The Committee on Radio Astronomy Frequencies (CRAF) is a committee of
the European Science Foundation (ESF).
Millimeter-Wavelength Astronomy: Radio astronomy at frequencies >30 GHz.
At millimeter wavelengths the non-thermal radiation which can be studied at longer wavelengths becomes weak and the cosmic signals are dominated by thermal radiation from cold material. This is just the "long wavelength extension" of the heat radiation produced by any hot body. In principle this is emitted over the whole electromagnetic spectrum from radio wavelengths to gamma rays. For example the thermal radiation from a room temperature body peaks in the infra-red region and is relatively weak in the radio bands. Thermal radiation from cold interstellar clouds has a maximum in the sub-millimeter band and the backround radiation left over from the "big bang", at an equivalent temperture of 2.7 K has its maximum at 150 GHz in the 2 mm band.
In the colder regions of space, matter can exist in molecular form if it is far away from the intense ultraviolet radiation from hot stars. The radiation from these molecules occurs at a series of discrete frequencies or spectral lines which are different for each type of molecule. The relative intensities of the lines emitted from a given molecule depend on the physical conditions such as density and temperature within the emitting region. Thus it is frequently necessary to observe several lines, or transitions, of a given molecule. This is also true if one wants to unambiguously identify a molecule since in general the observed frequency depends on the often unknown velocity of the object under study. Any given transition may be obscured by emission from some other molecule. However, by studying the frequency of several of the emitted spectral lines we can deduce which molecules are present. So spectroscopy is one of the main tools of the mm-wave astronomer.
However, some of the heavier molecules condense to form dust particles which radiate a continuum of frequencies. The study of this continuum radiation is the second tool available to the mm-wave astronomer. It is much more difficult to deduce the composition of the dust particles as there are few if any characteristic lines which can be used to identify its component molecules.
Thus, nature urges mm-wave astronomers to do spectral line as well as continuum observations. These spectral line observations serve in particular a new branch of astronomy: astro-chemistry.
The millimeter and sub-millimeter bands are unique in astronomy as they offer a window through which we can "see" and study these components of the universe which are otherwise invisible. The reasons for this are that:
The very high density of spectral lines in the millimeter spectrum sets this part of the spectrum apart from that studied at lower frequencies. Sensitive studies of molecular clouds have disclosed up to a hundred lines per GHz. In a recent (unsuccessful) search for the simplest amino acid - Glycine - the position of 98 lines was examined, but the density of other lines prevented an unambiguous identification. At such sensitivity the spectrum is completely filled by line emission.
As of January 1995 a total of 108 molecular species had been detected in interstellar and circumstellar gas clouds (Table of observed molecules in interstellar and circumstellar clouds). They include stable inorganic and organic molecules such as salt, carbon monoxide and ethyl alcohol, reactive molecules such as the strange carbon chains HC11N, radicals such as NH2, C2H, and several ions like HCO+, HCCCNH+. Several were discovered in space before being found in the laboratory. Strangely the aromatic molecules so important for life are absent so far, although searches will doubtless continue.
mm-Wavelength astronomy is thus the proper tool to study objects like comets, planets, interstellar clouds, stellar atmospheres, protostars, protoplanetary disks, galaxies, quasars and intergalactic clouds in which the material is largely molecular.
The radio emission from low temperature regions is naturally weak and very sensitive receivers are necessary for its study. For spectral line observations most observatories use super-conducting mixer elements as the first stage of their receivers. These are operated at the temperature of liquid helium and comprise a thin layer of insulating material sandwiched between two superconducting pieces. Hence the name "superconductor-insulator-superconductor" junctions (SIS). The tunnelling properties of these junctions provide the non-linear element necessary for mixing and translating the radio frequency signal to intermediate frequency for subsequent spectral analysis. Present designs have almost no pre-mixer frequency selectivity to allow tuning over a complete atmospheric window with the lowest loss. At any tuning they can examine a slice of spectrum of width typically 500 MHz to 1 GHz. This is usually done by digital autocorrelation spectrometers, filter banks or acousto-optics spectrometers.
For continuum observations of dust for example, very sensitive bolometer detectors of wide bandwidth have been developed. To attain the ultimate in sensitivity the bolometer elements are frequently cooled to 0.1 K and their bandwidth is several tens of GHz. "Staring" arrays of up to a hundred such bolometers are now coming into operation and allow an instantaneous picture of a section of the millimeter sky to be "seen".
Such equipement is extremely difficult to protect from interfering signals at nearby frequencies, i.e. out-of-band and spurious emissions. While also the broadband characteristics of the receivers imply that often frequencies outside the bands allocated to the Radio Astronomy Service are used.
This problem is because of the following reasons:
As at lower frequencies, single dish, connected element interferometers and also VLBI are used at millimeter wavelengths. However mm-observatories must be placed at high elevation, frequently on mountain tops in an attempt to get above the atmospheric water vapour which strongly attenuates mm-wavelength radiation. This has the disadvantage that such observatories often have line of sight paths extending to hundreds of kilometers so that they are open to terrestrial interference from a very large area, much larger than for instruments operating at lower frequencies.
Millimeter radio astronomy is now one of the most dynamic fields of astronomy. In Europe we mention the existance of single dish telescopes in Finland, Sweden, Turkey, Russia, France, Spain with important outstations in Hawaii and Chile. An interferomter array is operating in southern France on Plateau de Bure. On a world-wide scale plans are going ahead for the investment of several billion dollars in new millimeter facilities. These include a 50 m diameter single dish in Mexico and several large interferometer arrays. The USA plans an array of 40x8 m diameter telescopes, Japan aims for a 50x10 m array and the European countries have their sights set on an array of 50x15 m telescopes.
It is clear from the preceeding sections that the whole of the mm- wave spectrum is full of molecular line emission, each line potentially giving us information which is often unavailable by other means. Many lines are still unidentified and may prove of great interest in the future. The IAU list of important lines is an attempt to assign relative scientific priorities but at best it can only be a guide as we cannot anticipate future discoveries or needs. The situation becomes even more complex when one takes account of the Doppler shift acting on the radiation from distant objects. Even the few important lines may thus appear at practically any frequency in the mm-bands. This tendency is reenforced by the requirement of very large bandwidth needed for continuum studies by bolometer.
So astronomers need access to the whole of the mm-windows. It is difficult to see how the protection provided by the existing allocations, based on a few important lines and not taking adequate account of Doppler shifts, will be enough in the future as commercial exploitation of the spectrum moves to higher and higher frequency. It is clear that millimeter astronomers can share spectrum with several fixed services, by use of coordination or radio-quiet zones for example. However, this will be impossible in the case of the satellite services. Some careful thought might be necessary to set up "guard" bands around radio astronomy allocations which could be allocated to some of the fixed services which are more compatible with the strict requirements of the Radio Astronomy Service.
The problem of protection of mm-observatories is subject to further investigation. The way mm-observatories are protected may differ from methods used for "classical radio observatories" (i.e. operating at lower frequencies) and may be more like the protection of optical observatories.
Typical astronomical results obtained at the Institut Radio Astronomique Millimétrique, IRAM, in France and Spain) and various recent highlights illustrate that all individual spectral lines give their own image of the object studied. Combination of all these results obtained is an unavoidable and necessary prerequisite for the astronomer to study the structural and physical characteristics of the object.