SAO Guest Contribution


by A/Prof. Duncan Forbes

A/Prof. Duncan Forbes, over the years an instructor for HET603, HET604 and HET611, is a faculty member at Swinburne University. A New Zealander who completed his PhD at Cambridge, Duncan has also spent time at the Space Telescope Science Institute and Lick Observatory in California. Over the years, he has worked on various aspects of galaxy evolution with a recent fondness for globular clusters in external galaxies.

If you have any questions for Duncan, post them to the Astronomy News forum.

With my colleagues Mike Merrifield (Nottingham) and Ale Terlevich (Birmingham), we decided to put out a press release announcing our recent discovery of the growth of black holes. This was taken up by several newspapers, science magazines and the online media.  Below I describe how we made our discovery, what it means and how it led to a press release.

At the University of Birmingham in England, Ale Terlevich and myself have been working for the last year at `age-dating' galaxies. Determining the age of the stars in even nearby galaxies has proved very difficult. Only recently have techniques been developed (based on measuring absorption lines in a spectrum of a galaxy) to do this. So after a years work, we created the first large catalogue of ages for 180 galaxies. Knowing the age of a galaxy is clearly very important for `evolutionary' studies. For example, if we want to place galaxies in an evolutionary sequence we need to know what chronological order to place them in. So in many ways we have the equivalent of palaeontology - the ability to age-date (astronomical) fossils.

We forwarded an early draft of our age catalogue to Mike Merrifield for comment. (Astronomers come in two `flavours' - observers, like Ale and myself, and theorists like Mike.) Mike had been thinking about the relation between the mass of a central black hole and the mass of the galaxy it resides in. This has become known in the astronomical community as the `Magorrian relation' after John Magorrian who published the relation in 1997 ( Although bigger galaxies have bigger black holes on average, there was a lot of scatter in the relation. Mike (and many other astronomers) was looking for an explanation. There were 23 galaxies in the Magorrian relation for which ages were available. In turned out that age was the key property to explain the scatter in the relation. So older galaxies have black holes that are relatively large for their host galaxy, and viz versa for young galaxies.

To understand our discovery we need to consider exactly what our ages are measuring. The galaxy age is actually measuring the time since the last major episode of new star formation in the galaxy, which is probably caused by a merger of two smaller galaxies. Thus older galaxies formed by a merger that took place at an early epoch of the Universe. Back then, there was more gas and relatively fewer stars so the black holes gorged themselves on the abundant gas. The black holes in more recently formed galaxies have had to make do with less gas to fuel them.

Our results point towards an intimate link between central black holes and their host galaxies - it seems less likely that black holes formed independently of galaxies (a possibility suggested by some theorists). Understanding the connection between black holes and galaxies is important because we now think that all galaxies possess massive black holes which are probably the inactive remnants of quasars (the most energetic objects in the Universe).

The research paper detailing our work will soon be published in the Monthly Notices of the Royal Astronomical Society ( This result seems to have captured the interest of the media. After making a press release via the Royal Astronomical Society (, Mike and myself were contacted by several reporters. I answered calls from Italy and the USA. As well as the online media (eg BBC online) and science magazines, there were short news articles ranging from the Times of London to O Globo of Brazil!

So what's next? Ale and I are making new observations to extend the number of galaxies with age estimates. In fact, we are both at a telescope in northern Mexico right now, waiting for the clouds to clear. Fingers crossed for tomorrow night then...

Duncan Forbes
28 March 2000

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