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