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3 The Radio Astronomy Service
Pages 88-134

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From page 88...
... Many astronomical discoveries that have captured the imagination of astronomers and the public alike were made accidentally with radio telescopes; a list of such discoveries would include that of the primordial cosmic microwave background (CMB) , celestial masers, and pulsars -- the latter being the dense, fast-rotating, radio-emitting remnants of massive stars.
From page 89...
... (See Table 3.1 in §3.2, "Radio Observatories and Radio Telescopes," for the characteristics of the newer instruments.) The current scientific questions that are motivating the construction of these new telescopes are no less exciting than those that were resolved in the past.
From page 90...
... examples include the exploration of planetary systems in formation around other stars, measurements of neutral hydrogen in the early universe, and the study of star formation in distant galaxies. Furthermore, it is through radio observations that the discovery of life-indicating molecules in other planetary systems might be made.
From page 91...
... The planned Square Kilometer Array will enable detailed studies of such disks. The key strengths of radio measurements that will enable these studies are their ability to trace the distribution of gas and dust throughout the disk, to study the dynamics and temperature of the material involved in planet formation, and to follow the accretion of material as it develops from the tiny, submicron dust particles characteristic of the interstellar medium to centimeter-sized clumps, the first critical step in the formation of terrestrial planets.
From page 92...
... The radio spectrum is the place to pursue a connection between astrochemistry and prebiotic terrestrial chemistry because it gives access to the wealth of spectral lines. With the sensitivity and resolution of the coming generation of radio tele scopes, it will be possible to search for sugars and amino acids and to follow the flow of chemistry from molecular clouds into protoplanetary systems.
From page 93...
... It was through this simple observation that radio astronomy ruled out the rival, steady-state theory of a non-evolving universe and favored evolution ary models in which the universe has expanded from a compact, hot origin. Radio astronomy also provides the strongest evidence for the Big Bang through the discovery of the cosmic microwave background (CMB)
From page 94...
... After hundreds of millions of years, gravitational contrac tion of the material in the original density fluctuations produced the first stars, which gave off light, Figure 3-2 and so the "Dark Ages" ended. Further generations of stars formed, and galaxies and black holes coalesced from the stars.
From page 95...
... It will be very faint, and radio interference will be a serious concern. Such observations will have to be made from remote sites and will require careful attention to the mitiga tion of radio frequency interference (RFI)
From page 96...
... Ferrarese and D Merritt, "A Fundamental Relation between Supermassive Black Holes and Their Host Galaxies," Astrophysical Journal, 539(1)
From page 97...
... The pulsar is in orbit around a companion neutron star. General relativity (GR)
From page 98...
... The discovery and study of these powerful "radio galaxies" in the 1960s provided the first evidence for the existence of supermassive black holes -- evidence that was based on the energy conversion required. A major discovery from radio astronomy, made by the technique of very long baseline interferometry (VLBI)
From page 99...
... Spectral line emission from water vapor at 22 GHz has turned out to be an unexpectedly important probe of the environments of supermassive black holes in the nuclei of galaxies. Water vapor appears as a trace constituent in the accretion disks that surround these black holes, and it emits radiation by the maser (micro wave amplification by stimulated emission of radiation)
From page 100...
... Each "spot" in the lower portion represents a separate maser whose velocity, derived from the Doppler shift, is color-coded: red = –500 km/s; blue = 1,500 km/s. The thin curved distribution of masers with the observed velocity Figure 3-5 distribution traces a thin disk of material in orbit around an unseen black hole with a mass of about R01628 times that of our Sun.
From page 101...
... CO emission has been detected in many varieties of galaxies, including some with redshifts up to 6.4, so that the photons now observed were emitted when the universe was only a few percent of its present age. With radio telescopes, it is thus possible to study the properties of star formation in normal galaxies such as our own, in exotic galaxies with vigorous star formation accompanied by accre tion onto black holes (e.g., radio galaxies and quasar host)
From page 102...
... The CO image was R01628 made by combining 200 hours of observations at the Combined Array for Research in Millimeter-wave Astronomy with 40 hours of uneditable the Nobeyama Radio Telescope in Japan. Image courtesy observations at bitmapped images of Space Telescope Science Institute.
From page 103...
... . uneditable bitmapped image
From page 104...
... New radio telescopes, such as the ATA, will be used in a repetitive survey mode to search for transient and variable events. These phenomena are of broad significance and will contribute to the understanding of the life and death cycle of stars; the nature of exotic compact objects such as neutron stars, white dwarfs, 6 R
From page 105...
... Scientists cannot predict what will be found, but on the basis of past experience, they do expect to see new phenomena. The ability to distinguish between transients of cosmic origin and sporadic radio frequency interference will be a challenging enterprise.
From page 106...
... that are seen in the emission. Hence, radio observatories have a variety of telescopes and instruments with unique technical capabilities.
From page 107...
... The GBT and Green Bank, West Virginia, R01628 are in the National Radio Quiet Zone (NRQZ) ; see §3.6 in this report.
From page 108...
... When completed, ALMA will have up to 80 antennas, operating from 30 GHz to 960 GHz. ALMA is at an altitude of 5,000 meters, Figure 3-4-2 where the atmospheric water vapor is low enough that these high frequencies are useable.
From page 109...
... The large foreground building is the Angel R01628 Ramos Visitor Center, which receives more than 100,000 visitors per year. Image courtesy uneditable bitmapped image of the NAIC-Arecibo Observatory, a facility of the NSF.
From page 110...
... The be repositioned to different configurations, with a maximum baseline of 35 kilometers, to R01628 produce images of various angular resolutions. It currently is being upgraded to have more uneditable bitmapped image Very Large Array, it will sensitivity and better image quality.
From page 111...
... Another key factor in optimizing the capability of an observatory is its loca tion, and the broad spread of the radio spectrum results in a number of factors that can be important. At frequencies below 30 GHz, radio frequency interference (RFI)
From page 112...
... Hat Creek, California 0.5-11.2 1,230 Arecibo Observatory Arecibo, Puerto Rico 0.3-10 73,000 78 and 113b Arizona Radio Observatory Tucson, Arizona 68-500 Atacama Cosmology Telescope Chile 150-270 28 Caltech Submillimeter Observatory Mauna Kea, Hawaii 200-950 85 Combined Array for Research in Owens Valley, California 70-260 770 Millimeter-Wave Astronomy Green Bank Telescope Green Bank, West Virginia 0.3-100 7,850 Large Millimeter Telescope Mexico 85-275 1,960 South Pole Telescope South Pole 95-275 78 Submillimeter Array Mauna Kea, Hawaii 180-900 226 Very Large Array Socorro, New Mexico 0.07-50 13,250 Very Long Baseline Array 10 sites in United States 0.3-90 4,900 Selected Facilities Under Construction Allen Telescope Array (350 dishes) Hat Creek, California 0.5-11.2 10,220 Atacama Large Millimeter Array Chile 30-960 6,000 Long Wavelength Array 1+ New Mexico 0.015-0.09 20,000 @ 15 MHz Murchison Widefield Array Murchison, Australia 0.08-0.3 8,000 Selected Facilities in Planning Cornell Caltech Atacama Telescope Chile 200-900 490 Square Kilometer Array To be determined To be 1,000,000 determined a The collecting area listed in column 4 is the geometric area of the aperture for the dish-type telescopes.
From page 113...
... Using current spectrum allocations as well as many windows of opportunity, radio astronomers are able to learn fascinating information about the cosmos in which we live. Continuum and Line Observations Most radio astronomy observations fall into one of two categories: continuum observations and line observations.
From page 114...
... At high frequencies, especially above 100 GHz, broad bandwidths are often used in this application to encompass many spectral lines simultaneously. While broad bandwidths are often used at millimeter-wave frequencies to encompass many spectral lines simultaneously, recently there has also been renewed interest in making wide-bandwidth spectral scans at lower frequencies.9 Most of the frequency spectrum observed in these surveys has no protection against RFI.
From page 115...
... Manual of Regulations and Procedures for Federal Radio Frequency Management (Redbook) 10 and in the Federal Communications Commission's (FCC's)
From page 116...
... That will change in the future. Some of the bands at lower frequencies are more threatened by radio frequency interference than others.
From page 117...
... Pioneering efforts are underway using the 1420 MHz spectral line of hydrogen, to detect material that is heated by the first generation of stars in the early universe. That radiation now must be observed at much lower frequencies, owing to the large redshift.
From page 118...
... In addition, molecules in the 30-275 GHz frequency range provide essential diagnostics for star formation. The band from 65 to 115 GHz has relatively little absorption from the atmo sphere and is one of the best for both continuum and spectral observations.
From page 119...
... Hence the spectrum in this region is almost a "forest" of spectral lines in the direction of star-forming molecular clouds. Instruments such as ALMA will be able to image these regions with high angular resolution that will only be surpassed by infrared arrays in space, which are many decades from feasibility.
From page 120...
... Brightness tempera 12 S.S. Doeleman et al., "Event-Horizon-Scale Structure in the Supermassive Black Hole Candidate at the Galactic Centre," Nature, 455: 78-80 (September 2008)
From page 121...
... The radio power incident on the antenna is called the flux density and is usually denoted by Sf; it is also called the spectral power flux density or spectral pfd. Flux density is measured in janskys (Jy, named after the pioneer radio astronomer Karl Jansky)
From page 122...
... The usable bandwidth is often limited by RFI. Radio astronomers generally use very long integration times (hours or days)
From page 123...
... ; GMRT = Giant e uneditable bitmapped imag Metrewave Radio Telescope (India) ; EVLA = Expanded Very Large Array (New Mexico)
From page 124...
... The other factor in the noise calculation, the integration time, is assumed to be 2,000 seconds, whereas in modern practice the integration times often are 10 or 50 times longer. Again, if observations were limited to 2,000 seconds, much of radio astronomy, especially the new realms projected for the coming decade, would be impossible.
From page 125...
... Finding: Geographical separation of radio telescopes from transmitters (e.g., through the establishment of radio quiet zones and the remote siting of observa tories) is currently effective in avoiding much radio frequency interference, but the proliferation of airborne and satellite transmissions and the widespread deploy ment of mobile, low-power personal devices threaten even the most remote sites.
From page 126...
... When converted to standard radio astronomy units, the flux density during the short bursts is about 2,500 Jy. Image courtesy of CEPT and BNetzA.
From page 127...
... (Left) Image of a faint "OH/IR star" made in a narrow band at 1612.22 MHz, within the band 1610.6-1613.8 MHz that is allocated to the Radio Astronomy Service on a primary basis.
From page 128...
... Quiet Zones Around Observatories Only two observatories on U.S. soil benefit from Quiet Zone protection: NRAO (Green Bank, West Virginia)
From page 129...
... Local Radio Frequency Interference The NRAO engineering staff at Green Bank includes a team that tracks down instances of RFI that appear at the observatory. The team's equipment includes a portable interference system, which can trace interference originating within a few miles of the observatory.
From page 130...
... Triangulation then gives the direction of arrival of the radiation, meaning that the angular position of the radio source can be measured precisely. Furthermore, comparison of the arriving signals provides a method of imaging the source -- that is, of determining the angular structure of the emission, which reveals the struc ture and dynamics of the source.
From page 131...
... Communications Disruptions Energetic particles from the Sun, released in bursts called coronal mass ejections, arrive at Earth and can cause a disruption in radio communications, interference with GPS operation, surges on power grids, damage to Earth-orbiting satellites, and hazards to astronauts. The prediction of such events is important so that mea sures can be taken to ameliorate their effects.
From page 132...
... The measurement by radio astronomers of the timing of the rotations of pulsars in tight binary orbits about companion neutron stars, with exquisite preci sion, has been providing physicists with the strongest affirmative answer yet to the century-long question, "Was Einstein right? " (see Figure 3.3)
From page 133...
... They invented an electronic surveying technique, known colloquially as radio holography, which enables reflector surfaces to be set to an accuracy of a few microns. Methods that they developed for measuring antenna efficiency from observations of standard radio sources and solar system bodies are in wide use.
From page 134...
... This interest translates into an appreciation of science and technology, and draws students into technical subjects, helping to provide the personnel resources needed in today's world. Finding: In addition to the intellectual benefits that they provide, radio astronomy studies provide many technological benefits to American society.


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