red (metal-rich) globular clusters are found in galaxies with less than ~28 billion times the luminosity of the Sun. Basically, blue (metal-poor) globular clusters tend to reside around lower mass galaxies. A major contributing factor for this is that there are almost no red (metal-rich) globular clusters present around dwarf galaxies.

Figure 2: The fraction of all globular clusters within galaxies with luminosities ≤ L. The large solid dots indicate the median points along each curve. Harris (2016)

It appears that most globular clusters belong to galaxies with between a trillion to a hundred trillion solar-masses, with a peak near ~10 trillion solar-masses. Since galaxies grow by consuming their smaller neighbours, this peak is expected to shift gradually towards a higher mass as the universe gets older.

Overall, blue (metal-poor) globular clusters outnumber red (metal-rich) globular clusters by 4 to 1. In the early universe, matter was predominantly in the form of low-metallicity gas (i.e. gas with low abundance of elements heavier than hydrogen and helium). Those conditions were very favourable for the formation of dense massive star clusters that went on to become the blue (metal-poor) globular clusters observed in the present universe. That was the period when most of the blue (metal-poor) globular clusters formed.

Red (metal-rich) globular clusters formed at a later period when matter became more enriched with elements heavier than hydrogen and helium. This explains why red (metal-rich) globular clusters are associated with more massive galaxies as massive galaxies generally appeared during the later stages of galaxy formation.

Harris (2016), "Where Are Most of the Globular Clusters in Today's Universe?", arXiv:1603.00348 [astro-ph.GA]