Imagine having a telescope one hundred or even one thousand times more powerful than any previous telescope.
It might be hard to decide what to look at first, but for astronomers using the Allen Telescope Array (ATA) for conventional radio astronomy, it will not be too much of a problem. This new instrument will be the fourth largest telescope in the world, as gauged by its collecting area. But its real claim to fame is speed. For surveying the radio sky (a task necessary for SETI discoveries), the ATA will be almost 1,000 times faster than the Arecibo telescope, between 20 and 2000 times faster than the Very Large Array (depending on the scientific program) and 50 times faster than the Green Bank Telescope (GBT).
Moreover, the ATA will be able to resolve sources i.e., to see fine detail better than either the GBT (by a factor of 10) or Arecibo (by a factor of 3), and it will have spectral capabilities that simply arent available at the other telescopes: it can observe multiple spectral windows simultaneously within the entire spectrum to which the telescope is sensitive, a new capability available for the first time on the ATA.
So what will the radio astronomers want to do with this fabulous new instrument? In broad terms, the flagship science will be to determine the structure of the local universe, to search for new effects of black holes, and to seek out the primordial dark matter condensations of the universe. The ATA will be uniquely capable of discovering new phenomena related to the transient radio sky, that is, sources that vary in brightness on time scales of seconds to years. Many of these originate in processes involving ultramassive black holes in the centers of galaxies, distant supernova explosions near the edge of the observable universe, and gamma-ray bursts, the most energetic events in the cosmos. The telescope will also be a general-purpose radio telescope that will provide fundamentally new measurements and insights into the density of the very early universe, the formation of stars, the magnetic fields in the interstellar medium, pulsars, and a host of other phenomena of deep interest to astronomers. Lets look at two of the major areas of astronomical research for the ATA.
The Structure of the Nearby Universe
The forces that generated the density fluctuations in its early history shaped the large-scale structure of the nearby universe. Since dark matter, of unknown composition, is the largest mass component of the universe, it is the effect of forces on the dark matter that provides most of the observed structure. Mapping out the distribution of galaxies traces the dark matter distribution, something that cannot be determined any other way. Attempts to map the distribution of galaxies in the optical portion of the spectrum have been only partially successful because the presence of dust in the Milky Way obscures a large fraction of the sky. Another problem with the optical surveys is that they are not sensitive to faint, low mass galaxies which are often found first in surveys of atomic hydrogen (HI) in the radio portion of the spectrum and then later confirmed using large optical telescopes.
The ATA will make the first all-sky neutral hydrogen (HI) survey of the local universe, which will provide key information about how the universe evolved. An all-sky HI survey from Hat Creek will detect the hydrogen in galaxies similar to that of the Milky Way to distances as large as 20 times that of the Virgo cluster, the nearest rich cluster of galaxies. The survey will take three years to complete.
But the survey will also be sensitive to dark galaxies: the earliest matter concentrations to form. A dark galaxy is one that contains only atomic hydrogen and dark matter, but no stars. These galaxies are very difficult to detect with current radio telescopes, but may be quite common, and may represent the earliest concentrations of dark matter that have not yet coalesced into galaxies. A high-resolution image of a typical dark galaxy could be produced at the ATA in 5 minutes, but would take 10 days of time using the Arecibo telescope. Small dark galaxies can be detected with the ATA to distances as great as the Virgo cluster, which is about 20 times farther than the distance to the Andromeda Galaxy, M31. If these galaxies exist, the ATA will find them.
Black Holes
Black holes are among the most remarkable predictions of General Relativity, Albert Einstein's theory of gravity. They compress so much mass into such a small volume that gravity overwhelms all other forces and nothing, not even light, can escape. The unprecedented ability of the ATA to find transient radio sources will provide a powerful new probe of black holes in the universe. We highlight two examples:
- It is now believed that most nearby galaxies have supermassive black holes at their centers, with masses between a million and a billion times that of our Sun. However, the number of such black holes at higher redshift, that is to say, early in the life of the universe, is poorly constrained. When a star like the Sun passes close to a massive black hole, it is ripped apart by its strong gravitational force. The remnant gas falls into the black hole producing a bright, month-long flare of radio emission. The ATAs unprecedented ability to find transient radio sources will lead to the discovery of hundreds or thousands of such radio flares, providing a powerful new way to detect and study massive black holes. Such flares have distinctive signatures and are so bright that they can be detected with the ATA to distances as great as that of any known galaxy.
- When a massive star runs out of nuclear fuel it collapses under its own weight, giving birth to a neutron star or a black hole. The collapse also produces a powerful flare of radiation, a supernova. Historically, supernovae have been found by their emission at visible (optical) wavelengths. Over the past few years, however, it has become clear that they can also be detected as transient radio sources. The ATAs study of the transient radio sky will thus open up a new window into how black holes (and neutron stars) are formed in the collapse of massive stars.
These examples represent some of the more compelling areas of research that the ATA can do. But theyre just the tip of the iceberg.
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