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«The Scientific Legacy of the 20th Century Pontifical Academy of Sciences, Acta 21, Vatican City 2011 ...»

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The Scientific Legacy of the 20th Century

Pontifical Academy of Sciences, Acta 21, Vatican City 2011

www.pas.va/content/dam/accademia/pdf/acta21/acta21-swarup .pdf

Great Discoveries Made by Radio

Astronomers During the Last Six

Decades and Key Questions Today

Govind Swarup

1. Introduction

An important window to the Universe was opened in 1933 when Karl

Jansky discovered serendipitously at the Bell Telephone Laboratories that

radio waves were being emitted towards the direction of our Galaxy [1].

Jansky could not pursue investigations concerning this discovery, as the Laboratory was devoted to work primarily in the field of communications.This discovery was also not followed by any astronomical institute, although a few astronomers did make proposals. However, a young electronics engineer, Grote Reber, after reading Jansky’s papers, decided to build an innovative parabolic dish of 30 ft. diameter in his backyard in 1935 and made the first radio map of the Galaxy in 1940 [2].

The rapid developments of radars during World War II led to the discovery of radio waves from the Sun by Hey in 1942 at metre wavelengths in UK and independently by Southworth in 1942 at cm wavelengths in USA. Due to the secrecy of the radar equipment during the War, those results were published by Southworth only in 1945 [3] and by Hey in 1946 [4]. Reber reported detection of radio waves from the Sun in 1944 [5].

These results were noted by several groups soon after the War and led to intensive developments in the new field of radio astronomy.

In Section 2 are summarized radio observations of the Sun and of the massive coronal mass ejections that disrupt satellites and terrestrial power grids. In Section 3 are described discoveries of the powerful radio galaxies and quasars that indicate the presence of supermassive Black Holes of millions of solar mass at the centre of galaxies. In Section 4 is described the great controversy that arose between the Steady State theory and the Big Bang Model in 1961, after Martin Ryle and colleagues noted excess counts of weaker radio sources in the catalogue made by them using radio interferometers. I then describe observations of angular size of a large number of weak radio sources made with the Ooty Radio Telescope using the method of lunar occultation; their statistics indicated the evolution of the radio sources with cosmic epoch, consistent with the Big Bang Model. In Section 5 are described the important discovery of the Cosmic Microwave 74 The Scientific Legacy of the 20th Century


Background radiation (CMB) by Penzias and Wilson in 1965 and later its detailed observations with higher angular resolution by Mather et al. in 1990 with the COBE satellite and by Bennett et al. in 2003 with the WMAP satellite; these observations have given a firm support to the Big Bang Model, indicating that the Universe is dominated by 74% dark energy, 22% dark matter, and 4% ordinary matter. Observations of the HI emission from the spiral galaxies and attempts to measure the epoch of re-ionization are summarized in Section 6. The serendipitous discovery of the Pulsating Radio Sources (Pulsars) is described in Section 7. Observations of more than a hundred molecules in the interstellar medium and megamasers are summarized in Section 8. Developments of earth’s rotation synthesis radio telescopes for high-resolution observations of celestial radio sources are described in Section 9. In Section 10 are discussed some of the Key Questions today concerning the Universe. Conclusions are given in Section 11.

In this brief review, pioneering observations and discoveries are described at first, followed by descriptions of the current status.The references are not exhaustive and only indicative.

2. Radio Studies of the Sun and Solar Wind Soon after the end of the War in 1945, a few groups, particularly in Australia and the UK, started detailed observations of radio emission from the Sun, using existing radar equipment to begin with and later with interferometers. In 1946 and 1947, Pawsey and colleagues found that: (a) solar corona has a temperature of about one million degrees, (b) solar radio emission has a slowly varying component related to sunspot area and (c) there occur intense radio bursts associated with the flare activity [6]. Ryle and colleagues also measured angular sizes of solar emission associated with sunspots and also its circular polarization confirming the predictions by Martyn and by Appleton and Hey. These discoveries led to the development of two major facilities in Australia for investigating characteristics of the solar radio emission. Wild and colleagues [7] developed a swept frequency solar radio spectrograph that led to major classifications of solar radio bursts: (a) Type I, as noise storms, (b) Type II caused by outward ejections of matter with velocities of hundreds of km that cause plasma oscillations at successive higher levels of the solar corona and (c) Type III, caused by ejections of matter of ~1/3rd of the velocity of light. Type IV was later identified by French workers, Type V by Wild and colleagues and Type U by Maxwell and Swarup. In 1953, Christiansen and Warburton constructed an innovative grating array in order to make two-dimensional maps of the radio emission from the Quiet Sun [8]. During the last 60 years, these

The Scientific Legacy of the 20th Century GOVIND SWARUP

pioneering observations have been pursued in great detail by scores of workers and have provided very valuable information about the solar activity [9]. Of particular importance are the massive coronal mass ejections (CMEs) that derive their tremendous energy from the stressed magnetic fields by the sunspot activity on the Sun causing large disturbances on the earth. CMEs have also been associated with the coronal holes. Observations of the interplanetary scintillations of ~1000 compact components of distant radio galaxies and quasars are being done on a daily basis over a large part of the sky around the Sun by Manoharan and colleagues using the Ooty Radio Telescope in India [10].These observations provide information about variations of the solar wind and also acceleration of the coronal mass ejections affecting the earth. During the last 15 years, X-ray and coronagraphic observations of the Sun by the SOHO satellite of NASA have provided valuable data about the quiet and active Sun. NASA’s STEREO has revealed the 3D structure of the CMEs. Japanese and Russian agencies have also built solar observatories in Space.

3. Radio Galaxies, Quasars, and Black Holes

3.1. Radio Galaxies I describe firstly the remarkable story of the discovery of Cygnus A and its optical identification with a distant galaxy. In 1945 Hey, Parson and Phillips in the UK noted fluctuations in the intensity of cosmic radio noise towards the direction of the Cygnus constellation [11]. Their antenna had a very broad beam. In 1947 Bolton and Stanley determined its source size as ~8 arc-minute using a ‘sea interferometer’, consisting of an antenna placed on a hill at Dover Heights towards the Pacific Ocean in Australia that produced interference fringes as the source rose from the horizon [12]. In 1951, Graham-Smith measured its position to an accuracy of ~1 arc-minute using a radio interferometer [13].Thereafter, Baade and Minkowski made observations in that direction with the 200 inch (5 m) Mt. Palomar telescope and identified Cygnus A with a perturbed galaxy having a recession velocity of 17000 km s-1, corresponding to a redshift of 0.06 implying a distance of ~1000 million light years, much further than any other known optical galaxy at that time [14]. In 1953 using an intensity interferometer of about one arc-minute resolution, Jenison and Das-Gupta found that Cygnus A is a double radio source [15]. Since Cygnus A has a very high flux density, it became clear that it should be possible to detect much weaker radio sources up to large distances using sensitive radio telescopes and thus distinguish between various cosmological models, as discussed in the next Section.

76 The Scientific Legacy of the 20th Century


At that time there was great controversy about the physical processes giving rise to the very powerful radio emission. Brehmstrahlung radiation by hot bodies was totally inadequate. It was concluded in 1954 that radio emission is caused by ‘synchrotron radiation’ when electrons with relativistic velocities spiral in the presence of magnetic fields resulting in radiation of extremely high power. Observations of the predicted polarization gave support to the theory.

By 1950, using rather modest equipment, Australian and UK radio astronomers had catalogued ~50 discrete radio sources. A few were associated with known galaxies such as Virgo A and Centaurus A. Later, Martin Ryle and his group constructed radio interferometers using parabolic cylinders with large collecting area at Cambridge in UK and catalogued more than 250 radio sources across the northern sky by 1960. By then, Bernie Mill and colleagues in Australia also catalogued a few hundred radio sources, mostly across the southern sky, using the Mills-Cross using dipole arrays.

At first, there was great controversy concerning the overlapping portions of the two catalogues but it was resolved soon with better measurements by the Cambridge group, resulting in the well-known 3C catalogue.

Since the wavelength of radio waves is quite large, radio interferometers with spacing of many kilometers are required for making detailed radio images of celestial sources with arcsec resolution, as is now possible using synthesis radio telescopes that are described in Section 9. Today, thousands of radio galaxies have also been mapped with sub-arc second resolution. Very long baseline interferometers (VLBI) have provided even milli-arcsec resolution. About 13 years ago, Japanese astronomers placed a 10m diameter parabolic dish orbiting in space and combined it with ground radio telescopes on Earth in order to study a few compact radio sources with 0.0001 arcsec resolution.To date, millions of extragalactic radio sources have been catalogued by various workers. A major challenge has been to make optical identification, although it has become easier after the usage of CCDs on optical telescopes.

Yet, a large number of radio sources have remained unidentified with galaxies, as most of the weaker radio sources are likely to have much higher redshifts, requiring large optical telescopes to observe fainter galaxies.

We next summarize the nature of radio galaxies. As described earlier, radio galaxies are millions of time more energetic than normal galaxies. A radio galaxy is generally a double or triple source, with two outer radio lobes and a central component associated with a supermassive Black Hole at the centre of the galaxy. The central active galactic nuclei (AGN) give rise to jets of relativistic electrons and positrons, and also slower protons, in two opposite direction that result in radio lobes at the two opposite extremities (see Figure 1, p. 357).

The Scientific Legacy of the 20th Century GOVIND SWARUP

3.2. Quasars (QSO) 3C273 is a compact quasi-stellar radio source (quasar). In 1963, Martin Schmidt concluded from the known spectral lines of 3C273, which hitherto were found to be very puzzling as their occurrence could not be explained by any stellar process, that the spectra consisted of Balmer lines of hydrogen and were Doppler shifted corresponding to a redshift of 0.158, the highest known redshift at that time [17].This conclusion indicated immediately the existence of a new class of celestial objects in the Universe. 3C273 is an optically bright galaxy with a magnitude of 13. It is a compact radio source having not only a radio jet but also optical and X-ray jets. Subsequently, a large number of quasars have been discovered, the brighter ones mostly by Australian radio astronomers using the Parkes Radio Telescope at 5GHz.

Radio and optical surveys have indicated that quasars are associated with galaxies that have active galactic nuclei (AGN). Many AGNs are found only at optical wavelengths and are called Quasi Stellar Objects (QSO). A large number of QSOs have also been catalogued by optical surveys; a few have been identified even at redshifts 6. Many QSOs are found to be radio loud but a large number are radio quiet. Many QSOs are also strong X-ray sources, with the X-ray emission arising close to the Black Holes located at the centre of AGNs. X-ray observations have provided important information about Black Holes, such as their spin and properties of the surrounding Accretion disks that feed matter into the Black Holes from the associated galaxies.

Unified models of active galaxies indicate that jets of energetic particles emanate from the AGNs moving outwards at relativistic velocities. If the jet is beamed towards the observer, the radio emission from the central component, that generally has a flat spectrum, gets relativistically beamed and thus AGN is observed as a quasar with flat spectrum. If one of two jets is at a larger angle to the observer, radio emission only from that jet is seen as shown in Figure 1 (see. page 357). The unified model also explains observed optical spectra of the AGNs, which depend on the orientation of the axis of tori around the central Black Holes.

3.3. Black Holes in the Universe Although the presence of massive Black Holes at the centre of active galaxies was firstly established by radio astronomy observations, their existence has been firmly established by extensive radio, optical and X-ray observations. It has now been concluded that almost all galaxies have supermassive Black Holes of several millions, some even billions, of solar mass. Our Galaxy has a Black Hole of about one million solar mass. The 78 The Scientific Legacy of the 20th Century


matter from the outer parts of a galaxy spirals into the Black Hole, forming a torus and an accretion disk around the Black Hole. Although it is not yet clear as to how jets are created, it has been suggested that a large magnetic field near the centre of AGN gives rise to the jets of energetic particles.

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