Australia Telescope National Facility

Infrared Astronomy: in the Heat of the Night
The 1999 Ellery Lecture
J.W.V. Storey, PASA, 17 (3), 270.

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THE INFRARED ADVANTAGE

It was exactly 200 years ago, in 1799, that William Herschel proffered the somewhat tentative suggestion: ``Radiant heat will at least partly, if not chiefly, consist, if I may be permitted the expression, of invisible light.'' (See, for example, King 1955.) The year 1999 is therefore a particularly appropriate one in which to review the current state of infrared astronomy. The past 25 years have witnessed an extraordinary transformation in infrared astronomy, as it moved from being the preserve of a few physicists to become a mainstream activity at virtually every ``optical'' observatory around the world. Almost no new ``optical'' telescope is designed now without a major effort going into optimising the infrared performance. The so-called Next Generation Space Telescope (NGST) will probably not work in the optical at all, being purely an infrared telescope. To the astronomer, the infrared region offers substantial advantages. Some of the most important of these are:

Lower extinction. Interstellar extinction is a very steep function of wavelength. Heavily obscured regions, such as the centres of galaxies and star-formation regions, cannot be observed at optical wavelengths but are readily studied in the infrared.
The ability to see warm (as opposed to hot) objects. Planets, circumstellar discs, protostars and other warm objects emit negligible flux at visible wavelengths, and can therefore only be directly observed at infrared wavelengths and longer.
Better spatial resolution under seeing-limited conditions. The diameter of an image under seeing limited conditions goes as
$\lambda^{-1/5}$ . In addition, adaptive optics correction becomes much simpler and much more effective as the wavelength increases.
Unique phenomena. In any spectral region there will be phenomena that can be observed at other wavelengths only with difficulty, or perhaps not at all. In the infrared region, for example, lie almost all of the molecular vibrational transitions, plus the rotational lines of molecular hydrogen and HD.
Red-shifted spectral features. Important diagnostic lines whose rest frame wavelength is in the optical or ultraviolet are shifted into the infrared at high z.

Of course, there are also substantial difficulties to be overcome when working in the infrared. To begin with, the atmosphere is opaque except for a few wavelength bands or ``windows''. Strong, saturated absorption lines of molecules such as water, carbon dioxide and ozone are present at intervals across the spectrum, leaving many wavelength regions unobservable from the ground. Secondly, the sky is raining photons. Beyond about 2.2 microns, anything at room temperature and above is emitting copious amounts of black-body radiation. For the infrared astronomer it is as if the telescope were brightly illuminated and the sky itself glowing. Not only does the flood of photons create a high background-limited noise floor, but the sheer number of photons (up to 10¹⁰ per second per pixel) presents a significant technological challenge. Finally, and partly as a result of the last two considerations, the technology needed to work effectively in the infrared has lagged that of the optical region by a decade or more.

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