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INTRODUCTION Astronomy has taken a giant step thanks to the development of new technology in building large optical telescopes, radio telescopes and sensitive detectors. Cosmic radiation has been observed over a wide band of the electromagnetic spectrum, from the gamma rays of ultra short wavelengths through the visible and infrared radiation to the long radio waves. Optical astronomy dated back to the epoch of Galileo Galilei, while gamma astronomy, infrared astronomy and radio astronomy have been developed only in the last century. Multi-wavelength observations are necessary to understand the nature of astronomical objects. The hot environment around black holes emits energetic radiation in the form of gamma and X rays. Interstellar dust grains heated by stellar radiation emit in the infrared. Bright nebulae and supernova remnants in our Galaxy are strong radio sources. With large telescopes and radio telescopes on the ground together with the instruments launched in space, astronomers can probe deeply in the universe and make spectacular discoveries.
A wide assortment of large telescopes and radio telescopes installed on the ground and launched in space is now available to explore the universe (CNRS).
The sky observed in the infrared by the Planck satellite. Bright thin lane in the middle is due to the emission of hot dust grains in the plane of our Galaxy. Streamers of cold dust spread above and below the galactic plane. This dusty environment is the favourite site of star formation. Radio emission of extraterrestrial origin was first detected unexpectedly by Karl Jansky in 1933, while he was doing the research in the field of radio communication at Bell Laboratories (USA). Jansky was intrigued to detect in his antenna a “star noise”, which he interpreted later as coming from the Milky Way. A serendipitous but very important discovery in radio astronomy was the detection of the Cosmic Microwave Background Radiation. This discovery provides an experimental proof of the Big Bang theory. Quasars which can harbour black holes in their centre are the most remote radio emitters in the universe. Therefore, they can also be used for cosmological purposes. The observation of the 21 cm line emitted by neutral atomic hydrogen revealed that the diffuse band of light stretching across the night sky, the Milky Way, is in fact a spiral galaxy as are myriads of others in the universe. Galaxies and supernova remnants are the most powerful synchrotron radio sources. Synchrotron radiation is a non-thermal process produced by the interaction of relativistic (high energy) electrons with a magnetic field.The radio emission from pulsars originates in a rotating core, the remnant of a supernova explosion. Pulsars appear to be excellent laboratories to produce gravitational waves. The system of a binary pulsar consisting of two neutron stars revolving around each other should emit gravitational waves, according to Einstein’s theory of gravitation. For the first time, the monitoring of the radio pulses of a binary pulsar led to the detection of these waves. The discovery of interstellar molecules at radio wavelengths also marked an era in modern astronomy by creating the new branch of astrochemistry. Some molecular clouds are powerful cosmic masers capable of amplifying the background radiation by several orders of magnitude. THE COSMIC MICROWAVE BACKGROUND One of the very important contributions of radio astronomy is the serendipitous discovery of the Cosmic Microwave Background Radiation at the wavelength of 7.35 cm by Penzias and Wilson in 1965. This ocean of radiation spreads over the entire sky and appears to be the remnant of a much hotter and opaque universe created some 13.7 billion years ago. The intensity of this thermal radiation is governed by the Planck formula:I(n) = (2hn3/c2) / {exp (hn/kT) - 1}-1 n is the frequency, c the velocity of light, h and k are the Planck and Boltzmann constants.The spectrum of this radiation fits perfectly the black-body spectrum at 2.725 Kelvin and peaks at the wavelength ~ 2 millimetres ( n ~ 160 Gigahertz). Subsequent detailed observations by the satellites COBE and WMAP at millimetre wavelengths with high spatial resolutions revealed tiny temperature fluctuations ~ 0.0002 degrees around the average temperature of 2.725 Kelvin, indicating that the universe was clumpy. Statistical analyses of the temperature fluctuations obtained at different frequencies together with cosmological theories gave valuable information on the evolution of the primordial universe and revealed the existence of a spectacularly large amount of dark energy and dark matter.
THE 21 CM LINE OF ATOMIC HYDROGEN The energy of an atom and molecule is quantized into a series of discrete levels. The energy jumps from one level to another according to the energetic arrangement produced by the change of the electronic states in the atom or the rotation and vibration of the molecule. The most common line emission is the 21 cm line of neutral atomic hydrogen. This line arises from the transition between two hyperfine levels in the ground state. The hyperfine transition is due to the change of the spins of the electron and the proton from the parallel state to the antiparallel state. Since the energy difference between the hyperfine levels is small, the radiation is emitted in the radio range (low energy radiation) at the wavelength of 21cm. For a single atom, a spontaneous change from the upper state to the lower state is very rare, since it occurs every 11 million years. Collisions with other particles can trigger an induced emission and enhance the emission rate. Since hydrogen is the most abundant constituent of the interstellar gas, the 21cm emission appears to be the strongest thermal line emission observed in galaxies. The 21cm line is ubiquitous in the interstellar medium. It was used to reveal for the first time the spiral structure of the Milky Way and to detect remote galaxies. The observations of the Cosmic Microwave Background suggest that most of the matter in the universe should be in the form of invisible dark matter. The outer part of the rotation curve of galaxies observed with the 21 cm line of atomic hydrogen decreases as a function of the distance r to the galaxy centre more slowly than predicted by the Kepler law 1/r1/2. The anomalous shape of the rotation curve of galaxies indicates that there should be a halo of invisible matter around the galaxies and provide additional clues for the existence of dark matter.
Composite image of Whirlpool galaxy (Messier 51). Radio emission from neutral hydrogen (faint feature at large distance) extends well beyond the optical galaxy (bright spiral arms). (Courtesy
NRAO/AUI; Juan M. Uson).
QUASARS AND SUPERLUMINAL MOTION Quasars
are extremely compact and intrinsically very bright. They are thousands
of times more luminous than the brightest galaxies, yet they have the
appearance of very faint stars because of their large distance.
Therefore, it is difficult to find them in the optical sky, since they
are easily confused with billions of other stars in the Milky Way. Some
quasars are strong synchrotron radio sources and appear extremely bright
in the radio sky. This is the reason why the first optical quasars were
identified with the help of their radio counterparts.
Sporadic explosions in quasars eject clouds of relativistic electrons
emitting synchrotron radiation. The intensity of the quasar is highly
variable. There should be a time lag for the radiation to reach the
earth from the back side and from the front size of the quasar. From the
measured variability time scale The electron clouds ejected from the quasars move at relativistic velocities (close to the velocity of light). The apparent cloud velocities projected onto the plane of the sky can be much larger than the velocity of light (superluminal velocities), though their true velocities are subluminal. The paradox of superluminal motion can be explained by the theory of special relativity. THE COSMIC MIRAGE General relativity predicts that the radiation from a remote galaxy or quasar can be deflected by the gravitational field of an intervening massive object lying by chance along the line of sight. The intervening object can be a galaxy or a cluster of galaxies. The gravitational lensing mechanism is similar to the focusing effect of an optical lens. The discovery of a twin optical quasar which is also a double radio source raised the question about its origin. The two components are separated from each other by only a few arcseconds. Furthermore, the fact that they have the same redshift (the same distance) implies that they are very close in space. They are actually the images of a very faint single quasar at a distance of ~ 10 billion light-years. The remote quasar happens to be almost aligned with a foreground galaxy located at about half way between us and the quasar.
Upper: the gravitation field of a foreground galaxy (red spiral) bends the light of the background quasar (blue dot). Lower: this phenomenon gives rise to a double quasar image (blue dots). Gravitational lensing not only amplifies the background object, but also distorts its image. It creates a cosmic mirage depending on the more or less complex distribution of the galaxies in the cluster present in the foreground. If the foreground and the background objects are single and perfectly aligned along the line of sight, the lensing effect results in a ring, the “Einstein ring”, surrounding the position of the objects in the sky. The radius of the ring is r = (4GMd/c2)1/2, where G is the gravitational constant, M the mass of the deflector and d a function of the distance of the deflector to the background source and to the observer.
Upper: A distant galaxy 1938+ 666 appears in the infrared as a perfect Einstein ring around the lensing galaxy (bright spot in the centre of the ring). Lower: The radio image of this galaxy is an arc instead of a complete ring, since the radio counterpart is not perfectly aligned with the lensing galaxy along the line of sight. (Images: Hubble telescope and MERLIN). The lensing effect can be used to investigate intergalactic clouds of dark matter and remote galaxies in formation which would remain otherwise undetectable.
INTERSTELLAR MOLECULES AND ASTROCHEMISTRY Molecules emit spectral lines when they rotate and vibrate in space. The excitation of low-lying rotational and vibrational energy levels leads to the emission of radio and IR lines. Hydrogen in the interstellar dense clouds is almost entirely in molecular form. Therefore, the 21 cm line of atomic hydrogen cannot be used as a tracer of molecular clouds. The discovery of interstellar molecules has been helpful to probe this very important component of the interstellar gas. The first interstellar diatomic molecules CH, CN and CH+ were detected in 1937 – 1941 through the ultraviolet absorption lines. The detection of diatomic OH molecules in space at radio wavelengths was made in 1963. These 18 cm OH transitions arise between two hyperfine levels of a doublet in the ground energy state. Radio lines are easier to excite than optical lines because they arise from low-lying molecular levels. The improvement of radio astronomical techniques resulted later in the detection of polyatomic molecules, such as NH3, H2O, … So far, about 150 molecular species, mostly organic compounds, have been detected in space at millimetre wavelengths. The heaviest molecule ever detected is HC11N belonging to the long-chain carbon molecules, the cyanopolyynes HC2n+1N.In an attempt through experiments in the laboratory to understand the mechanisms of formation of cyanopolyynes in space, chemists discovered fullerene, a very stable molecule made of carbon atoms distributed on the surface of a cage consisting of polygons with 60 vertices and 32 faces. A carbon atom is placed at each vertex of this structure. The discovery of fullerene has an important impact on nanotechnology.
The Egg Nebula: a star is hidden in the central dusty disk (dark lane in the middle) and diffuses its light through a hole (NASA).
Spectral lines of ammonia and cyanopolyyne HC7N detected in the Egg Nebula (Nguyen Quang Rieu, David Graham, Valentin Bujarrabal; Astron. Asrophys. Vol. 138, 1984). Since acids and amines have been detected separately in the interstellar medium, it is tempting to search for amino acids. Glycine (NH2CH2COOH), the simplest amino acid may be the best candidate. Searches for interstellar glycine began as soon as the frequencies of radio lines were measured in the laboratory. Since three decades, many attempts at detecting glycine, including our attempt towards the centre of our Galaxy and the Orion Nebula, turned out to be unsuccessful. The glycine lines are indeed very weak and their spectrum can be contaminated by other faint molecular lines. Scientists at NASA announced recently that they detected glycine in a sample of dust gathered in the tail of a comet. Radio astronomers detected interstellar amino acetonitrile (NH2CH2CN) which may be a precursor of glycine. Searches for prebiotic molecules are very important in astrochemistry, since they are the building blocks of living organisms. The detection of these molecules would have a great impact not only on interstellar chemistry, but also on the origin of life and the possibility for extraterrestrial life to exist. Life can be present on planets with appropriate atmospheric conditions. Hundreds of extraterrestrial planets have been detected and intense searches are under way to find rocky planets similar to the earth. MASER ACTION IN SPACE Radio astronomical observations have led to the discovery of unexpected and spectacular phenomena in physics. One of these is the ability for the dense molecular clouds harbouring stars in formation or the envelope of old dying stars to greatly amplify the background radiation. This maser action is a well-known phenomenon in the laboratory. Masers are built to amplify weak radio signals received on radio telescopes. In space, the clouds containing H2O, OH and SiO molecules are the most powerful masers. Methanol (CH3OH) and silicon sulphide (SiS) exhibit weaker maser emission.
The SiS maser from the envelope of the star IRC+10216 was detected as a spike (on the left in the spectrum) whose intensity varies in time. Black dots represent a synthetic spectrum calculated with a theoretical model to fit the observed spectrum in order to determine the physical parameters inside the stellar envelope. (Nguyen Quang Rieu, Valentin Bujarrabal, Hans Olofsson, Lars Johansson and Barry Turner; Astrophys. Journal., Vol 286, 1984). Interstellar clouds are in general in “local thermodynamic equilibrium” (LTE) state, in which the population n of molecules is governed, according to their energy E, by the Boltzmann law: n ~ exp(-En/kT). A maser effect is produced when the population of molecules is not in the LTE state. This situation occurs when the molecules are pumped to higher energy levels by collisions with other particles or by the infrared photons from stars and warm interstellar dust grains. This population inversion results in an excess of molecules in high energy states as compared to the LTE population distribution. The maser emission is then induced by photons inside the cloud or by an external radiation. The investigation of the interstellar maser emission requires the resolution of the statistical equilibrium equations, which control the level populations, coupled with the radiative transfer equations, which describe the intensity of the radiation during its propagation through the cloud.
ASTRONOMY IN VIETNAM In tropical countries like Vietnam, the humid and hot climate is not favourable to optical observations. Cosmic radio waves at centimetre and longer wavelengths propagate almost freely through the atmosphere even saturated with water vapour. Therefore in such an environment, radio astronomy would be appropriate to the observation of the sky. This was the choice of Indian astrophysicists who built a Giant Metrewave Radio Telescope (GMRT) near the city of Pune. This interferometer consists of thirty 45-m antennae and operates from 150 MHz (l = 2 m) to 1500 MHz (20 cm). It can be used to detect remote embryonic galaxies formed at the very early stage. Their 21 cm line emission is expected to be redshifted to metre wavelengths due the expansion of the universe.
After three decades of wartime, Vietnam was faced with many important problems of economy. Nowadays it seems eager to develop both applied and fundamental sciences. Space science is one of the fields having priority. At the beginning it is not necessary to build large radio telescopes. Astronomers of any country are allowed to use the instruments built abroad if they have reasonable scientific proposals. In principle, they can observe remotely from their laboratory and reduce their data without being present at the telescope site. Model calculations can also be done with computers to interpret the observations. Astrophysics involves several fields from physics and chemistry to mathematics. A concrete measure to take is to form young astrophysicists inside Vietnam or abroad. A radio telescope of modest size should be useful for teaching purposes. So far, a few vietnamese astrophysicists were formed at the Paris Observatory and came back to the country. They form a core of specialists who should be encouraged to do research and to teach in the field of astrophysics.
PROSPECTS Important discoveries in astrophysics have been made. The spatial resolution of a telescope (the smallest angular separation between two points in the sky that can be distinguished by the telescope) is ~ l/D, where l is the operating wavelength and D is the size of the instrument. Since radio wavelengths are at least a thousand times larger than optical wavelengths, radio astronomers must build giant single antennae of hundreds of metres and interferometers extending over tens to thousands of kilometres, in order to get a spatial resolution comparable to optical telescopes. State-of-the-art receivers built with high electronic technology have been installed at their focus. The ALMA (Atacama Large Millimetre and Submillimetre Array) is an interferometer being built and installed on a desert plateau in Chile at the altitude of 5000 metres. It consists of about sixty 12-m antennae which can be spread over distances from 150 m to 16 km. ALMA will be the best ground-based instrument ever built in radio astronomy. With its spatial resolution ten times better than that of the Hubble Space Telescope and its large collecting area, the ALMA interferometer will be used to investigate distant objects at the edge of the observable universe. The space telescopes Planck and Herschel launched in 2009 and operating in the far infrared and submillimetre bands will be devoted to a detailed observation of the Microwave Background Radiation and stars and galaxies still in formation. These data combined with those obtained with large optical telescopes are expected to reveal further important secrets of the primordial universe and the cold world of nascent stars and galaxies. Nguyen Quang Rieu
Article
published by the author in the “Proceedings of the
Conference on Nuclear (Hanoi-Vietnam November 2010).
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