The Role of Images in Astronomical Discovery Read online

  produce the apparent spiral pattern. Credit: Wikipedia Commons, Dbenbenn/User: Mysid.

  But spirals are complex, with various components. We saw how Walter Baade could

  distinguish families of stars by their spatial distribution, colours and kinematics, suggesting

  different chemical evolution stages and ages.28 The structural components are identified

  using Baade’s criteria. The most extended component of a galaxy is the exponential disk,

  the flattened region whose brightness diminishes exponentially from the center to the edge.

  They often have a centrally inflated bulge where stars revolve around the center on orbits

  similar to those of elliptical galaxies. They also possess a halo that comprises older and

  high-velocity stars. The spirals with large bulges are more massive than those with small

  bulges. Moreover, the strength and amplitude of the spiral arms is inversely proportional to

  the size of the bulge, so galaxies with small bulges have the most prominent arms.

  Galaxies Hit with a Jolt

  Recognizing barred spirals as a distinct class of spirals, Heber Curtis introduced the subclass

  that he named “-type spirals.”29 Indeed, in addition to their spiral pattern, disk galaxies are

  prone to another important instability or mode of vibration, the bar instability. The trigger

  for this mode has various causes, for example the presence of a companion galaxy or the

  passage of a nearby galaxy, which distorts their gravitational potentials. A majority of the

  stars in the central part of the main galaxy are slowly deviated from their circular orbits

  around the center and move into elongated trajectories, along near-radial orbits. Stars, gas

  and dust clouds then swing from one side of the disk to the other, as if carried by a huge

  pendulum. This is the bar (Plate 6.4).

  This creates havoc in the traffic. Because interstellar clouds are relatively large, they

  collide, leading to the compression of gas clouds, shocks in the dust and loss of angular

  momentum. The gas and dust clouds fall toward the center of the galaxy where they feed

  a circumnuclear ring of enhanced star formation. More spectacularly, the gas is slowly

  28 Walter Baade, Evolution of Stars and Galaxies, Cecilia Payne-Gaposchkin, ed., Cambridge: Harvard University Press, 1963.

  29 H. D. Curtis, Descriptions of 762 Nebulae and Clusters Photographed with the Crossley Reflector, Publications of the Lick Observatory, 1918, Vol. XIII, Part I, p. 11.

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  sucked in by a central black hole. The visual effect of this instability can be quite strik-

  ing. The collective appearance of the stars on those radial orbits is a prominent elongated

  feature, called a bar. This is not a solid feature but a herd of billions of stars moving collec-

  tively along weird orbits. The bar may be hidden, especially in highly tilted or dusty disks.

  Therefore, it is more tractable at infrared wavebands. The bar can be a conspicuous feature.

  Our Milky Way is a barred galaxy probably triggered and maintained by the presence of

  our nearby companions the Magellanic Clouds, themselves irregular galaxies (Fig. 2.9).

  Islets Adrift

  If ellipticals and spirals seem to fill the universe, spread between and around them is a

  multitude of smaller systems. Many display amorphous shapes, as if they had been left

  over during the making of the larger galaxies. They are hard to sort (Chapter 9). Small

  galaxies, “irregulars,” cannot be classified as either elliptical or spiral, and irregulars are

  indeed a mix bag. Best known are the small companions to the Milky Way, the Large and

  Small Magellanic Clouds. Interestingly, recent observations at infrared wavelengths have

  shown that the Large Magellanic Cloud gas and dust distribution resembles that of disk

  systems (see Plate 6.5).

  Small galaxies can be gas-rich or almost devoid of any gas. In the latter instance, this

  is due to the fact that the masses of the small galaxies were probably too weak to gravita-

  tionally retain their warm or hot interstellar gas, which boiled off. Once the intense star-

  formation episode has waned, and the stellar orbits have stabilized after a few billion years,

  they became dwarf ellipticals.

  Galaxies in Excited States

  Finally, many galaxies do not fit the simple categories of ellipticals, disk spirals or irregulars

  because they are in strongly perturbed states, due to a close interaction or a recent merger

  (see Plate 6.6). The perturbed states involve galaxies of all classes and masses. They give

  rise to a wide range of shapes, some of them really weird (Chapter 10).

  Deep observations by large ground-based telescopes, and in space by the Hubble Space

  Telescope and the Spitzer Space Telescope, reveal that “peculiar” galaxies were much more

  common in the early universe.30 In these images, we see objects as they appeared 6 to 12

  billion years ago when most galaxies seem to appear most peculiar. This is due both to

  their high rate of star formation and the more frequent merging and interaction in a smaller

  universe.

  In the wake of Australian astronomer David Malin’s pioneering work during the 1980s,

  new imaging techniques and processing have evolved into even more powerful discovery

  30 The principal large ground-based telescopes capable of reaching to the edge of the observable universe are the two 10-m Keck telescopes (Hawai’i), the two 8-m Gemini telescopes (Hawai’i and Chile), the Subaru 8-m Telescope (Hawai’i) and the four 8-m telescopes making up the Very Large Telescope (Chile).

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  Fig. 6.7 Elliptical galaxy NGC 474, 100 million light-years distant, shows multiple structures, likely

  of tidal origin. Credit: Canada-France-Hawaii Telescope Corporation.

  tools. Objects that had been imaged dozens of times before turned out to be much more

  complex (Fig. 6.7). Faint, giant stellar rings and tidal tails were found around many normal

  galaxies, indicative of a convoluted dynamic history. Images have played a crucial role

  in viewing and understanding interactions between galaxies. Ubiquitous large-scale tidal

  effects had to be taken into account, and hence the role of merging and galaxy cannibalism in

  galaxy evolution. This opened a new research field, with astrophysicists modeling galaxies

  using computer simulations (Fig. 6.8).

  After the first decades of the twentieth century, the “riddle of the nebulae” faded quite

  suddenly. Centuries-old controversies, such as the variability of “nebulae” and their resolu-

  tion into stars, just evaporated as the nature of “nebulae” was finally resolved. New imaging

  techniques and ways of analyzing starlight, for example spectroscopy, resulted in a spec-

  tacular leap in our understanding of the world of galaxies. Powerful giant reflectors located

  at exquisite mountain sites and telescopes in space finally allowed the resolution of stars

  in the nearest galaxies. The transformations in physics pulled astronomy away from “the

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  Fig. 6.8 Galaxies generated by computer simulation. Numerical simulation of the evolutionary stages

  (at 0
.18, 0.54 and 0.90 billion years after the start of the merging) of two disk galaxies merging: left

  panels show the gas and right panels, the stars (compare with photograph of NGC 474, Fig. 6.7).

  Courtesy of Amélie Dumont and Hugo Martel.

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  classical, Herschelian tradition of astronomy.”31 With the discovery of the expansion of

  the universe, Einsteinian spacetime superseded the Newtonian cosmos. Quantum theory

  became the engine for the new astrophysics, furnishing precise tools to interpret the spectra

  of stars and galaxies. The new physics enabled technologies that opened a new window

  into the universe, leading to surprising new ways to look at galaxies. And gave birth to new

  disciplines, radio astronomy and X-ray astronomy.

  31 H. S. Kragh, Conceptions of Cosmos, From Myths to the Accelerating Universe: A History of Cosmology, Oxford: Oxford University Press, 2007, p. 120.

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  A Symphony of Waves

  In retrospect, the serene astronomical landscape holding sway before

  World War II had begun to show fault lines even before the war – although

  few seemed to notice. The first signs came from a puzzling observation

  of radio noise emanating from the Milky Way’s center that Karl Jansky,

  a radio engineer at the Bell Telephone Laboratories in Homldel, New

  Jersey, had discovered in 1932 . . .

  Martin Harwit 1

  Using his own funds, he [Grote Reber] was able to design and construct

  novel apparatus which he could use, driven by his own curiosity, experi-

  mental skills and uncanny insight, to identify and interpret important new

  research areas that have changed our view of the universe and its contents

  in a fundamental way.

  K. I. Kellerman 2

  My determination to image the universe with high-angular-resolution

  instruments, although not shared by other experimenters and opposed on

  theoretical grounds by some theoreticians, drove me to push hard for the

  development and use of a telescope.

  Riccardo Giacconi 3

  How Can Galaxies Be Imaged at Radio Wavelengths and in X-Rays?

  Grote Reber (1911–2002) was “the first person who knowingly built a radio telescope.”4

  This was in 1937. For almost ten years, he was also the only radio astronomer in the world.

  Reber was a creative engineer who explored multiple technologies to improve everyday

  life.

  1 M. Harwit, In Search of the True Universe: The Tools, Shaping and Cost of Cosmological Thought, Cambridge: Cambridge University Press, 2013, pp. 132–133.

  2 K. I. Kellerman, Grote Reber’s Observations of Cosmic Static, in The Astrophysical Journal: American Astronomical Society Centennial Issue, 1999, Vol. 525, p. 372.

  3 R. Giacconi, Secrets of the Hoary Deep, A Personal History of Modern Astronomy, Baltimore: Johns Hopkins University Press, 1998, p. 114.

  4 P. Edwards, in Biographical Encyclopedia of Astronomers, T. Hockey, V. Trimble, and T. R. Williams (editors), New York: Springer, 2007, pp. 956–957.

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  Fig. 7.1 Grote Reber and his home-made 160-MHz radio receiver used to detect radio emission of

  cosmic origin. Credit: Image courtesy National Radio Observatory/AUI.

  Reber had been trained as an electrical engineer with interests in ham radio (amateur

  radio communication) and astronomy. Intrigued by the pioneering radio work conducted at

  Bell Laboratories by Karl Jansky a few years earlier, he built a parabolic antenna 9 m in

  diameter that he erected in his home backyard in Wheaton, Illinois (Fig. 7.3). He used his

  facility to map the sky. He discovered important discrete radio sources, among others the

  strongest radio source in the sky, Cygnus A, making him the first to observe a radio galaxy.

  Always aiming for the simplest design, Reber stayed away from expensive instruments.

  He later lived and worked in Tasmania, Australia, where he studied the ionosphere and its

  effects on the propagation of low-frequency radio waves. We have reason to believe that

  Reber, with his sharp insight, knew that radio astronomy would develop into the thriving

  discipline it became. He lived to see the explosive growth of radio astronomy.

  Riding the Waves or Multiwavelength Imaging

  Evolution by natural selection has matched the sensitivity of the retina of our eye to the

  visible portion of the electromagnetic spectrum. This sounds like a tautology but this coin-

  cidence makes our eyes most sensitive to the wavelengths of the maximum sunlight in the

  spectrum. We call the eye’s spectral range the optical domain, a wavelength range of about

  0.4 to 0.75 micron, matching that of a thermal source with a temperature of about 5,700 K,

  close to the temperature of the solar surface.

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  In 1800, William Herschel carried out a slightly different version of Isaac Newton’s

  experiment with sunlight. Letting sunlight through a prism and holding a thermometer just

  beyond the red part of the visible spectrum, William Herschel registered a rising temper-

  ature. He thus discovered that the Sun emitted “infrared radiation,” beyond the red part

  of the visible spectrum. The radiation was invisible to the eye but detectable by other

  means. Astronomers have found over the course of the last 100 years that our own star,

  the Sun, emits much more than light and infrared radiation. These are small fractions of the

  whole electromagnetic spectrum, ranging from the shortest-wavelength radiation – gamma

  and X-rays during solar flares – to the longest, radio wavelengths, also produced during

  flare-associated disturbances. Physicists and astronomers have learned how to track these

  non-visible wavelengths and have created ways to detect them, leading to the develop-

  ment of innovative tools to “view” the sky at the different wavelengths. Using a variety of

  techniques, present-day astronomers register images over the whole electromagnetic spec-

  trum. We can observe cosmic sources with almost all wavelengths: from the shortest wave-

  lengths or highest frequencies – gamma rays and X-rays – to the longest or least energetic

  ones, the kilometric radio undulations. Because the Earth’s atmosphere blocks the shortest

  wavelengths and the longer mid-infrared domain, it is necessary to get above the Earth’s

  atmosphere to capture these emissions from cosmic sources; more on this at the end of the

  chapter.

  Radio images of astronomical sources provide an extraordinary source of information.

  Several techniques were developed as the new field opened up. It has also been helpful

  that most radio waves can get through the Earth’s atmosphere with minimal disturbance.

  They are not affected by atmospheric turbulence or by the presence of clouds. Because

  radio waves correspond to “long” photons, using them to make images has been very chal-

  lenging. Meanwhile, the phenomenal onset of the Space Age in 1957 gave access to the

  short-wavelength domain by enabling the placement of telescopes and detectors above the

  Earth
’s atmosphere. Astronomers could take advantage of the stunning developments for

  detecting and recording shorter wavelengths, X-rays and gamma rays, which took place in

  the wake of World War II. These breakthroughs led to X-ray and gamma-ray astronomy.

  How did this happen in such a short period of time?

  But let’s begin with cosmic radio waves and look at the radio universe to find how

  astronomers learned to make images in the non-optical domains (see Plate 7.1).

  From Cosmic Hiss to Radio Maps

  The German physicist Heinrich Rudolf Hertz (1857–1894), who died tragically at the young

  age of 37, had set up experiments to produce electromagnetic waves and study them. In

  doing so, Hertz proved the existence of airborne electromagnetic waves and showed that

  they were propagated at the same speed as light. In his honor, these waves were called

  hertzian waves; now, they are simply known as radio waves. Hertz also discovered that shin-

  ing ultraviolet light enhanced the production of sparks in the electrode gap of his receiver.

  This effect was the manifestation of the photoelectric effect that Albert Einstein would

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  explain by quantum theory in 1905. The standard unit of frequency – cycles per second,

  hertz (Hz) – is named after him; one megahertz is written as 1 MHz. Hertz’s findings turned

  out to be conclusive confirmations of James Clerk Maxwell’s (1831–1879) electromagnetic

  theory of light. Soon after, other physicists found more links between light and electricity.

  In the late nineteenth century, there were attempts to detect hertzian waves from the Sun;

  these were unsuccessful, as the Sun did not turn out to dominate the radio sky as it does

  in visible light. The first unambiguous detection of radio waves from the Sun was made by

  British physicist and astronomer James Stanley Hey (1909–2000), on 26, 27 and 28 Febru-

  ary 1942, when investigating signals thought to be produced by the Germans to jam British

  radars.5 In fact, radio waves had been registered from a far more distant source almost ten

  years before World War II, in a most unexpected way. The American radio engineer Karl

  Jansky (1905–1950) of Bell Laboratories was the first to detect cosmic radio waves in the