The Dawn of Multi-wavelenght Astronomy

Jose Ribeiro

HET607 1st semester 2002

Introduction

Man is the real proof of the evolutionist theory. As multicellular beings in the top of the evolutionary scale we will probably be the most adapted being to the environment we belong to. The constant stimulus we are submitted to are peripherally received by our sensorial organs and then processed centrally in our brain. Perception of whatever surround us depends therefore on our sensorial organs, as well as on our capacity of memorisation and of association of the new information in relation to the experienced before – knowledge and intelligence. Curiously, cognitive aspects are more developed in Man than the sensorial ones, being science a good example. Due to our brain capacity today we have knowledge of things that would transcend our sensorial capacities. For instance, our sense of vision is well adapted to the kind of illumination existing naturally in the Earth. The eyes are an optical system that focus light in the plan of the retina. The retina has two types of sensorial cells, ones that detect colour, the cones, and the other more sensitive to light intensity, responding to black and white, the rods. At low light intensities only the rods are stimulated, and therefore in the dark our vision happens in black and white. The spectral sensitivity of the rods has a maximum at 510 nm and the cones have a maximum at 550 nm [1]. Our visible light varies from 400 nm (blue) to 700 nm (red), so our eyes reach the maximum sensitivity precisely in the middle of this scale. Curiously, the Sun has a maximum of intensity precisely at 500 nm. Therefore, it is most likely that our sense of vision developed in accordance with the colour of our Sun.

For millenia Man constructed his knowledge based in his senses. Still today, the unlearned man does so. For the learned and modern Man there is a lot more than the sensorial organs indicate him. Astronomy is a paradigm of this. Probably one has learned more about the surrounding Universe in seventy years of radio astronomy, in fifty years of X-ray astronomy, in forty years of infrared astronomy, ultraviolet astronomy and gamma-ray astronomy, than during millenia of positional astronomy or during two centuries of visual telescopic astronomy. In this text I will talk about the history of radio astronomy, its evolution, science achieved, and bout the men who interacted in this process. To they I dedicate this work.

Background

In 1860 James Clerk Maxwell developed four equations that relate electricity with magnetism. Electric charges in motion generate magnetic fields and magnets in motion generate electric fields:

rot E = - dB/dt rot H = J

Maxwell equations describe that electric and magnetic forces are interconnected in a single phenomenon, the electromagnetism. An oscillating electric charge causes an electromagnetic field that radiates at light speed. The oscillation frequency of that charge defines the frequency of the radiation. The radiation can be represented by its frequency f in Hertz or by its wavelength l in meters. They are both related to the speed of propagation of the wave c by the following formula: lf = c.

Depending on l or f, the radiation takes a certain place in the electromagnetic spectrum that is divided as follows:

Radio waves	100 km < l < 10 cm
Micro waves	10 cm < l < 1 mm
Infrared	1 mm < l < 700 nm
Visible	700 nm < l < 400 nm
Ultraviolet	400 nm < l < 10 nm
X-rays	10nm < l < 10 ^–2 nm
Gamma-rays	l >10^-2 nm

[2]

From the above, the visible light is only a tiny fraction of the whole electromagnetic spectrum.

The Quantum particle equivalent to electromagnetic radiation is the photon. Its energy is given by E = hf , in which h is Plank’s constant. The higher the frequency is, the more energetic the photon is.

The electromagnetic radiation can be of thermal or non-thermal origin. A dense object radiates in accordance with its temperature: the higher the temperature T is, the lower the wavelength of the maximum of energy is.

l _max = 0,0029 / T

Some examples of non-thermal radiation are the synchrotron radiation when an electron enters at great speed in a strong magnetic field, and the stimulated emission of radiation that happens in lasers and masers.

From the Quantum mechanics one knows that atoms radiate at well defined frequencies when the electrons change of energetic states. On the other hand, moving ionised atoms also produce radiation as well as molecular collisions.

One defines luminous matter all matter that emits electromagnetic radiation, whether it is or not visible. The great majority of baryonic matter in the Universe is luminous although its detection is not always an easy task.

When a radiation source moves in relation to a detector, its frequency suffers a shift. This phenomenon is known as the Doppler-effect. If the source approaches, the frequency of its radiation increases and a blueshift takes place (in analogy with the visible spectrum in which frequency increases towards blue). When the source recedes its frequency decreases and a redshift takes place. After the works of Edwin Hubble, one knows that the majority of the galaxies are receding from us, and so their radiation reaches us at a lower frequency than the one it would happen should they be motionless. According to Hubble, the more distance an object is from us, the quicker it recedes (higher redshift). Another source of redshift is gravity. A mass bends light and forces it to loose energy. According to E = hf , if the energy decreases then f must decrease. This phenomenon is known as gravitational redshift. At last we still have the cosmological redshift, a redshift caused by the expansion of the Universe.

The extreme example of the effect of redshift is the cosmic microwave background radiation (CMBR), that comes from 300,000 years after the Big Bang, when the Universe became transparent to radiation. That date corresponds to a redshift of 1000 and in that date the temperature of the background radiation was of 3000 K. That same radiation reaches us today at a temperature of 2.726 K. Therefore, due to the redshift, a lot of distant events that took place at high energies reach us today in the inferior part of the electromagnetic spectrum as radio waves.

History

The first work I found referenced about electric detection in astronomy dates from 1896, from Wilsing and Scheiner, under the title “On the attempt to detect electrodynamic solar radiation and on the change in contact resistance when illuminating two conductors by electric radiation” [3]. Up to 1930, I found two more works, both from 1902 about the search of hertzian waves emanated by the Sun [3].

It is with Karl Guthe Jansky (1905-1950) that for the first time radio electromagnetic waves were detected, from an extraterrestrial source [4]. Jansky was graduated in Engineering by the University of Wisconsin and started its professional life at Bell Telephone Laboratories in New Jersey. He was assigned with the task to obtain information about the characteristics of the atmospheric statics at 14.6 meters wavelength [5]. To do so, he had to design and built a rotating antenna in azimuth [6]. Jansky detected three signals. Two of then where undoubtedly attributed to atmospheric statics. The third one was fainter, sounded like a steady hissing and was detected when other noises were at minimum. At first, Jansky attributed this hiss to the Sun. Yet, more accurate measurements lead him to relate the origin of these radio noises with the sidereal time. In fact, they came from a specific region in the sky, every 23 hour and 56 minutes. Jansky related this signal with the Milky Way, and that it was generated by interstellar ionised gas. This discovery was serendipitous due to the fact that it was achieved during a very low solar minimum. In fact, the frequency in question, the 20.5 MHz, favours the detection of bursts from Jupiter that is what he probably detected in the first place [5]. But should the work of Jansky have been made during higher solar activity, probably the ionospheric noise would be superposed over the extraterrestrial signals and he would have missed them. As Grote Reber writes “Jansky is an example of the right man at the right place doing the right thing at the right time” [5].

Karl Jansky published some articles between 1932 and 1937 in publications from the Institute of Radio Engineers, IRE. Unfortunately, the scientific community of his time did not pay him the right tribute. Yet, Gennady Potapenko (1895-1979) together with Donald Folland, his student, confirmed in 1936 the discoveries of Jansky [6,7].

Also in 1937, a work is published from Fred Laurence Whipple (b.1906) and Jesse Leonard Greenstein (b.1909) under the tittle “The origin of interstellar radio disturbances” [4,8,9]. In this work they tried to relate the radio signal coming from Sagittarius to the Galactic centre.

The first antenna with parabolic reflector was designed by Grote Reber (b.1911) [10]. This engineer born in Chicago is one of the main references in the development of radio astronomy. His antenna, with 31.5 feet of diameter and 20 feet of focal length, allowed the reception of a larger range of frequencies than the arrays of wire antennas, as well as to obtain a better angular resolution [5]. One can consider Grote Reber as the “father” of the radio telescope. In tribute to Jansky, Reber named the radio waves from space the “cosmic static” that he used as tittle in many of his papers since 1940. With his new antenna, Reber widened the range of frequencies to 160 MHz and 480 MHz. For this he had to develop more sensitive receptors as in cosmic radio emissions the larger the frequency is the lower the intensity of the received signal is.

During Second World War the scientific activities were used in the war effort. Yet, the works on radar were implemented, and this device benefited from a great evolution in this period, mainly in England. Hey, Philips and Parsons adapted a radar device to detect cosmic static in the wavelengths between four and six meters. Their works were published in Nature in 1946 [3]. Hey discovered a small region in Cygnus of two degrees diameter. Today we know it as Cygnus A, a classical radio galaxy, one of the strongest radio sources. Other works from Hey focused in solar studies. The main conclusion of his works is that at 64 MHz the obtained results are coherent with those of Reber at 160 MHz.

In the meantime, the still student Hendrik Christofell van de Hulst (b.1918) [11], predicted that atomic hydrogen could be detected by radio techniques. The hydrogen atom is composed by one proton and one electron. The direction of the magnetic field of the electron depends on its spin. If the magnetic field of the electron is opposed to the magnetic field of the proton, the atom is in a lower energetic level than when both fields coincide with. When the spin of the electron changes making the atom go from a higher energetic level to a lower one, a photon is emitted with a wavelength of 21 cm. This phenomenon happens once every 10 million years [2]. Yet, as the hydrogen is the most abundant element of the Universe, the radiation at 21cm (or 1420 MHz) is the best way to detect objects in radio astronomy.

Only later on, in 1951, Ewen and Purcell detected in practice this radiation, publishing this fact in a work in Nature under the tittle “Observation of a line in the galactic radio spectrum - radiation from galactic hydrogen at 1420 Mc/sec” [3]. At the same time, Jan Hendrick Oort (1900-1992) confirmed the works of Purcell in the detection of the radiation at 21cm.

The radio astronomy was established in Australia in 1940 by Joseph Lade Pawsey (1908-1962) [13]. Pawsey worked in CSIRO (Commonwealth Scientific and Industrial Research Organisation), in the Division of Radiophysics, between 1940 and 1962, date of his death, as a research physicist. He dedicated to the study of the Sun, to the radio emissions from the Milky Way and external galaxies, and developed radio detection technology, mainly in the techniques of interferometry in order to improve angular resolution. Australia has been given probably the main contribution in the evolution and discoveries of radio astronomy. Part of that initial history is told by Pawsey himself in a 1953 publication, “Radio Astronomy in Australia” [14].

The detection of certain discrete sources, such as Cygnus A, increased the need to improve the angular resolution of the radio telescopes. The angular resolution is given by a = 1.22 l/D, being l the wavelength and D the diameter of the objective, that in this case is the reflector dish of the antenna. The lower a is the best the angular resolution is. In the visible radiation one can obtain good angular resolutions because the wavelength is low, and so with small objectives one can already obtain a good definition of the discrete objects. The wavelengths in radio are extremely high, and so large diameters of objectives are required, in order to obtain angular resolutions allowing the definition of discrete sources. Many pioneers studied and developed techniques to improve angular resolution in radio astronomy. One of them was Sir Martin Ryle (1918-1984) [15]. Ryle used a lot of his time working in discrete sources, having completed the 2C and 3C surveys with an interferometer of his own design. Interferometry is based in the principle that the angular resolution obtained by two or more objectives separated by a distance D is equivalent to the angular resolution of one single objective with the diameter D. Due to his work in the development of the techniques of radio astronomy, Ryle was awarded the Nobel Prize of Physics of 1974. I enhance a passage of a work published in 1950 [16] in which he states that “The detection of discrete sources of radio waves presents considerable difficulty, partly because of the small radio frequency power which is intercepted by the aerial system, but chiefly because of the problem of distinguishing between the radiation from an individual source, and the general background radiation from the galaxy (...) the use of an interference system of considerable resolving power had great advantages over a conventional pencil-beam aerial system”. Bernard Yarnton Mills (b.1920) [17], an Australian Engineer and Astronomer, developed in 1952 a system, the cross-type radio antenna, today known as the Mills Cross. This system aimed to improve angular resolution in large wavelengths, higher than 1 or 2 meters [1,18,21].

Another important name of the beginnings of radio astronomy is John Gatenby Bolton (1922-1993) [19]. He began his career as radar officer, having researched in this area, developing the system during the Second World War. Part of his work was done in Australia, having worked with Pawsey in the CSIRO. According to Bolton “one of the principal aims of the radio astronomer working in the field of radio stars is to identify the stars with visible objects” [20]. His long scientific career touched almost all the fields in radio astronomy. Yet, I think his most important milestone was the identification in the visible of radio sources. Namely he distinguished himself in the identification of the Crab Nebula. According to Bolton “The identification of the Crab Nebula was a turning point in my own career and non-solar radio astronomy. Both gained respectability as far as the “conventional” astronomers were concerned” [19]. The Crab Nebula had been studied in simultaneous in 1942 by Wilhelm Heinrich Walter Baade (1893-1960) [22,29] and Rudolph Leo Bernhard Minkowski (1895-1976) [23,28]. Yet, the true origin of its radiation, synchrotron radiation, [24] was only found in 1953 by Iosif Samuilovich Shklovskii (1916-1985) [25], an ucranian scientist.

The structure of our Galaxy was deeply studied by Oort [12] and by Frank Kerr (b.1918) [26]. In a work published in 1958 they wrote: “The view that the Galaxy might have a spiral structure has been expressed almost since the first discoveries of spiral structure in Nebulae” [27]. In this work to access the spiral structure of our Galaxy the 21cm line radio emission of hydrogen was fundamental. [30].Oort also studied the radio source Sgr A, associating it to the Galactic centre [31].

The first Quasar was identified by Maarten Schmidt (b.1929) [32], a former Oort student. His work showed that Quasars are very distant and that only existed in a certain phase of the evolution of the Universe, what denies Hoyle’s theory of a steady-state Universe.

In the same year, Thomas Mattews measured the redshift of the radio source 3C 48 in 0.37 [33].

The Cosmic Microwave Background Radiation was discovered in 1964 by Arno Penzias (b.1933) [34] and Robert Wilson (b.1936) [35]. Both worked in Bell Laboratories. This was also a serendipitous discovery and curiously a very important one: the steady-state theory was no longer valid, and the Big-Bang theory became a most plausible one. Arno Penzias and Robert Wilson were awarded the Nobel Prize of Physics of 1978. The CMBR had already been predicted by Philip Janes Edwin Peebles (b. 1935) [36] and by Robert Henry Dicke (1916-1997) [37]. I should like to underline an edition of the Astrophysical Journal from 1965 [38], in which there are two articles, one from Peebles and Dicke “Cosmic Background Radiation”, and the other from Penzias and Wilson “A measurement of excess antenna temperature at 4080 Mc/s”. Dicke also developed systems for radio detection (Dicke switching).

Anthony Hewish (b. 1924) [39] was another scientist who greatly developed the technological capacities of the radio wave detection. He shared in 1974 the Nobel Prize of Physics with Ryle, with whom he co-worked, not due to his achievements in technological radio astronomy developments, but for the discovery of Pulsars. Pulsars are neutron stars, extremely dense, with short rotational periods, that emit radiation periodically . In a paper written in 1970 Hewish stated “As 1968 drew to a close the discovery of the rapid Pulsars in Vela and the Crab Nebula caused an overwhelming swing of opinion in favour of the spinning neutron – star model (...) the Pulsar in the Crab Nebula, was optically identified with the star from which the supernova is believed to have originated” [40].

There is however an issue regarding the discovery of Pulsars. The first detection of a Pulsar was achieved by a Hewish student, Jocelyn Bell Burnell (b.1943), [41]. Some people wonder if Hewish should have shared the Nobel Prize with her. Yet, about this, Jocelyn said “First, demarcation disputes between supervisor and student are always difficult, probably impossible to resolve. Secondly, it is the supervisor who has the final responsibility for the success or failure of the project. (...) Thirdly, I believe it would demean Nobel Prizes if they were awarded to research students. (...) Finally, I am not myself upset about it”. In the last thirty years, radio astronomy became a routine procedure, and mainly the observation capacities have been improved. With the development of the computers, new algoritms were created in order to isolate the signal from the noise, such as averaging techniques and the application of the Fast Fourier Transform. More sensitive systems were created and the baselines of interferometry were enlarged. Projects, such as the Space Long Baseline Interferometry, are in process, placing radio telescopes in orbit, thus obtaining extraordinary angular resolutions [42]. Land-based giant interferometry arrays are also in process, such as ALMA (Atacama Large Millimeter Array), the expansion of the VLA (Very Large Array) [43], and SKA (Square Kilometer Array).

Conclusion

Due to radio astronomy a lot of science was achieved. The major discoveries were:

- The existence of radio galaxies, active nuclei galaxies, and of the respective jets and

lobes. This discovery lead to the idea of the existence of supermassive black holes in the centre of those galaxies.

- The existence of Quasars and the determination of their distance. As previously mentioned, this discovery had implications in the cosmological theories.

- The discovery of CMBR.

- The discovery of neutron stars, Pulsars, and Supernovae. Due to the possibility of measurement of the orbit of certain binary systems, in which one of the elements is a Pulsar, it has been verified that the orbital period of these binaries decreases, and that can be due to the loss of energy by radiation of gravitational waves (Taylor and Hulse).

- The study of the structure of our Galaxy and the eventual existence of a back hole in its centre, Sgr A *.

- The study of molecular clouds. Due to the submillimeter wavelengths (microwaves) it has already been possible to identify over 100 different molecules in the molecular clouds.

- The Sun and the Planets studies. Sunspots were associated to radiation. The determination of Mercury rotation was measured by radar. The discovery of radio emissions of Jupiter and its electromagnetic interactions with Io.

Nothing of the above mentioned was possible to have been known if based only in visible radiation.

I could not conclude this work without a special remark. When making my research for biographies I noticed that the great majority of the scientists working in radio astronomy show high longevity, and most of them published works within periods longer than forty years! I am happy to conclude that radio astronomy gives you physical and mental health.