The electromagnetic spectrum comprises “radiation” of many types whose fundamental similarity is usually overlooked – from radio at the low-energy end to gamma rays at the high end. Whether one measures it in frequency, wavelength, or photon energy, the difference from one end to the other is more than 15 orders of magnitude. In principle, there is neither an upper nor lower limit, though the extremes are uncommon in nature.
All forms of EM radiation result fundamentally from the oscillations of electrical charges. All forms carry information encoded in these oscillations. Radio waves have lengths typically measured in meters. They were the first type of EM radiation used for communication, because the atmosphere is transparent to them. The length of microwaves is measured in centimeters, and although the atmosphere is mostly transparent to microwaves, they do not easily penetrate solid objects. There’s an atmospheric window for visible light and infrared radiation, with wavelengths measured in a few hundreds of nanometers. But at shorter wavelengths (ultraviolet, ~400 nm) the atmosphere again becomes opaque – fortunately for living things.
Astronomy can, in principle, be done at any wavelength, from radio to gamma rays, and there is astronomically interesting information at all wavelengths. In cases where the atmosphere is transparent to the radiation, astronomy can be done from the ground (though higher elevations are better, to be above dust, water vapor, and clouds). Otherwise it must be done from space.
One especially interesting case is the millimeter and sub-millimeter wavelength band, lying between microwaves and far infrared. Very little astronomical work has been done in this band, because the atmosphere is mostly opaque to those wavelengths, and because it has been technically difficult to build receivers for this range.
However, by using the latest technology and locating facilities at high elevations above most of the atmosphere, astronomers are about to gain access to this range for the first time.
Humanity’s most complex ground-based astronomy observatory, the Atacama Large Millimeter/submillimeter Array (ALMA), has officially opened for astronomers. The first released image, from a telescope still under construction, reveals a view of the Universe that cannot be seen at all by visible-light and infrared telescopes. Thousands of scientists from around the world have competed to be among the first few researchers to explore some of the darkest, coldest, furthest, and most hidden secrets of the cosmos with this new astronomical tool.
At present, around a third of ALMA’s eventual 66 radio antennas, with separations up to only 125 metres rather than the maximum 16 kilometres, make up the growing array on the Chajnantor plateau in northern Chile, at an elevation of 5000 metres. And yet, even under construction, ALMA has become the best telescope of its kind — as reflected by the extraordinary number of astronomers who requested time to observe with ALMA.
What types of astronomical questions can be investigated in this newly opened territory?
The types of astronomical objects that radiate significantly in the millimeter and sub-millimeter band are rather cold – less than 100 K (-173° C) – especially the dense clouds of dust and gas from which stars and planets form. For instance, the image of the Antennae Galaxies (above) shows regions of cold gas resulting from the galaxy collision.
Another priority astronomical target is AU Microscopii, a star 33 light-years away that is only about 12 million years old. Astronomers are looking for clumps of matter around the star (planetesimals) that will eventually form planets. It may be possible to identify specific types of molecules present in such dusty regions, including organic molecules.
The Milky Way’s central black hole, Sagittarius A*, is 26,000 light-years away, but observation of the immediate vicinity has been difficult because of intervening galactic dust. ALMA will make it possible to observe cold gas clouds near the black hole and study their motion. Although our black hole isn’t the center of an “active galactic nucleus” at present, it’s a whole lot closer than any of the real thing, and it may exhibit similar behavior in some respects.
After many more dish receivers are added to ALMA, for a total of 66 with a maximum separation of 16 kilometers, the technique of interferometry will enable observation of even very distant galaxies at high resolutions. This will make it possible to study the internal structure of galaxies when they first began to form in the very early universe, several hundred million years after the big bang.