Massive Blackholes & Nuclear Star Clusters: Old Friends or New Partners?

Blackholes are, well some of the really fascinating objects of our universe. And they come in different sizes from Stellar-mass Blackholes to Supermassive Blackholes. Both these ends are well understood in their formation and evolution. But in between them, are the Intermediate and Massive Blackholes which are not yet well understood as to how they are formed.

A recent study focused in this direction may throw some light on the process of formation of Massive Blackholes. This study, conducted by researchers from the Washington State University aims at understanding the mechanism for the formation of Massive Blackholes.

Nuclear Star Clusters: Ever heard about them?

Nuclear star clusters(NSC) are the densest known stellar environments in the universe. They consist of a high density of stars within them and the luminosities of the member stars are also very high.

The nuclear star cluster of our own Milky Way Galaxy is seen with adaptive optics in the infrared with the NaCo instrument on the VLT. Credit: https://en.wikipedia.org/wiki/Nuclear_star_cluster

They reside in the center of most of the galaxies and, according to the recently published study, are a promising candidate for the formation of Massive Blackholes(MBH) and/or Intermediate mass Blackholes(IMBH). The mass of a typical NSC is in the range of 10⁵-10⁷ Mₛᵤₙ.

How is an NSC formed?

NSCs are present in every galaxy but how are such massively dense structures formed in the first place? There are two proposed mechanisms for the formation of these NSCs which are proposed.

1. Globular Cluster infall — The globular clusters in a galaxy is attracted to the center due to the gravitational pull and collide with the stars there to form a dense star cluster.
2. Star formation — The formation of stars in a galaxy is such that they form a dense cluster and due to the gravitational effects, it migrates to the center of the galaxy.

NSC could form around a pre-existing Blackhole or could provide an environment in which massive blackholes form more easily.

How are Blackholes Formed in a Galaxy?

Almost all galaxies with a mass > 10¹⁰ Mₛᵤₙ are thought to host an MBH. But, the formation of these MBH requires some mechanism through which they could form and grow. There are two mechanisms through which this could be possible.

A rendering by artist Yu Jingchuan of the accretion of gas onto a stellar black hole from its blue companion star, through a truncated accretion disk. Credit: Jingchuan/Beijing Planetarium via the China Academy of Sciences/AFP via Getty Images

The first mechanism operates at a high redshift and involves the death of population III stars and/or a direct collapse. Both of these mechanisms require a low-metallicity condition — here metalicity means the abundance of other atoms with respect to a hydrogen atom, which was found in abundance in the early universe.

The second mechanism results in the continuous production of MBHs throughout the cosmic time scale. This mechanism is invoked by the gravitational runaway process — in which due to the dense gravitational field of an object, the other objects lose mass to this object with a denser gravitational field. This process requires a dense star cluster.

Do NSCs help MBH formation?

As discussed above, the second mechanism which involves the gravitational runaway process is of particular importance here as it requires a dense star cluster. The paper published focused on this mechanism of Blackhole formation for their study.

This mechanism operates at any redshift, unlike the first method which occurs only at high redshifts. It is of particular interest because of its promising ability to form IMBH or MBH.

The observational evidence is to compare the X-ray emission from NSC to the measured velocity dispersion in the NSC and it is found that NSC with a velocity dispersion > 40km/s are twice likely to be detected by X-ray observation.

What the researchers found was that an NSC could form around a pre-existing Blackhole or could provide an environment in which an MBH could be formed. The limit of velocity dispersion > 40 km/s is because, above this limit, the system would not be able to prevent itself from a core-collapse which is the basis of the start of the formation of a Blackhole.

How will MBH form From NSC?

Once the system of NSC crosses the threshold of 40 km/s, the formation process could take one of the two paths:

1. It will produce a blackhole subcluster which will leave a single stellar-mass blackhole.
2. It will leave no blackhole and then, one will from the stellar mergers due to the core collapse.

Thus, both these mechanisms lead to the same final step — the formation of a stellar-mass blackhole which will proceed to form an MBH given the conditions in the NSC.

What did they find?

The results of the paper published found that :

1. NSC with higher velocity dispersion had pre-existing MBH and NSC formed around the MBH.
2. High-density, high-velocity NSC is more efficient at fueling the formation of MBH.

Thus, the results point to a new direction that can link the formation of IMBH and MBH to the NSC present in the galaxies, and probing in this direction can help uncover more about the origins and formation of IMBH and MBH.