ScienceWise - Spring 2010

The astronomy opportunity of the century

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An integral field spectrograph records the spectrum of an object at every pixel of the image plane creating what astronomers refer to as a data cube
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Professor Peter McGregor in the massive instrument assembly hall of the Advanced Instrumentation and Technology Centre at Mt Stromlo
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Artist’s concept of the Giant Magellan Telescope next to the Sydney Opera House

The Giant Magellan Telescope holds great promise for young Australian astronomers

You might not immediately think of Australia as a country of astronomers, but when you look at the quality and quantity of our scientific publications on a per capita basis, we are one of the highest ranking nations in the world in this area. But until recently there has been a disturbing downward trend in the facilities available to Australian astronomers. It’s not that our telescopes are becoming fewer or smaller, it’s that the rest of the world is building bigger and better instruments. However, a recent government decision to fund Australian partnership in the Giant Magellan Telescope will guarantee our astronomers a 10% share in what will be by far the largest and most powerful telescope the world has ever seen.

With seven giant 8 metre mirrors the effective aperture of GMT will be a staggering 22 metres enabling it to collect almost ten times the light of current generation super-telescopes. The large aperture coupled with advanced adaptive optics will also enable the GMT to achieve very high-resolution images of the cosmos. But straight imaging of the universe is only a small part of the science mission of the GMT. The vast majority of its time will be spent directing the light it collects into an array of specially designed instruments. One of these, the GMT Integral Field Spectrograph (GMTIFS), is being designed at the ANU Research School of Astronomy and Astrophysics.

Spectroscopy (splitting light into its component colours) has been of fundamental importance to astronomy for more than a century. It enables astronomers to determine the chemical composition of stars and nebulae by studying the sharp emission and absorption lines present in starlight. Because motion Doppler shifts the frequency of these lines – in the way the pitch of a moving siren can be heard to change – they can also be used to measure the velocities of stars and rotation of galaxies.

In its simplest form a spectrograph places a slit at the focal plane of the telescope and disperses the light using either a prism or more commonly a diffraction grating. The problem with this design is that if you’re interested in something extended like a galaxy, it only enables you to measure the spectrum of one section through it – defined by the slit. What’s much more useful it to be able to measure the spectrum of every part of the galaxy. With a traditional spectrograph, this can be done by moving the slit across the object and combining the data. But this approach is very slow, laborious and wastes a lot of precious telescope time.

An Integral Field Spectrograph takes the concept of spectroscopy one step further, enabling astronomers to record the spectrum of each point in the galaxy simultaneously. This makes it a much more useful and powerful instrument.
Professor Peter McGregor is leading the design team for the GMTIFS. “What we’re aiming to do is build a dedicated instrument that will take full advantage of the incredible light grasp and resolution of the GMT.” He says. One of the key things with the GMTIFS is that it will sit after the adaptive optics module that corrects for the constant distortions created by the Earth’s atmosphere. This means that it will be able to take full advantage of the GMT’s enormous spatial resolution when generating its spectroscopic maps of objects. “With adaptive optics and IFS working together, we’ll be able to do science that simply can’t be done with current generation telescopes.” Professor McGregor says.

One area of particular interest to Professor McGregor is the formation of galaxies. Because light travels at finite speed and the universe is very big, when we look at the most distant galaxies we are in effect looking back in time. Expansion of the universe means that generally, the further away a galaxy is the older it is, but of course larger distance also means a dimmer galaxy with less light to work with. Current studies using present generation large telescopes such as Gemini, have shown that in the very early universe galaxies were lumpy and irregular.

“From the work we’ve been able to do with existing telescopes, it would seem that galaxies evolve from these clumpy structures to the spirals and ellipticals we see in more modern galaxies.  But because the early galaxies are so small and faint we’ve only been able to study a couple of the brightest ones and whether these are really typical of the whole population is very much debatable,” Professor McGregor explains. “Using the GMTIFS we should be able to not only see the structure of early galaxies, but also measure the motions of the different parts of them and get a much better idea of how they evolve.”

Closer to home, the GMTIFS will enable astronomers to see nearby galaxies in such detail that the motions of individual stars can be mapped. This will enable them to study how the giant black holes that lie at the core of most galaxies influence the motion of the stars that lie within their spheres of influence. “Our current thinking is that the presence of super massive black holes at the centre of most galaxies, including our own Milky Way, has a profound influence on the way the galaxy evolves and behaves. So having access to this kind of data will really help us to understand this process.” Professor McGregor says.

The GMT and its Integral Field Spectrograph should also enable astronomers to make great advances in the study of the formation and nature of planets orbiting other stars. “Using the GMT we should be able to not only directly image nearby planetary systems, but to do spectroscopy on individual planets in such systems.” Professor McGregor says. “And that could be the critical step in detecting life outside the solar system.”

However in spite of his enthusiasm for the GMT and its instruments, Professor McGregor isn’t expecting a lot of personal benefit from the project. “Realistically first light will be around 2020,” He says, “And by then I’ll have more or less retired. But what this instrument and Australia’s partnership in it really mean is that the next generation of Australian astronomers, kids that are in high school and college now, will have a wealth of opportunities at their disposal. This is a really great time for young scientists in Australia to get involved in astronomy.”
GMTIFS is supported in part by a grant from the US National Science Foundation

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