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10. What Next?

10.1. Prospects for the Development of Experiment Cold
 
The deep surveys at the RATAN-600 will undoubtedly continue. Although the material already obtained between 1980 and 1982 has not yet been completely reduced (it still remains to compile a catalog of all the weak radio sources, obtain estimates of the spectra √ there are several hundred sources in common at 3.9, 7.6, and 31 cm √ determine how variable the sources are, and carry out a complete optical identification using the Palomar Sky Survey and, selectively, using the 6-m telescope), it is already clear that the potential of the RATAN-600 is far from being exhausted. The novelty of the situation stuns observers who had become accustomed to the idea that the centimeter sky was empty and that it was necessary to search for objects to study. Instead, the sky has turned out to be literally "filled" with radio sources. Under the conditions of Experiment Cold, they occupy about 20% of the sky, and extended objects and background emission must be studied by "peeking through the gaps" between the numerous sources.

In Experiment Cold, we reached a level of 30 mK in brightness temperature when studying the anisotropy of the 3-degree background. Today's theories require a precision of 1 mK (the so-called very-small-fluctuation variant of the theory of entropy perturbations). Constructing a radio image of the Universe at the hydrogen recombination epoch (z ~ 1000) is a basic goal of modern natural science. This information not only allows one to trace out how all of the structural forms in the Universe which are observed by mankind were formed; but also to gain access to the very early stages in the evolution of the Universe by approaching right up to the singularity. Through this information, problems in cosmology and elementary particle physics turn out to be related to one another. The 3-degree background and the Grand Unification Theories are undoubtedly the clearest example of mankind's use of the cosmos as a "laboratory apparatus" which accelerates particles to 10¹⁹ GeV.

So, let us see how far away a limit of 1 mK is from us. We shall list the set of tasks which we plan to carry out in order to realize the next stage in the deep surveys:
 

(1) Further improvement of the radiometer. It is possible to reduce the noise temperature at the input by a factor of 2.5 and further improve the sensitivity by a factor of 1.4 √ 2 by increasing the receiver bandwidth and using a design with two or more channels;

(2) Further "cooling" of the radio telescope, i.e., protecting the primary focus from the direct penetration of radiation from the ground by using anti-noise screens in order to decrease the contribution of ground radiation to the total noise temperature by several degrees;

(3) Increasing the integration time to 1 year will become practicable after the work on automating the radio telescope is completed over the next two to three years;

(4) Improving the algorithms for removing interference due to terrestrial (and extraterrestrial) civilization, the atmosphere, and faint discrete sources from the data;

(5) Our experience from Experiment Cold shows that the search for small-scale fluctuations is complicated by confusion noise √ it is necessary to either go to a shorter wavelength (a wavelength of 2.6 cm is planned) or get around the confusion noise problem via a sharp increase in the resolving power at 7.6 cm using two-dimensional synthesis of the region of the sky being studied. The reader may find a description of the three alternative synthesis methods ("azimuthal", "circumpolar", and "circumzenithal") in the monograph by Esepkina, Korol'kov, and Pariiskii (1973).

What is necessary for a sharp increase in the amount of data on intrinsically faint radio sources? According to the estimates of Syunyaev and Longair, this should occur for flux density sensitivity higher than 100 mJy. Then, the steep downward slope in the log N √ log P curve should be replaced by a new, rapid upturn at faint fluxes because of the dominant role of low-luminosity objects (the appearance of a "new population"). The maximum flux density sensitivity is achieved when using the entire useful surface of the RATAN-600 radio telescope, i.e., in the so-called "zenith" mode (circumzenithal synthesis, see Section 2.1). In this mode, the RATAN-600 is a coherently operating section of a paraboloid 600 m in diameter. We remind the reader that the height of the elements of the RATAN-600 is 7.4 m, and the circumference is 1.80 km. Thus, the physical area of the surface is 9000 m'. The "anti-noise" screens increase this area to 14,000 m' at wavelengths longer than 6cm, which makes the physical area of the RATAN-600 similar to that of the giant dish at Areceibo.

So, the RATAN-600 has a large collecting area in the "zenith" mode. Background sources play a reduced role in this mode because of the high resolution; for the same reason, the atmosphere can filter out well using a double-beam receiving mode. The major (and not very simple) problem will be obtaining a long integration time. A radio source will remain within the field of view of the radio telescope for approximately 1 h, and it is necessary to make full use of this hour. Thus, when all of the measures listed above are completed, we expect an increase of a factor of 5 √ 6 in collecting area relative to that in Experiment Cold, and we hope to increase the sensitivity of the radio-meters by a factor of 2 √ 3, and the integration time by a factor of 15 √ 20, so that the sensitivity to the flux density of sources can be improved by at least a factor of 60 in the "zenith" mode.

On completion of the work (which has already started) towards improving the surface of the RATAN, about 9000 m² of high-accuracy surface (with a surface error of approximately 0.1 mm) will be available, and we look forward to opening up the millimeter region with the very high capabilities of the radio telescope. The prospects described here are breathtaking. Indeed, in the time which was allotted to Experiment Cold-1 (about 100 days of observations, ~ 65 of which were used), using the new-generation radiometers (for which the noise will be determined mainly by the sky temperature) and after the automatic tracking mode is in operation, it will be possible to reach a flux density sensitivity of order 10 mJy over a wide wavelength range from 10cm to 1.5mm, but, naturally, over a limited region of the sky. The maximum sensitivity in this case will be obtained near a wavelength of 6 √ 8 cm. This is connected with the fact that for the RATAN, as for any real telescope, there exists an optimum in sensitivity, since the fluctuations due to unresolved point sources increase almost as l³ toward longer wavelengths, and fluctuations of atmospheric origin increase almost as  l^-2 toward shorter wavelengths (shorter than 6cm). This leads us to the concept of a "limiting" sensitivity √ the extremely faint objects detected on the RATAN in this deep noise minimum cannot be detected at any other means over the entire region of the electromagnetic spectrum from fractions of a millimeter to kilometer wavelengths. This pertains to all objects which have a standard non-thermal spectrum, a flat spectrum, an inverse spectrum, or a blackbody spectrum, independent of the nature of the objects being studied.

 
10.2. Experiment Cold and the "New-Generation" Radio Astronomy
 

In this section, we will look ahead somewhat √ how radio observations like Experiment Cold, using modern radiometers similar to the 7.6 cm cooled radiometer described here, and large radio telescopes like the RATAN-600 and the VLA, may change the face of radio astronomy.

As is well-known, radio astronomy is no longer the youngest branch of astronomy: infrared, X-ray, gamma-ray, neutrino, gravitational and even "acoustical" astronomy (low-frequency noise in the circumsolar plasma) were developed later than radio astronomy. However, in terms of rate of development, it still occupies one of the leading places; only X-ray astronomy is developing more rapidly today. This rapid development is, on the one hand, caused by progress in the techniques for receiving radio signals, and, on the other hand, by the unique role of radio wavelengths in studying the Universe. We remind the reader that such basic trends in modern natural science as

-the history of the formation of the Universe which surrounds us (very distant objects, the "relic" background of the Universe);

-the problem of the phenomenon of quasars and the nuclear activity of galaxies;

-the initial and final stages of stellar evolution are directly and immediately connected with radio astronomy, just as before.

Thus, the advent of very long baseline interferometry allowed one to think of developing "three-dimensional" radio astronomy √ an extrapolation of the capabilities of the new methods to solar system scales leads to the fact that practically all of the observable Universe is in the near field of a multi-element interferometer:

We shall dwell for a moment here on another curious leap in the capabilities of radio astronomy: the development of the actual capability of seeing radio sources at any distance from the observer, using a sensitivity similar to that attained in Experiment Cold.

As is well-known, the further an object is from us, the greater its red shift. Observing an object at a fixed wavelength  l₀, we record the radiation which it emitted at a wavelength it l₀(1+z)^-1. Therefore, it is necessary to take into account not only the curvature of space, but also the characteristics of the spectrum of the emission from the object.

For nearby objects, (z<= 1), these corrections are small, and the observed flux density

 

For z > 1, the situation changes: it is necessary to take both the metric of the Universe and the nature of the object's spectrum into account. However, the assumption that  which simplifies calculations a great deal, has been used in all previous work. In this case, as is well-known, the correction to the energy and number of photons (the K-correction) E = (1 +z)^1-a is equal to unity. As observations in the last decade have shown, these simplifications are, first of all, incorrect for some classes of objects (sometimes a = √ 2.5, rather than +1). Secondly, for z >> 1, it is necessary to take the "infrared peak" observed in the spectra of many objects in the 1970's into account. Although the measurements in the region around the peak l  100 mm are still insufficiently reliable, the fact that the phenomenon is real in many objects is not open to question. A summary of the results of the 1970's for such different objects as quasars, radio galaxies, Seyfert galaxies, H II regions, and the nucleus of our Galaxy is given in Kaplan and Pikel'ner (1979). In some cases, the IR spectral flux density exceeds a simple extrapolation of the radio data by 5 √ 6 orders of magnitude. One example of this type of object √ galaxies with a high dust content √ is shown in Fig. 10.1.

Let us now carry out the following thought experiment: we shall take a particular object with a well-studied spectrum and move it way from
 

 us, placing it at successively larger values of the red shift z (i.e., further away from us). Using the well-known analogy between the Universe and a black hole, one can visualize the experiment in another way. We pick up an "object" (for example, a quasar), let it go, and it will

A completely diferent situation occurs for an object with a sharply defined infrared excess. The function P(z) for two such objects is shown in Fig. 10.3 (the quasar 3C273) and Fig. 10.4 (NGC 4151). It is evident that for large z, the K-correction not only dominates over the effects of the metric, but also over the law P  R^-2, where R is the photometric distance to the object. Although for small z, the flux density falls off like z² (similar to the case for Cygnus-A, Fig. 10.2), for z > 1, this falling oft slows down, and then an increase in the flux density is even observed!

 
Of course, this is connected with the obvious fact that the radio telescope begins to record the emission around the infrared peak, where the spectral flux density is large. Since the flux density increases more rapidly than v²(closer to v³) when approaching the peak from the radio region the K-correction in fact increases faster than z². Therefore, after z > 3, a Seyfert galaxy like NGC 4151 begins to increase its monochromatic brightness as it becomes further away from the observer. NGC 4151 is a relatively weak radio source. If it is placed at a distance z = 0.1, it is already below the level attained in Experiment Cold (1 mJy = 10^-29 watt/m² Hz). However, for z > 30, it can once again be detected using the RATAN-600 under the conditions of Experiment Cold (P ~ 1mJy), and remains so up to the recombination epoch! Beyond z = 1000, electromagnetic waves of any wavelength experience absorption. The ability for mankind to see discrete objects in the Universe ends at this point. However, the ability to observe a class of objects which exists nearby at any distance, all the way up to the recombination epoch, with the sensitivity already achieved is amazing in itself. We note that this ability only exists in radio astronomy √ in all other regions of the spectrum, the K-correction does not compensate for the decrease in flux density with distance. This characteristic of radio astronomy can be traced in Fig. 10.5, which is from Meyer and Syunyaev (1979) and is based on the calculated emission from first-generation objects within the framework of the standard evolutionary cosmology. Thus, for observations in the central portion of the optical region, when a source is at a distance z ~ 5, its flux density falls practically to "0", i.e., the K-correction is almost zero.

Figure 10.5 The calculated spectrum of the emission from first-generation objects (Mayer and Syunyaev, 1981).

 
Thus, modern radio astronomy can detect sources of radio emission at any distance in the Universe, if objects with a spectrum like NGC 4151 exist at these distances. Standard cosmology does not guarantee this: such objects should appear at z < 5√100. However versions with early formation of stars and galaxies, and, consequently, objects with infrared peaks, which require heavy elements (which come from stars during their evolution) do exist. The extremely high degree of isotropy in the 3-degree background forces one to pay more attention to versions with early galaxy formation. In this case, one can hope to see extremely distant objects with infrared peaks using radio telescopes.

Such a program has been begun in a joint experiment between the USSR and the FRG. Extremely deep surveys with the RATAN-600 allow a rather large number of radio sources with flux density greater than 1 m Jy to be selected, while the joint efforts of the RATAN-600 and the 100 m paraboloid in EfTelsberg (FRG) make it possible to estimate the spectra of these objects. Objects with a maximum in their spectrum between 4 and 11 cm could either be infrared sources shifted into the radio region or ordinary sources of synchrotron emission, with self-absorption. The surface brightness of the objects can serve as a check, since for the first case, one would expect a brightness temperature ~ 1 K, and for the second case √ 10¹⁰ K (the "Compton limit"). It is possible to show that the relativistic corrections do not change this difterence by more than a few orders of magnitude, and it is completely possible to distinguish between these classes of objects in practice. Determining the frequency of the infrared peak also allows the problem of distances in radio astronomy to be solved.

The high sensitivity of present-day radio telescopes (higher than 0.3 mJy in the USSR, and 0.1 mJy in the U.S.) not only increases the penetrating power to the "limit" imposed by the recombination epoch, but also allows us to hope for the development of a "standard candle" in radio astronomy.

We shall dwell on this in more detail:

As von Hoerner (1976) showed, the radio source luminosity function f(L) (i.e., the number of radio sources of a given luminosity per unit volume) not only covers an unusually wide range in luminosity (8 orders of magnitude), but also has a "dangerous" slope: F(L)  L^-3/2. The "danger" in this slope consists of the fact that in an Euclidean universe, this means that a source with a given flux density may be at any distance from the observer with equal probability.

The ideal case is a luminosity function F(L) = d(L √ L₀), i.e., a "monochromatic" luminosity function. In this case, knowing the luminosity of the "standard candle", we are sure that a source with flux density P is at a distance given by the expression

Evidently, it is precisely this circumstance [the dangerous slope in the actual F(L)] which makes it difficult for radio astronomy to see clusters  of radio sources, since integrating the inhomogeneities in the radio source distribution along the line of sight smooths out inhomogeneities. This effect is significantly weaker in the optical.

We shall now show that with a sensitivity higher than 1 mJy, the "radio Copernican Universe" can be transformed into a "radio Ptolemaic Universe". If we adopt the orthodox evolutionary Universe, where the galaxies form from protoclusters at z < 5 √ 30, then, in contrast to the preceding case of "infrared radio astronomy", there must be a period of wholesale formation of radio sources at  z = 5 √ 10. Before this period, there were none at all. This means that even very faint galaxies must be nearer than this limit. This places limitations on the luminosity function: it must be "truncated" at high luminosity. Thus, by observing fainter and fainter objects, we keep shrinking the dispersion in the luminosities of the sources which produce the flux density observed by us. It is possible to show that for P  0.1 mJy, there will already be almost no quasars or radio galaxies of typical luminosity; the luminosity function becomes "monochromatic", leading us to a radio Ptolemaic Universe. This phenomenon has evidently been observed in recent American surveys.

In conclusion, it is necessary to rehabilitate optical astronomy. For the present, the sky is filled with optical objects, and not radio sources (or X-ray and gamma-ray sources). The basic quantitative information (and, above all, the distance) has thus far been obtained by optical means. A person (without a telescope) sees a sky studded with stars. Not one radio telescope on the Earth has yet been able to detect the radio emission of a single normal (solar-type) star.

It is interesting to trace the dynamics of the change in the penetrating capability of optical and radio astronomy. In optical astronomy, the penetrating power has changed with time in a more or less "natural" way: the more powerful the means, the more distant the objects one can observe: first, stars, then galaxies, and, finally, quasars; i.e., the most distant objects in the Universe. We observe the opposite in radio astronomy. The first thousand discrete radio sources turned out to be so distant that only a few percent of them were successfully identified by 1958 √ the penetrating power of optical telescopes turned out to be insufficient. Since then, the sensitivity of radio telescopes has increased by a factor of 10,000, and the nearer objects (of lower luminosity) have become accessible to radio astronomy. A typical representative of the first generation (1940s) of radio sources is Cygnus A, which is 100 million parsecs away from us, and not until 10 years later was the emission from the Andromeda Nebula, at a distance of 1 Mpc, observed. In order to observe stars in massive numbers (in the same number as a person sees with the naked eye) the sensitivity of present-day radio telescopes must be increased by another factor of 1000 (to 10^-32 watts/m² Hz)!

Acknowledgements

What was presented above sums up a stage in the life of the RATAN which occupied several years, and, naturally, the authors were only partners in a large group. Here, in conclusion, we would like to recall the main sections of the work, each of which was an integral part of Experiment Cold, and list the participants. A large number of staff members of the Special Astrophysical Observatory of the Academy of Sciences and several other institutes took part in preparing for and carrying out Experiment Cold. The development of the SHF cooled amplifier and cooled radiometer √ the basis of the experiment √ was carried out by L. G. Gassanov, V. I. Lebed', and Ya. V. Yaremenko, as well as A. B. Berlin, Y. Ya. Gol'nev, N. A. Nizhel'skii, and G. E. Spangenberg; the latter four also ensured that the complex receiver system worked during the observations. The development and maintenance of the microcryogenic system was carried out under the leadership of L. L. Shtein and E. D. Baranov.

B. M. Lebed', V. D. Voronkov, E. D. Krameshchenko, L. O. Kopteva, U. K. Kunevich, R. I. Vergasov, and their colleagues took part in the construction of the SHF components for the radiometers with which the observations were carried out.

The measurements of the antenna parameters and the experiments in "cooling" it were carried out by G. M. Timofeeva, and the calculations by E. K. Maiorova and A. A. Stotskii. The obervations themselves, which lasted for many months around the clock were carried out by the group of RATAN-600 observers under the supervision of M. G. Mingaliev. Finally, the mathematical treatment of the enormous data set is being carried out by V. V. Vitkovskii, V. K. Kononov, and V. I. Kateneva.

In conclusion, the authors should thank V. M. Proleiko for assistance in the development of the cryo-electronic instruments and in understanding the important role of electronics (cryoelectronics, as well as digitial electronics) for the RATAN-600, and L. G. Gassanov, P. P. Kurbatskii and their co-workers for their assistance in equipping the RATAN with modern receivers. The technical provisions for the experiment were organized by the Operating Department of the RATAN-600; the authors thank V. A. Kapranov and his co-workers for the successful organization of this work. The contribution of Yu. N. Konovalov and A. N. Dudoladov to the fine alignment and mechanical work is priceless.

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