Supermassive black holes are impossibly big – and still growing

 ·12 Jul 2015

“Here be dragons” was a phrase once used on ancient maps, often accompanied by mythical sketches, to highlight an unexplored or potentially dangerous area.

Astronomers might want to borrow this warning to label the centre of galaxies, which contain supermassive black holes.

There’s a lot we don’t know about these monsters – and scientists have just found one that even defies the laws that are meant to govern its growth. By growing to a huge mass at an exceptionally fast rate, this black hole indicates that there could be more – and bigger – supermassive black holes out there than we previously thought.

No mythical creatures

These proverbial “dragons” of astronomy are no mere myth. We have many lines of evidence supporting their existence. These range from infra-red observations of very distant galaxies and quasars to infer their masses, through to observations of our very own Milky Way.

The animation below shows the orbits of stars near the centre of the Milky Way. They were observed over more than a decade to be orbiting around a dark, unseen object.

By applying simple orbital physics, we know that this dark object must have a large mass to cause these orbits by its gravitational pull – perhaps more than 4m solar masses.

However it must take up a tiny amount of space as it doesn’t “hit” the orbiting stars. Only a black hole can really fit the bill as an explanation for what this unseen object can be.

Disturbingly though, this is just a middle-of-the-road mass range for supermassive black holes. Discoveries of black holes having ten billion solar masses are being made in other galaxies.

Growing a supermassive black hole

Unlike your average black hole, supermassive black holes are not simply the product of the death of a star – they are more massive than any star could produce on its own. So where do they come from?

There are a number of theories. Arguably the main one is that they must have been “born” in the younger moments of the universe from the earliest star clusters. From there, they need to grow rapidly to gain the amount of mass they have.

In part, this can be explained by accretion of gas and in-falling materials from the galaxy. Indeed, recent observations by the European Southern Observatory suggest that the Milky Way’s own supermassive black hole once had such a dinner.

But the growth of these supermassive black holes are probably not entirely due to accreting material from their own galaxy. If two galaxies were to collide with each other, their black holes gain a new source of food to fuel their growth. They might also merge with one another to create an even bigger black hole.

In this manner, the growth of black holes is understood to be intimately tied to the growth of their parent galaxy. Indeed, the mass of a supermassive black hole appears to be tightly correlated with how fast stars orbit inside of the host galaxy.

At an even deeper level, the black hole can act as a kind of thermostat for star formation inside its host galaxy.

Energy and/or momentum that is generated by the black hole can cause star formation to stop in its host galaxy. This is because the energy heats up the gas inside a galaxy so that stars may not form as readily.

CID-947: the rebel

But in a new study published in Science, astronomers report the discovery of a supermassive black hole that seems to break the synchronisation of galaxy growth with black hole growth.

Using Keck telescope in Hawaii, the authors found a galaxy with an atypically large supermassive black hole in its centre (CID-947). Its ratio of black hole mass to stellar mass is enormous – almost an order of magnitude bigger than what might otherwise be expected.

With a mass of 7 billion solar masses, this supermassive black hole compares favourably to some of the most massive known.

Here is where it gets really interesting.

Using a measurement known as redshift, which is tells us how far away – and thereby how old – a galaxy is, the team could see that this supermassive black hole must have grown exceptionally fast. Most models predict black hole masses in the range of millions of solar masses, rather than billions, for a galaxy of this age.

Meanwhile, the galaxy is still in the process of forming stars, its black hole has not (yet) caused this to stop. The study concludes that CID-947’s black hole is in the final stages of forming and we are witnessing the cessation of accretion on to it.

This is significant, as CID-947 can be interpreted as an early version of the most (extremely) massive galaxies in the present day universe. That is because if we have humongous galaxies in the universe today, then they must have come from something massive in the earlier universe.

Importantly, they’re in place at early times and have not yet shut down their host galaxy star formation – meaning they could well grow even bigger with time.

In other words: there may be more (and bigger) “dragons” than we thought lurking out there. As we are exploring bigger and bigger parts of the universe, maybe a map with clear warning signs is in order?

By Kevin Pimbblet, Senior Lecturer in Physics at University of Hull

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