Radio Astronomy with the Arecibo 305-meter Telescope:
The Benefits of a
Large Single Dish
While great advances in the mapping of astronomical objects have been accomplished using radio telescope arrays, large single-dish telescopes such as Arecibo have a unique and important role to play in radio astronomy. This is particularly true for the study of faint, extended, or time-variable sources. A large single-dish telescope, particularly when equipped with multiple receivers to maximize sky coverage, can provide a more sensitive and rapid look at a larger region of the sky.
The unparalleled sensitivity and excellent single-dish angular resolution of Arecibo allow a number of unique investigations which will not be possible with any other instrument through at least the next decade. Below are some examples of such unique contributions:
- Pulsars are city-sized, rotating neutron stars, with a radio beam generated in a region that is smaller than a kilometer (probably much smaller). Such small objects remain point sources even at the highest resolution achievable with interferometer arrays, and hence there is no benefit in mapping them with such instruments. On the other hand, they tend to be faint, which calls for large single dishes like Arecibo. As a result, many advances in pulsar science have been made with Arecibo, such as the discovery of the first binary and the first millisecond pulsar and their use for fundamental studies in physics (see other pages for further details).
- As the most sensitive instrumentation available, only Arecibo can provide adequate follow-up observations of the faintest and most transient radio sources discovered in surveys currently underway. For example, Arecibo provides the only avenue for exploring the faintest "dark" galaxies, those objects comprised largely of dark matter. The telescope has also detected galaxies with extremely low masses, more objects than all previous radio surveys combined, and is uniquely poised to place sensitive limits on the amount of gas in the regions between galaxies.
- With a large collecting area and excellent frequency coverage between 1 to 10 GHz, Arecibo enables sensitive study of a variety of molecules in clouds throughout the universe. Spectral lines studied are both simple and complex, and can be detected in the comae of comets in the solar system, in star-forming regions within the Milky Way Galaxy, and in other galaxies, even at great distances. The more complex molecules may be important in forming a greater understanding of the origins of life.
While interferometric arrays provide higher angular resolution on small areas on the sky, faint and extended objects that fill the interferometer beam are "resolved out" and do not appear in interferometric images. Also, interferometers depend upon an imaging technique that takes advantage of the rotation of the Earth. The time required to make an interferometric image may average out emissions that are transient or variable. In contrast, the Arecibo dish can detect emission from extended, faint sources; and in shorter times than interferometers can, rendering it an excellent instrument for the characterization of these kinds of emissions from astronomical sources.
- These unique strengths of Arecibo have led to the discovery of new types of interstellar structure, including blobs that are probably gravitationally bound by dark matter (unknown material that constitutes the majority of the universe!), shards ripped off of high-velocity clouds, and high-energy filaments produced by magneto-gas-dynamic instabilities.
Arecibo even plays a powerful role as part of intercontinental arrays of radiotelescopes.
When used in combination with other telescopes across long baselines, including into space, the large aperture of Arecibo adds considerably to the ability to map extremely fine structure in faint objects, and to detect time variations. Such variations are observed on several-hour timescales in active galactic nuclei (AGN), which are tied to black holes whose masses are billions of times the Sun's.