Astron. Nachr./AN 323 (2002) 6, 530–533

The development of radio astronomy

W. REICH and R. WIELEBINSKI

Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53121 Bonn, Germany

Received 2002 May 28; accepted 2002 October 2

Abstract. Following the detection of extraterrestrial radio waves in 1932 by Karl Jansky, radio astronomy developed quicklyafter World War II. It established itself soon as a new branch of astronomy with today’s outstanding record in the detectionof new phenomena in space. These have been honoured by a number of Nobel prizes. Radio astronomy largely depends ontechnical developments in receiver technology, antenna systems, electronics and computing power. Ever shorter wavelengthsdown to the submm-wavelength range became accessible, resulting in new exciting discoveries. However, now and in futurecare must be taken, in particular for the lower frequency range, of harmful man-made interferences, which might maskthe weak signals from space. New international facilities with orders-of-magnitude higher sensitivity like ALMA and SKAare planned or under construction. Space-borne observatories like PLANCK will detect weak fluctuations of the cosmicmicrowave background, which will constrain cosmological models with an unprecedented accuracy.

Key words: radio astronomy – history

1. The thirties and forties

Starting in 1932, Karl Jansky, an engineer of the Bell Tele-phone Laboratories, investigated transcontinental radio com-munications at 20.5 MHz (14.6-m wavelength) with his rota-ting antenna. After a few years of observations he concludedthat a ‘static’ radio signal from the Milky Way close to theGalactic Centre exists. Although these unexplained observa-tions interested the daily press and Jansky gave some lecturesto engineers, the astronomical community did not react. OnlyGrote Reber, an ‘amateur’ radio astronomer, observed withhis 9.4-m backyard antenna and constructed the first map ofthe sky at 162 MHz (1.9-m wavelength) with an angular reso-lution of ��Æ. This result was published in 1944, after a four-year delay due to the lack of referees (Reber 1944).

Radio emission from the Sun was detected in 1942 byJ.S. Hey in England and independently by G.C. Southworthin the United States. The publication of both these results wasdelayed for reasons of military secrecy.

From that time on, radio astronomy was an issue forthe astronomical community, and professional astronomersstarted observations. In 1949, Bolton and co-workers (Boltonet al. 1949) optically identified the exceptional strong andcompact radio sources Virgo A and Centaurus A as ‘radiogalaxies’. They also identified another source, Taurus A, withthe Crab nebula, the remnant of a supernova explosion seenin 1054. A rather important technical step was made in 1948

Correspondence to: wreich@mpifr-bonn.mpg.de

by Ryle and Smith by inventing the two-element interferom-eter (Ryle & Smith 1948), thus being able to increase sub-stantially the inherently low angular resolution of single-dishantennas and to identify discrete sources within the dominat-ing diffuse large-scale emission from the Galaxy.

The book The Early Years of Radio Astronomy – Reflec-tions Fifty Years after Jansky’s Discovery (Sullivan 1984)presents details of the pioneering time of radio astronomy.Unfortunately, it does not contain a contribution on the Ger-man history, which develops later than in other countries dueto restrictions after the war.

2. The fifties

In spite of well settled observational results of the intensitydistribution and the spectrum of the galactic radio contin-uum emission in the meter and decimeter wavelength range,the emission process was still under discussion. Explanationsby ‘radio stars’ or thermal free-free emission failed. In 1950,K.O. Kiepenheuer proposed that synchrotron radiation is ableto explain the observations (Kiepenheuer 1950). The finalproof that this explanation was correct was made by obser-vations of linearly polarized emission, which characterisessynchrotron emission.

A very important discovery was the detection of a spectralline from neutral hydrogen by Ewen & Purcell (1951). ThisH I line is observed at 1.42 GHz (21 cm) and had been pre-dicted theoretically by Henk van de Hulst in 1944. Neutral

c�2002 WILEY-VCH Verlag Berlin GmbH & Co. KGaA, Weinheim 0004-6337/02/0612-0530 $ 17.50+.50/0

W. Reich and R. Wielebinski: The development of radio astronomy 531

hydrogen is the most abundant component of the interstel-lar medium. Subsequent surveys of H I emission revealed thespiral structure of the Milky Way from Doppler shifted lines,and present-day sensitivities permit rotation curve observa-tions of distant spiral galaxies.

Instrumental developments resulted in the constructionof various interferometers aiming at increasing the angularresolution of radio observations. Furthermore, fully steerableprecise single-dish telescopes were completed. In 1956, theStockert 25-m telescope of Bonn University started observa-tions. In 1957, the 76-m telescope located at Jodrell Bank wascompleted, which was the largest radio telescope at that timeand is fully operational even today.

3. The sixties

Outstanding discoveries in the sixties were the identifica-tion of ‘Quasars’, standing for quasi-stellar radio sources.The prototype source is 3C273, identified by Schmidt (1963).These very compact objects turned out to be highly redshiftedcores of galaxies with an extremely high luminosity. Cosmo-logical investigations became possible by studying these dis-tant sources from the early universe.

A Nobel Prize was given to Ryle and Hewish in 1974 forthe invention of aperture synthesis and for the discovery ofpulsars (Ryle & Hewish 1960, Hewish et al. 1968). Pulsarsare very fast and extremely precise rotating neutron stars be-ing remnants of supernova explosions of massive stars in theMilky Way. The aperture synthesis telescope is a techniqueto calculate radio images of high angular resolution. The size(= angular extent) of these images is given by the beamsizeof the individual telescopes of the arrays.

An observational result of very high cosmological rele-vance was the discovery of the 3 K background radiation byPenzias & Wilson (1965), which is the afterglow from the‘big bang’ observed today. Fluctuations in this 3 K back-ground are one of the main topics of current research. Thesevery faint fluctuations reflect details of the evolution of theearly Universe. Penzias and Wilson were honoured for thisdiscovery in 1978 by the Nobel Prize.

4. The seventies

Progress in receiver technology made observations at cen-timeter and millimeter wavelengths possible. New telescopeswere constructed with higher surface accuracies than wereneeded for meter and decimeter-wavelength observations.These telescopes had to be located on high mountains be-cause of the decreasing transparency of the atmosphere to-wards shorter wavelengths (see Fig. 1).

In 1963, the hydroxyl molecule (OH) was detected at18 cm, followed by the detection of ammonia (���) at1.3 cm and water (���) at 1.4 cm in 1968. Formaldehyd(����) was observed at 6.2 cm one year later. At the be-ginning of the seventies interstellar chemistry became a newand exciting branch of radio astronomy when a large numberof spectral lines from different molecules were discovered. In

1970, carbon monoxide (CO) was detected at 2.6-mm wave-length, which subsequently became the most important tracerof the molecular component of the interstellar medium, andlarge-scale surveys were carried out. From 1970 onward, thenew Kitt Peak 12-m telescope was particularly successful indetecting molecules in the millimeter wavelength range. Intotal, 7 new molecules were detected in 1970, 10 more in1972, and the detection of increasingly complex moleculesand additional transitions of already known molecules andtheir isotopes continues until now.

A remarkable increase in sensitivity was also achieved fordecimeter and centimeter wavelengths. In 1971, the GermanEffelsberg 100-m telescope, located in the Eifel mountainsand operated by the Max-Planck-Institut fur Radioastronomiein Bonn was put into operation. Until 2001, it was the largestfully steerable single-dish telescope in the world and wasonly recently succeeded by the slightly larger Green-Bank-Telescope in the USA. The Effelsberg telescope realized forthe first time the concept of ‘homology’, where the gravita-tional deformation of the dish was always kept in a parabolicshape at all elevations. This principle allows a ‘light’ con-struction with about 3200 tons, which is about three timeslower as compared to classical telescopes. The Effelsbergtelescope was constructed to operate at a shortest wavelengthof about 3 cm, but because of permanent improvements, it isnow used even at 3 mm wavelength.

Important new synthesis telescopes were added in theseventies as well. In the Netherlands, the Westerbork tele-scope with ��� ��-m dishes started operations, and in Cam-bridge/GB a telescope of ����-m dishes. Both could be usedat a shortest wavelength of 6 cm, at which an angular resolu-tion of a few arcseconds was achieved.

In the 1970s, much progress was achieved based on thenew observational possibilities. Without claiming complete-ness, we list some of them in the following:

1. The technique of Very Long Baseline Interferometry(VLBI) starts to become a standard observational method.A highlight of VLBI measurements was the detectionof apparent superluminal motions, a relativistic effect ofcomponents in the cores of some extragalactic sources(e.g. Cohen et al. 1971).

2. By monitoring the binary pulsar 1913+16, which was de-tected by Hulse & Taylor (1975), the effect of gravita-tional wave emission was measured (Taylor et al. 1976).This finding was honoured in 1978 by the Nobel Prize.

3. Multiple images of some very distant extragalacticsources led to the discovery of gravitational lenses causedby massive galaxies located in the foreground (Walsh etal. 1979).

4. Radio galaxies with lobes ranging into the Mpc-rangewere identified by Willis et al. (1974). This class com-prises the largest objects detected so far in the Universe.

5. Sensitivity improvements in measuring linear polariza-tion made studies of the magnetic field structure in nor-mal nearby galaxies possible. Mathewson et al. (1972)presented a first map of the face-on galaxy M51 madewith the Westerbork telescope, and more results for othergalaxies were added quickly.

532 Astron. Nachr./AN 323 (2002) 6

Fig. 1. The radio window in perspective (taken from Kraus, 1966). The direction of technical developments allowed to extend measure-ments from m-wavelengths towards cm and mm-wavelengths. The mm-wave observations need high-altitude observatories to minimizeatmospheric absorption. Commercial use of the radio spectrum develops – although delayed in time – towards cm and mm-wavelengths aswell.

5. The eighties until now

Radio astronomy meanwhile is a well established branch ofastronomy with hundreds of published papers every year. Itis difficult to give a fairly complete list of highlights, wherea weighting of the more recent results is not always possible.What can be clearly stated is that significant progress is beingmade in nearly all fields of research:

1. Galactic radio emission: Sensitive surveys of the conti-nuum, spectral line and polarized emission were made,and also systematic searches with many new detectionsand subsequent follow-up work of pulsars and supernovaremnants. Detailed investigations of star-forming regionsand cold dark clouds were done by tracing their physicalproperties by observing various transitions of molecules.Some lines were identified as masers as well indicatingextreme physical conditions. A new picture of the con-stituents of the interstellar medium emerged. The weakradio emission from various classes of stars is being in-vestigated, as well as its spectral and temporal variations.High-resolution and high-frequency studies of the Galac-tic Centre region revealed unique properties of its emis-sion components and unusual, very strong magnetic fieldstructures.

2. Studies of all kind of galaxies revealing their continuum,linear polarization, H I, CO and emission from othermolecules were made for a large number of objects us-ing synthesis and single-dish telescopes. These data giveinput to models describing the evolution of the global pro-perties of galaxies.

3. Details of radio galaxies, quasars and active galactic nu-clei were collected, improving the theoretical understan-ding of their physics. Numerous clusters of galaxies werestudied, where in particular the technically difficult obser-vations of the Sunyaev-Zeldovich effect, the absorption of

the 3 K radiation of the cosmic microwave background byhot cluster gas, received much interest.

4. Recently, studies of fluctuations of the cosmic microwavebackground gave first results and proved that theoreticalpredictions of cosmology can be tested observationally.

A long list of technical developments and new observingfacilities needs to be added:

1. New synthesis telescopes with unprecedented sensitivityand angular resolution were completed: the Very LargeArray (VLA) in New Mexico, USA consisting of ����-m antennas and the Australian telescope with � � ��-mdishes, which gives access to sources in the southern sky.

2. Large mm-wave telescopes were opened: the 45-mNobeyama telescope in Japan, the IRAM 30-m telescopenear Granada, Spain, and the SEST 15-m telescope lo-cated in Chile. Synthesis telescopes in the same wave-length range were operated in Nobeyama and by IRAMon the Plateau de Bure, France.

3. High altitude sub-mm-telescopes observing through theatmospheric windows (see Fig. 1) started their obser-vations. We just mention three large telescopes of thatclass: the German/US 10-m Heinrich-Hertz-Telescope onMt. Graham, USA at 3200 m, the British/Canadian 15-mJames-Clerk-Maxwell-Telescope located on Mauna Kea,Hawaii at 4000 m altitude, where also the 10.4-m CSOtelescope (Caltech) is located. The high telescope surfaceaccuracy required for sub-mm-observations forces to usenew materials (like carbon fiber) with little deformationdue to temperature changes.

4. The COBE satellite produced sensitive all-sky maps inthe mm and sub-mm-wavelength range, which in spite oftheir coarse angular resolution of several degrees, gavethe input data for studies of the cosmic microwave fluctu-ations.

W. Reich and R. Wielebinski: The development of radio astronomy 533

5. Low noise receiver technology developments at mm-wavelength and bolometer arrays provided adequate sen-sitivity for sub-mm-wavelength observations. Wide-bandspectrometers with different technologies, digital multi-channel receiver backends, correlator backends and datarecording systems with very high time resolution im-proved the observing capabilities at most telescopes.

6. VLBI observations were organized in various networks,which at present observe down to a shortest wavelength of3 mm, where angular resolutions of typically 0.1 milliarc-seconds were realized for transatlantic baselines. Sub-stantially increased bandwidths allow to study mJy ra-dio sources today. The inclusion of the 8 m antenna ofthe Japanese HALCA satellite for VLBI observations in-creased the angular resolution by realizing baselines ofseveral earth diameters also for dm and cm-wavelengthobservations.

6. Coming next

Upgrades are planned for nearly all operating larger facilitiesto include recent technical developments. However, it has tobe realized that some effort is needed to preserve the sensitiv-ity already achieved. The direction of technical developmentas explained in Fig. 1 points towards shorter wavelengths,which subsequently became accessible for radio astronomi-cal investigations. Commercial use of the radio spectrumpoints in the same direction, although it is significantly de-layed in time. International regulations protect certain wave-length ranges as passive bands, where all transmissions areprohibited. This is needed for radio astronomy and other pas-sive services like remote sensing. However, these bands arelimited and special backend developments are required to fil-ter out man-made interference. Although some success is al-ready reported on that issue, strong transmitters, in particu-lar from space, must be restricted to their allocated bands.So-called ‘spurious’ or ‘out-of-band’ emission showing up inadjacent wavelengths must be extremely well suppressed inview of present sensitivities being achieved by modern radiotelescopes.

Based on the outstanding scientific results as sketchedabove the radio astronomical community is heading for futurefacilities aiming to open new fields of research. All these newobservatories are planned as international facilities due to fi-nancial requirements and we list the most important projects:

1. Already approved is the Atacama Large Millimeter Array(ALMA), which is in the course of construction by anUS/European consortium with an expected participation

of Japan in the near future. It will be a synthesis telescopeconsisting of � � ��-m telescopes for mm and also forsub-mm-observations and will be placed in the extremelydry Atacama desert (Chile) at an altitude of 5000 m.

2. Another project with a large number of participating in-stitutions around the world is the Square Kilometer Ar-ray (SKA), an ambitious project to construct a telescopefor dm and cm-wavelength observations with a collectingarea of � ���. Such a telescope will improve the sensi-tivity of existing facilities by about two orders of magni-tude. Various concepts in different countries exist for itsrealization and are presently under intensive debate. Nu-merous prototype developments were already started.

3. Two space projects, the already observing US-AmericanMAP and in particular the European PLANCK satellite,to be launched in 2007, are expected to improve all pre-vious attempts of precision cosmology by measuring thefluctuations of the 3 K cosmic microwave backgroundwith unprecedented accuracy and by including linear po-larization. Observations of the entire sky will be made atmm and sub-mm-wavelength and are expected to decideon the various already existing models of the Universe, itsearly evolution and its future development.

References

Bolton, J.G., Stanley, G.J., Slee, O.B.: 1949, Austral. J. Sci. Res. A2, 139

Cohen, M., Cannon, W., Purcell, G.H., Shaffer, D.B., Broderick, J.J.,Kellermann, K.I., Jauncey, D.L.: 1971, ApJ 170, 207

Ewen, H.I., Purcell, E.M. : 1951, Nature 168, 356Hewish, A.R., Bell, S.J., Pilkington, J.D., Scott, P.F., Collins, R.A.:

1968, Nature 217, 709Hulse, R.A., Taylor, J.H.: 1975, ApJ 195, L51Kiepenheuer, K.O.: 1950, PhRv 79, 738Kraus, J.D.: 1966, Radio Astronomy, McGraw-Hill Book Company,

New York, p. 2Mathewson, D.S., van der Kruit, P.C., Brouw, W.H.: 1972, A&A 17,

468Penzias, A.A., Wilson, R.W.: 1965, ApJ 142, 419Reber, G.: 1944, ApJ 100, 279Ryle, M., Hewish, A.: 1960, MNRAS 120, 220Ryle, M., Smith, F.G.: 1948, Nature 162, 462Schmidt, M.: 1963, Nature 197, 1040Sullivan III, W.T. (ed.): 1984, The Early Years of Radio Astronomy

– Reflections Fifty Years after Jansky’s Discovery, CambridgeUniversity Press

Taylor, J.H., Hulse, R.A., Fowler, L.A., Gullahorn, G.E., Rankin,J.M.: 1976, ApJ 206, L53

Walsh, D., Carswell, R.F., Weymann, R.J.: 1979, Nature 279, 381Willis, A.G., Strom, R.G., Wilson, A.S.: 1974, Nature 250, 625