Globular clusters are dense spherical collections of stars. Every large galaxy, such as the Milky Way, contains a system of globular clusters. Observations of globular clusters show that they do not contain gravitationally bound dark matter. Most of the matter in the universe is in the form of dark matter. Dark matter neither emits nor absorbs light, and its presence can only be inferred from its gravitational effects on normal matter and radiation. Nevertheless, the existence of dark matter is important because it provides the gravitational framework for normal matter to come together to form galaxies and clusters of galaxies. As a result, it remains a challenge to explain how normal matter could gravitate so tightly together to form globular clusters.
The globular cluster NGC 1806 located within the Large Magellanic Cloud as observed by the Hubble Space Telescope. Image credit: ESA/Hubble & NASA.
A study by Noaz & Narayan (2014) suggests that globular clusters can form naturally whenever there is some relative velocity between normal matter and dark matter. In this scenario, the formation of a globular cluster begins with a collapsing clump of normal matter in a dark matter halo which is itself also collapsing. The gravity that is driving the collapse comes mostly from dark matter. However, the collapsing clump of normal matter eventually finds itself outside the dark matter halo due to the relative velocity between the normal matter and dark matter components. If the relative velocity is small, then the clump of normal matter remains in the dark matter halo and forms a typical dwarf galaxy with somewhat comparable proportions of normal matter and dark matter, albeit more dark matter.
As a consequence of the relative velocity between the normal matter and dark matter components, the collapsing clump of normal matter becomes a long-lived dark matter-free gravitationally self-bound object (i.e. a globular cluster). Such a clump of normal matter can have a mass ranging from roughly a hundred thousand to a few million times the Sun’s mass, consistent with the masses of present-day globular clusters. On the contrary, the corresponding dark matter halo, depleted of normal matter, could become a dark satellite galaxy or an ultra-faint satellite galaxy. Such a galaxy would be comprised almost entirely of dark matter and would contain extremely few stars, possibly none at all, since stars are made of normal matter.
Noaz & Narayan (2014), “Globular Clusters and Dark Satellite Galaxies through the Stream Velocity”, arXiv:1407.3795 [astro-ph.GA]