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An ‘overmassive’ black hole that breaks all the rules – so what?

For a number of reasons – not least its ‘overmassive’ black hole – NGC 1277 is a little different to other galaxies. David W. Hogg, Michael Blanton, and the SDSS Collaboration

An ‘overmassive’ black hole that breaks all the rules – so what?

Supermassive black holes weighing millions or billions of times as much as our sun appear to be ubiquitous in large galaxies.

We don’t fully understand how these supermassive black holes form and grow, but it’s clear they have an important role to play in the evolution of their host galaxies.

But a paper published by Remco van den Bosch and colleagues in Nature last week could provide valuable new information to help us understand this process of galaxy evolution.

Mass and more

As far as we know, the mass of a black hole seems to depend on a number of properties of the central part of the host galaxy (known as the “bulge”), including:

  • the total mass of the stars in the bulge
  • the luminosity (or brightness) of those stars, and
  • the spread in velocities of the light from those stars (known as the velocity dispersion, or sigma value.)

This relationship indicates that the host galaxies seem to “know” how massive their central black holes are, implying an important co-evolutionary link between the growth of galaxies and their supermassive black holes.

These mass scaling relationships appear to hold for the vast majority of galaxies where we have been able to measure the black hole masses. But it is not clear that the relationships hold for the outliers – that is, for really low- or really high-mass galaxies.

Nor is it clear if these relationships are the same now as they were at much earlier stages in the life of the universe.

Testing these scaling relations at the extreme ends of the population of galaxies is a very hot topic in modern day astrophysics, and this is precisely what van den Bosch and colleagues have attempted to do in their paper.

Using the Hubble Space Telescope and the Hobby-Eberly Telescope, van den Bosch and colleagues weighed the central black hole in the galaxy known as NGC 1277.

NGC 1277

At a measly 200 million light years away, NGC 1277 is actually a relatively close neighbour to the Milky Way … in astronomical terms. That said, the first dinosaurs were walking the Earth when the light we are now seeing from NGC 1277 was first emitted.

NGC 1277 is what we call a lenticular galaxy. It is dominated by a disc of gas and very old stars but doesn’t have the beautiful spiral arm structure we see in other nearby galaxies (such as Andromeda and our own Milky Way).

The Hobby-Eberly Telescope gleams in silver and gold against a deep blue night sky. Damond Benningfield

In their work, van den Bosch and colleagues were able to fit models to the velocities of the starlight leaving the galaxy in order to measure the black hole mass. Their result: the discovery of a black hole – dubbed an “overmassive” black hole – weighing in at a whopping 17 billion times the mass of our sun.

This is right up there with the previous record holder NGC 4889, making it one of the most massive supermassive black holes observed so far.

Beyond records

More interesting than the sheer size of the supermassive black hole is the fact it was found in NGC 1277 at all – a very compact and relatively low-mass galaxy. This black hole behemoth weighs 14% of the total stellar mass of the galaxy and 59% of the mass of the central bulge.

In contrast, most supermassive black holes constitute only 0.1% of their galaxy’s bulge mass, with the previous largest outlier being the small galaxy NGC 4486B with a black-hole-to-bulge-mass fraction of 11%.

NGC 1277 also lies far off the typical trends in black hole mass vs bulge luminosity … by a factor of roughly 200. Its M-sigma value – that is, the correlation between stellar velocity dispersion and the mass of the supermassive black hole – is also ten times higher than is typical.

These characteristics make the black hole in NGC 1277 a truly unusual specimen.

An artist’s conception of a supermassive blackhole at the centre of a galaxy. NASA

So what does this all mean?

Why exactly NGC 1277 bucks these observed trends is not immediately clear, particularly as no evidence was found for any tidal stripping – mass being stolen by a nearby galaxy – that could perhaps account for the lower-than-expected mass in the host galaxy.

In addition, the stars in NGC 1277 are very old, indicating that the black hole must have been as massive as it is now for at least the past 8 billion years.

Why? The growth of the black hole through the accretion of gas is highly unlikely to occur without triggering the formation of new young stars or the formation of a larger bulge.

Learning more

Important clues about this bizarre black hole are likely to be found through studying other galaxies with similar properties.

To this end van den Bosch and colleagues have identified five other nearby compact galaxies that look extremely similar to NGC 1277 with very high velocity dispersion (i.e. sigma) values, which are typical indicators of the presence of supermassive black holes.

These galaxies are unusual in the present-day universe, but appear to be very similar to the “red and dead” galaxies found at much earlier times.

This lead van den Bosch and colleagues to suggest that their six compact systems may be local analogues to these older high-redshift galaxies.

Such distant galaxies are very difficult to study even with the most powerful telescopes (given they’re so far away) so this new, nearby bunch could provide an enormous boost to our understanding of how galaxies evolved from the dawn of time until the present day.

Further reading: