Where Are the Universe’s Globular Clusters?


Observations of globular clusters — gravitationally-bound, spherical clusters of stars that orbit galaxies as satellites — are critical to studies of galactic and stellar evolution. What type of galaxies host the largest total number of globular clusters in today’s universe? A recent study answers this question.

GCs per host gal luminosity

Total number of globular clusters vs. host galaxy luminosity for a catalog of ~400 galaxies of all types. [Harris 2016]

Globular Favoritism

Globular clusters can be found in the halos of all galaxies above a critical brightness of about 107 solar luminosities (in practice, all but the smallest of dwarfs). The number of globulars a galaxy hosts is related to its luminosity: the Milky Way is host to ~150 globulars, the slightly brighter Andromeda galaxy may have several hundred globulars, and the extremely bright giant elliptical galaxy M87 likely has over ten thousand.

But the number of galaxies is not evenly distributed in luminosity; tiny dwarf galaxies are extremely numerous in the universe, whereas giant ellipticals are far less common. So are most of the universe’s globulars found around dwarfs, simply because there are more dwarfs to host them? Or are the majority of globular clusters orbiting large galaxies? A scientist at McMaster University in Canada, William Harris, has done some calculations to find the answer.

Finding the Peak

Harris combines two components in his estimates:

  1. The Schechter function, a function that describes the relative number of galaxies per unit luminosity. This function drops off near a characteristic luminosity roughly that of our galaxy.
  2. Empirical data from ~400 galaxies that describe the average number of globulars per galaxy as a function of galaxy luminosity.
Where are the GCs?

Relative number of globular clusters in all galaxies at a given luminosity, for metal-poor globulars only (blue), metal-rich globulars only (red), and all globulars (black). The curves peak around the Schechter characteristic luminosity, and metal-poor globulars outnumber metal-rich ones 4 to 1. [Harris 2016]

He finds that globular clusters are most commonly found in galaxies within a surprisingly narrow range around the characteristic luminosity of the Schechter function. This means that, at the current time, the collection of galaxies similar in brightness to the Milky Way or Andromeda host the largest total number of globulars in the universe.

Metal-Poor Dominance

Harris extends these calculations by examining two subpopulations of globulars: blue (metal-poor) and red (metal-rich). Metal-poor globular clusters are found in all galaxies, but metal-rich ones reside preferentially in massive, bright galaxies. Strikingly, Harris finds that this preference results in metal-poor globulars making up almost 80% of all globular clusters in the universe, outnumbering the metal-rich ones by nearly 4 to 1.

This result implies that the earliest stages of hierarchical galaxy mergers — when most of the available gas was low-metallicity — provided the most favorable conditions for the formation of dense, massive star clusters. This early environment birthed the majority of the globular clusters we see today.


William E. Harris 2016 AJ 151 102. doi:10.3847/0004-6256/151/4/102