Are globular clusters home to dark matter?

One of the most spectacular things to spy through a telescope is a globular cluster, a dense ball of about a million stars held together by their mutual gravity. About 150 globular clusters inhabit the tenuous stellar halo that envelopes the spiral disk of the Milky Way, some buzzing around the Galactic Centre, whilst the orbits of others can take them out to hundreds of thousands of light-years.

Globular clusters appear to be simple objects, each formed in a single burst of star formation. They also appear to be amongst the oldest objects in the Milky Way, born in its initial stages and then simply aging over billions of years. 

Globular clusters are found in the haloes of most galaxies. Whilst the Milky Way has about 150 of these objects, other galaxies can contain more - for example, our nearest large spiral neighbouring galaxy, Andromeda, has about 500 and the massive M87 elliptical galaxy, has about 13,000.

Furthermore, globular clusters that are bound to the Milky Way can be categorised into two distinct groups, based on their location from the galactic centre.

The first group tend to reside near the central galactic bulge of the Milky Way, orbiting roughly within a small radius from the centre, and present metal-rich and old stellar profiles. These objects also orbit in more or less the same plane and direction as the rest of the central bulge of the galaxy.

The second group is found at much greater distances and include metal-poor bluer stars, orbiting with varying inclinations and direction, around the Milky Way’s flattened disc. These clusters form part of the larger halo around the galaxy. 

The formation of globular clusters, however, remains mysterious. Most objects in the Universe, galaxies and clusters, are born when gas pools into the gravitational wells created by collapsing dark matter, but the simplicity of globular clusters suggest that they formed from collapsing gas clouds without the aid of dark matter. Clearly, finding, or not finding, evidence for dark matter within globular clusters will be crucial to unravelling their formation. 

Of course, dark matter is invisible to our telescopes. Instead, astronomers have hunted for its signature in the motions of stars within globular clusters, revealed with the Doppler shifting of light. Most studies have focused upon the inner regions of clusters, where the density of stars is the greatest, but no conclusive detection of dark matter has been made.

Dark matter can’t be observed through our conventional methods and instruments, which rely on electromagnetic radiation (across the whole spectrum) to be detected. Instead, dark matter is inferred due to its gravitational influence on the objects we can see (and their surrounding regions). 

When Zwicky first applied the virial theorem to observations of the Coma Galaxy Cluster and coined the term dark matter, he noted that the velocities of the galaxies were moving too fast for the amount of matter present (which could be observed). 

Similar observations were made through the work of Vera Rubin, Kent Ford and Ken Freeman in the 1960s and 70s when reviewing the rotational velocities of spiral galaxies, which showed that for all the matter we could see with normal electromagnetic light, there was still about six times the mass of matter that we couldn’t see.

Soon enough radio telescopes would start to trace the huge masses of neutral hydrogen gas that galaxies contained, out to great distances beyond the visible edges of galaxies, and also confirmed that the rotational velocities of this gas were being induced by a greater mass than what could be observed. 

These days, several additional methods of indirectly observing dark matter are utilised - such as carefully measuring the x-ray emissions from galaxy clusters, or using gravitational lensing techniques to infer the presence of the unseen mass. 

In all cases, dark matter has not been directly observed to date - but it is well accepted across by astrophysicists because it also supports Einstein’s General Theory of Relativity favourably through the observed dynamics produced by gravitational interactions, compared to any modified gravity theories. 

One of the most interesting questions in astrophysics, over the decades, has been to look for any signs of dark matter presenting in or around globular clusters, just as dark matter is present in a halo around galaxies. 

Now, a new program to understand the dynamics of globular clusters is underway. Using the AAOmega spectrograph at the 3.9m Anglo-Australian Telescope near Coonabarabran in NSW, astronomers are able to target almost 400 stars in a single exposure, allowing them to survey the stellar speeds in the outer reaches of the cluster, where the influence of dark matter would be the most obvious.

In a new paper, University of Sydney PhD student Zhen Wan, presented the first results from this survey, targeting the globular cluster, NGC 3201, located at about 15,000 light-years in the southern constellation of Vela.

The cluster is estimated to have about 250,000 times the mass of the Sun, and like most globular clusters contains a collection of older stars, many just over 10 billion years old. These stars are contained within a diameter of approximately 80 light-years, which is smaller in both size to the Milky Way’s largest globular cluster Omega Centauri (roughly 150 light-year diameter) and its mass (Omega Centauri contains just over 4 million solar masses). 

Almost uniquely, however, NGC 3201 has an inhomogeneous stellar distribution - in that the stars closer to its core are redder and coolers whilst increasing the distance from the centre and moving outwards their temperature increases (and indirectly, their colour starts to progress towards the blue side of the spectrum). 

And as part of this latest study by Zhen Wan and the team, the speeds of almost 700 stars were measured over several degrees, covering an area much larger than the full Moon. But what did the stellar dynamics reveal?

Whilst outward appearances of globular clusters are simple, the speeds of their stars are not. Firstly, NGC 3201 is clearly rotating, complicating an assessment of its dark matter content. Other effects are also confounding, with the orbits of binary stars can inflate the velocity signature, and presence of black holes within the cluster can fling stars out at high speeds. And it is apparent that NGC 3201 is being steadily dismembered by the gravitational influences of the Milky Way. 

The result, however, is that NGC 3201 is solely stars, and there is little room in the observations to hide a significant quantity of dark matter. 

Globular clusters remain mysterious, but we are steadily chipping away at their mystery.