Central Black Holes

Black holes are relatively simple objects, compared to the wide variety of objects present in the universe. The ones expected to be found astrophysically can be described by just two parameters. But where black holes get interesting is when we start studying the fascinating ways that they interact with their environments. Probably the most useful aspect of astronomy is that objects that astronomers like to study are never quite alone and isolated. Their surroundings interact with them to produce even more amazing effects we can study. In this same way, it is the habitat of black holes that make them some of the most interesting objects to study in the universe.

The Centers of Galaxies

There is a substantial amount of evidence indicating that most galaxies in the local universe harbor very massive black holes in their centers. The masses for these black holes range from a million to a billion times the mass of the Sun. Despite their very high masses, these black holes are only a tiny proportion of the amount of mass in their entire host galaxy. Usually only about 0.1% of the mass of a galaxy is taken up by the black hole. Still, the properties of a black hole correlate surprisingly well with the properties of their host galaxies, suggesting a deeply tied relation between galaxies and their central black holes.

Black holes are relatively simple systems. All the properties of a black hole can be described by just the three parameters of mass, angular momentum, and charge. Charged black holes are not expected to form in nature, so astrophysically we can ignore even that parameter. The fact that the mass for all known central black holes is nearly 0.1% of their host galaxies is very remarkable. There is also a very close correlation between the mass of central black holes and the velocities of the stars in their galaxies. All this suggests that galaxies and black holes are closely linked, sharing similar evolutionary paths and perhaps even similar roots for formation.

This leads to the question of how these massive black holes formed in the first place. Smaller and lighter black holes, those more closer to the mass of the Sun1, are comparatively easier to account for. They are the endpoint in the stellar evolution of the most massive stars in the universe. But the formation for larger black holes is more difficult to accommodate.

Massive and Early

One way we can learn more about the formation of black holes is through observations of early quasars. Quasars are extremely luminous objects located in the centers of some galaxies. They are believed to be powered by supermassive black holes as they accrete a large amount of mass2.

Interestingly, quasars are often observed to be located in galaxies that are distant (read “further back in time”). Observing and measuring the energy outflow from these quasars tells us that massive black holes (about a billion times the mass of the sun) already existed when the universe was just a billion years old. This fact places some constraints on models that may help explain how central black holes developed.

Some current theories suggest that the first generations of stars3 resulted in creating a population of stellar mass black holes. These black holes could perhaps join together to create a larger black hole and migrate towards the center of its host group of stars. There are still uncertainties in this model though, and further work on the earliest stars is required to see if they could generate the necessary black holes. Only a range of masses for the black hole are stable at the center of a group of stars: too light and it could wander from the center of the galaxy; too massive and the star required to form it would be unstable.

Another possible route suggested by researchers has been through the collapse of dense gas in the early universe. Metal-poor gas that made up the earliest galaxies could have resulted in gas not fragmenting into smaller stars but instead collapsing to form supermassive stars in the center of galaxies. The supermassive star near the center of each galaxy would then eventually collapse and its core would subsequently become a black hole.

Once these black holes form, there is still the requirement to turn them supermassive: from masses of about a few times the Solar mass to a million or billion times the mass of the Sun. Models of early galaxies suggest that gas reaches very high densities near their centers, enough to promote rapid accretion onto the black hole and growing it larger. There are still some uncertainties on whether or not this method would be feasible for the majority galaxies and their central black holes. These questions can really only be solved through more observational evidence and simulations of gas in early galaxies.

Collecting data from quasars and active galactic nuclei is not the only method to gain observations of these central black holes. Gravitational waves could also do the job4. As smaller seed black holes merge to create more massive holes, they are expected to emit gravitational waves. The amount of events detected would help place constraints on the models, and perhaps even suggest different mechanisms for the creation of supermassive black holes.

Smaller Stellar Systems

The entire discussion raises the question about the point at which groups of stars stop hosting black holes in their centers. We do have evidence for the existence of black holes in nearly all large galaxies, and it is widely believed that they do exist in all large galaxies. But the smallest groups of stars, such as open clusters consisting of a few thousand stars, are not believed to have black holes in their centers. Recently, there have been suggestions that large galaxies should not be alone in harboring massive central black holes, and that dwarf galaxies and globular clusters should posses black holes in their centers as well if they can undergo core collapse (see Miller (2012)).

Velocity dispersion measures the statistical variation of the velocities of stars in a large group. Since the velocity of each of the stars corresponds to the amount of kinetic energy it possesses, the velocity dispersion can act as an analog to the kinetic energy of the entire group stars. The recent research finds that above a velocity dispersion of about 40 km/s in a stellar system, massive black holes will form. If the stellar system has reached an equilibrium state, it will evolve to contain a high density core. The stars and the already existing black holes located in the core will begin a runaway process of falling in towards the center of the stellar group of stars, and with a velocity dispersion greater than 40 km/s, there will be no process possible to stop the runaway core collapse.

Depending on how many black hole interactions there are in the center of a stellar system, there will be either one or zero black holes remaining. This depends on if there were originally an even or odd number of black holes5. When two black holes merge, the resulting gravitational radiation is not expected to be symmetric. Therefore, the resulting single black hole is expected to be ejected from the stellar system due to this gravitational radiation. If there were originally an even number of black holes, there would be no black holes remaining since all mergers would result in the ejection of the resulting black holes. However, if there were originally an odd number of black holes, there would still be a black hole remaining that would not have undergone a merger, and later survive from being ejected out of the system. This black hole would be expected to accrete matter and grow into a larger massive black hole in the stellar system center.

In the case where there are no black holes remaining after the merger and ejection stages, the stars falling in towards the center of the group of stars would be expected to collide together and create a massive star, the core of which would later collapse into a stellar mass black hole. This black hole at the stellar system center would similarly also accrete matter and grow into a larger and more massive black hole.

This research sets a loose lower limit on the size of a group of stars that can host a black hole in their centers. It serves as a limit since, if shown to effectively explain globular clusters and small galaxies, it provides a boundary using velocity dispersion above which the formation of a central black hole is inevitable. Forming a black hole below the boundary is not prohibited, on the other hand. A central black hole may still appear and become massive through other means in these smaller groups of stars. While it does help provide an answer to the question of where a group of stars starts possessing a central black hole, it also suggests that there isn’t a hard limit.

An Exciting Future

As is probably obvious by now, there are a huge number of uncertainties about black holes in the centers of star groups. Like many other cases in astronomy, what starts as a seemingly simple situation, a black hole describable by only two parameters, morphs into a more interesting and complex situation due to considerations about the interactions with its surroundings. It also shows how theory and observational sides of astronomy play off of each other. Observations have provided evidence for interesting black holes in the early universe, and theory is working to explain their presence. On the other hand, theory has provided a suggestion of the presence of central black holes in smaller groups of stars that now have to be supported or disproven observationally. Both lead towards an exciting time ahead, as we begin to answer more questions about central black holes and undoubtedly form newer ones.

Sources and Further Exploration

  1. And subsequently called “stellar mass black holes”. 

  2. One mechanism is thought to be through the tidal disruption of stars. If a star ventures too close to a black hole, the gravitational force on its side closest to the black hole will be much larger than the gravitational force on the side facing away from the black hole. This creates a tidal effect, which, if powerful enough, can rip apart (“disrupt”) a star. This matter collects in an accretion disk around the black hole, which can emit radiation, and may be also shot up into relativistic jets, which can emit directed emission. 

  3. The first generations of stars were formed from the remnants of the big bang, which was primarily hydrogen and helium. Their constituency and surrounding environments have led astronomers to believe that they were very massive (on the order of 100 times the mass of the Sun) and lived very short lives. 

  4. Gravitational waves are expected to be first detected some time in the next ten to twenty years. If they are detected, they will definitely revolutionize the field astrophysics. It will become one of the few ways astronomers can learn about the universe outside the electromagnetic spectrum, and allow a way to study some of the most interesting objects in the sky through their gravitational radiation: black holes and neutron stars. And like any new observational tool, it will introduce more questions than it can answer. 

  5. Yes, I was surprised and amused by this result when I first read about it.