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.
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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.
Reference:
Noaz & Narayan (2014), “Globular Clusters and Dark
Satellite Galaxies through the Stream Velocity”, arXiv:1407.3795 [astro-ph.GA]