Astronomy and Astrophysics for the 1980's, Volume 2 Reports of the Panels (1983) / Chapter Skim
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IV. Scientific Priorities
IV. Scientific Priorities
Pages 229-257
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From page 229... ...
The greatest use of Arecibo at present is in the observation of gas in distant galaxies; not only would the observable number be increased 2.5 times, but the red shifts that could be reached would be increased by 40 percent. The ability to detect radio spectral lines in very distant quasars would be greatly improved.
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From page 230... ...
Radio astronomy also pointed the way to quasars. These enigmatic sources have high red shifts and, if at the distances implied by their red shifts, their output of light and radio waves is enormous.
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From page 231... ...
me 3 K background is the thermal radiation left over from the compact and hot early Universe. The subsequent expansion has cooled the radiation (by red shifting wavelengths)
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From page 232... ...
Radio measurements of the 21-cm hydrogen line are particularly useful on large nearby galaxies, where the rotation curve can be followed out beyond the visible edge of the galaxy. The motion of hydrogen clouds around galaxies is another promising way
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From page 233... ...
Then gravitational forces overcame radiation drag, and matter condensed into clouds. Galaxy formation could have taken place at any red shift between z = 1000 (when the temperature dropped below 3000 K)
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From page 234... ...
234 allows hundreds of galaxies at large distances to be measured. Not only is the technique giving Hubble's constant, but it shows promise for answering old questions about the dynamics of galaxies in our neighborhood.
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From page 235... ...
That our Milky Way is a spiral system was first suggested over a century ago, but observational evidence supporting large-scale spiral structures has been obtained only in the second half of this century. Application of radio spectral-line techniques to the distribution of interstellar matter supplied the necessary information.
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From page 236... ...
m e enigmatic properties of galactic nuclei have been extensively studied at optical and at radio wavelengths. However, we lack detailed knowledge of the structure of the central energy source -- not only in normal galaxies such as our own but also in the active nuclei of radio galaxies and quasars.
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From page 237... ...
The significant data that come from radio studies often suffer from poor angular resolution, a property inherent in the relatively long wavelength of such electromagnetic radiation. However, interferometric techniques on a large scale such as with the VLA, or on an even larger scale with VLBI, yield a very high resolution, often better than at optical wavelengths.
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From page 238... ...
VLBI at radio wavelengths has shown that the nucleus is typically only a few tens of astronomical units in size. So far, these size measurements seem to be limited to scattering of the radio emission from surrounding clouds of ionized gas; VLBI measurements at wavelengths of 1 cm and shorter are under way to avoid the scattering effects and reveal the actual size and shape of the active region.
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From page 239... ...
In a similar way one can imagine that the superluminal velocities in quasars arise from moving "searchlight beams" of relativistic particles that only radiate when they encounter surrounding clouds of gas and magnetic fields. Many models have been proposed, but to test them requires very accurate, high-contrast maps from a dedicated VLBI array.
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From page 240... ...
Unforeseen obstacles to the search may be encountered, such as the absorption of their own radiation by the relativistic particles that generate the radio emission, but if history is a guide, every advance in angular resolution brings new and unexpected phenomena to view. When starlight passes the Sun, it is deflected by the Sun's gravitational field, and a similar phenomenon has now apparently been discovered on a cosmic scale.
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From page 241... ...
m e beams are well collimated, supporting large-scale ordering at the nuclear source, and terminate at the outer edges of the cavity, where the magnetic fields in the compressed IGM are higher. The nature of the IGM is still elusive, but its interaction with the ejecta of radio galaxies provides one of our few clues to its nature.
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From page 242... ...
At greater red shifts, the angular size corresponding to a given length should grow or at the very least should stay constant. Two classes of tests can be projected: the angular size of large double-lobed radio galaxies can be studied as a function of red shift, and an independent test can be performed in a similar way on the compact structure near the radio cores of quasars and galactic nuclei.
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From page 243... ...
D Interstellar Matter and Star Formation Every decade discoveries have been made in radio astronomy that have led to major advances in our understanding of interstellar matter and how stars are born and die.
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From page 244... ...
Important first steps have been made in answering these questions. It has been shown for example that cosmic rays probably play an important positive, not destructive, role in molecular synthesis, by producing molecular ions that react rapidly at low temperature.
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From page 245... ...
and so has been largely overlooked in past surveys of the Galaxy. One of the major accomplishments of molecular radio astronomy has been to reveal just how extensive and complex this new phase of the interstellar gas is.
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From page 246... ...
The spiral arm structure of the Galaxy, revealed first by the 21-cm emission from atomic hydrogen, shows even more sharply in the molecular gas. To a much greater extent than the atomic gas, the molecular gas lies in welldefined condensations, or "molecular clouds." The largest molecular clouds are formidable objects, containing 100,000 solar masses or more and stretching for hundreds of light-years along the Galactic plane.
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From page 247... ...
Radio studies continue to expand our understanding of the interstellar medium. Early observations of the polarized low-frequency radio emission from the Milky Way demonstrated the pervasive character of the cosmic-rays and magnetic fields in the Galaxy.
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From page 248... ...
To elucidate this view, the crucial radio studies in the 1980's will be very sensitive hydrogen absorption observations and accurate pulsar distance determinations to find the electron density. From a technological standpoint, the discovery of interstellar molecules has been part of one of the major themes of radio astronomy and indeed radio science -- an advance to ever higher frequencies.
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From page 249... ...
For some time now, our understanding of the life cycles of stars has been generally satisfactory, except for the earliest and latest phases: star birth and star death. As described elsewhere in this chapter, star formation takes place in the cold, neutral gas clouds of interstellar space, and radio observations of the more abundant molecular species are very effective probes of this intricate process.
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From page 250... ...
Several major new classes of stellar phenomena were first recognized and widely studied with radio techniques during the 1970's, including the expanding shells around stars that recently have undergone nova outbursts, shells of relatively cool gas and dust in circumstellar envelopes around red giant stars, mass exchange processes between members of close binary pairs, steady mass outflow from highly luminous, isolated stars of different types, and pulsars or neutron stars, thought to be remnants of supernova explosions. Much of this work involves the ways in which stars shed substantial fractions of their mass before collapsing to form white dwarfs or neutron stars.
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From page 251... ...
As the x rays are sensitive to the temperature and emitting volume and the radio emission to magnetic fields, the combination can probe the stellar coronas in great detail. Such information is necessary to understand how much the coronas affect the stellar winds and ultimately to understand the origin of magnetic activity in stars.
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From page 252... ...
Theoretical advances over the past decade have emphasized the importance of comparing the calculated masses of neutron stars with observations and the possibility of determining the structure of neutron stars from observations of pulsar periods over extended intervals of time. Radio-frequency timing measurements, and recently hard x-ray spectral observations, have shows that pulsars possess huge magnetic fields (up to 101 times that of the Earth)
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From page 253... ...
F The Sun Over the past decade, solar radio astronomy made many advances: the detection of coronal holes, the observation of very small structures associated with the primary energy release in solar flares, and both observational and theoretical advances in the understanding of the complex plasma processes caused by electron streams moving from the Sun through interplanetary space.
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From page 254... ...
As an example, the most extensively studied problem in astrophysical plasma physics is probably the Type III solar radio burst. This common type of radio emission is caused by a stream of electrons moving from the inner corona to the orbit of the Earth and beyond.
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From page 255... ...
Further progress in understanding solar flares will require very high spatial and temporal resolution at several radio wavelengths, accurate polarization measurements, concurrent hard x-ray data with high spatial resolution, and better theory. Before and after flares we need accurate maps in circular polarization to determine the strength and structure of the coronal magnetic field.
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From page 256... ...
Detailed measurements of the shock parameters are just becoming possible: particle densities, field variations, electron distribution functions, plasma wave intensities, and kilometer-wavelength radio emissions. From these measurements we can expect much better theories of the structure of collisionless shocks, particle acceleration within them, and the nonlinear wave-particle interactions that give rise to the varied phenomena.
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From page 257... ...
Except at centimeter wavelengths, most of the studies have not been made with enough angular resolution to distinguish spatial features. Nevertheless, large-scale properties such as the cloud-top pressure in the Jovian atmosphere and the CO2 partial pressure at the surface of Venus have been deduced.
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