The Role of Images in Astronomical Discovery Read online

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  Part II – Images as Galaxy Discovery Engines

  Fig. 6.1 Walter Baade in 1926. Credit: University of Chicago Photographic Archive, [apf6–01309],

  Special Collections Research Center, University of Chicago Library.

  Resolving Stars in Galaxies

  Using his wartime isolation “privilege,” Baade pushed the depth and resolving power of

  photography on the 100-inch as no one else before. “There is a very considerable, broad,

  pretty faint, small nebulae near it; my Sister discovered it August 27, 1783, with a New-

  tonian 2-feet sweeper.”5 This is how William Herschel described NGC 205, one of the

  small elliptical galaxy satellites of Messier 31. Almost 160 years later in 1944, Baade

  managed to resolve the stars in the central region of Messier 31, the Andromeda Galaxy,

  and also in its elliptical companions Messier 32 and NGC 205.6 Despite numerous claims

  by past observers, this was the first time individual stars could be seen in a galaxy. Allan

  Sandage included a Palomar 200-inch image of NGC 205 obtained by Baade in The Hub-

  ble Atlas of Galaxies to illustrate the elliptical morphological type of galaxies. In a side

  5 W. Herschel, On the Construction of Heavens, Philosophical Transactions of the Royal Society of London, 1785, Vol. 75, pp. 213–266.

  6 W. Baade, The Resolution of Messier 32, NGC 205, and the Central Region of the Andromeda Nebula, The Astrophysical Journal, 1944, Vol. 100, pp. 137–146.

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  note to the photograph, Sandage wrote “ . . . resolution does not occur until a critical expo-

  sure time is reached, at which time the entire smooth image of the galaxy breaks up into

  individual stars.”7

  In a companion paper of the same issue of the journal, Baade presented stunning images

  of NGC 147 and NGC 185 as new members of the Local Group of galaxies.8 He made

  history with his exquisite photographs of the galaxy. Baade’s photographs of these nearby

  galaxies are some of the most important galaxy images of twentieth-century astronomy.

  Being able to resolve individual stars, Baade could determine their luminosities and colours.

  He clearly distinguished two families of stars, Populations I and II, shortened as Pop I

  and Pop II, as they came to be called. For the first time, a case was made for true cosmic

  evolution, “a process that results from the change in the ‘genetic’ material of successive

  generations of the ‘species,’ not just the aging of individual members of the species.”9

  In this case, the species are the different stellar populations that Baade had managed to

  separate very clearly: (i) yellowish and reddish stars of low luminosity and a low content of

  chemical elements heavier than helium (i.e. “metals”), which were dominant in the central

  bulge; and (ii) relatively bluish stars of higher luminosity and higher metal content, which

  were located in the extended galaxy disk and spiral arms. In his paper, Baade commented

  on the resolution: “The plate reveals incipient resolution of NGC 205 quite unmistakably;

  but the prevailing pattern is still very soft, and the smallest elements are not yet stars but

  small-scale fluctuations in the stellar distribution. The resulting impression is very irritating

  to the eye.”10

  The images Baade had obtained were densely crowded with myriad stars. It was diffi-

  cult to reproduce the details with ordinary halftone illustrations. “Baade therefore requested

  that actual photographic prints of NGC 185 be used in his paper in The Astrophysical

  Journal, and the editor agreed to this procedure. . . . ”11 This was certainly an extremely

  rare case of an actual photograph being bound with the text of a printed article (Fig. 6.2).

  An unusual editorial note introduced the special image: “The photographic reproduction

  on the opposite page is from a negative of NGC 185 by Dr. Baade. North is toward the

  binding; west is at top of page. The prints were produced at Yerkes Observatory by the

  Misses Maude Laidlaw and Doris Blakeley, under the supervision of Dr. W. W. Morgan

  from a duplicate negative prepared at Pasadena by Mr. E. R. Hoge. – Editor.”12 Baade had

  engaged the best professionals to prepare the most appropriate production of his unique

  image.

  7 A. R. Sandage, The Hubble Atlas of Galaxies, Washington: Carnegie Institution of Washington, 1961, p. 3.

  8 W. Baade, NGC 147 and NGC 185, Two New Members of the Local Group of Galaxies, The Astrophysical Journal, 1944, Vol. 100, pp. 147–150.

  9 D. Mihalas, Baade’s Resolution of M32, NGC 205, and M31, The Astrophysical Journal: American Astronomical Society Centennial Issue, 1999, Vol. 525, Number IC, Part 3, p. 360.

  10 W. Baade, NGC 147 and NGC 185, Two New Members of the Local Group of Galaxies, The Astrophysical Journal, 1944, Vol. 100, p. 140.

  11 O. Struve and V. Zebergs, Astronomy of the 20th Century, New York: The Macmillan Company, 1962, p. 450.

  12 W. Baade, NGC 147 and NGC 185, Two New Members of the Local Group of Galaxies, The Astrophysical Journal, 1944, Vol. 100, p. 148.

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  Part II – Images as Galaxy Discovery Engines

  (a)

  (b)

  Fig. 6.2 Transformational Image: Baade’s Photograph Resolving for the First Time Individ-

  ual Stars in Nearby Galaxies. (a) NGC 185, “best described as a slightly elongated, giant glob-

  ular cluster,” photographed in red light by Walter Baade using the Mt Wilson 100-inch telescope.

  From a photographic insert in Baade (1944b), The Astrophysical Journal. C

  AAS. Reproduced with

  permission. (b) NGC 185, in a fine halftone reproduction, resolved into stars and showing patches

  of obscuring material. From Baade (1944b), The Astrophysical Journal. C

  AAS. Reproduced with

  permission.

  Walter Baade was not only a master at producing exquisite images, he also had a pro-

  found influence on our understanding of galaxies. And as an advocate for fine astronomical

  imaging, he went on to promote a new type of imaging telescope. Enjoying a deep friend-

  ship with the Estonian optician Bernhard Schmidt (1879–1935), Baade discussed new tele-

  scopic designs for improved astrophotography. He was looking for optical systems with an

  enlarged field of view without off-axis aberrations of coma (Chapter 3, Part B). Astonish-

  ingly, Schmidt met the challenge with a revolutionary optical design. At Baade’s urging,

  a 18-inch Schmidt optical system was built in Pasadena and installed at Mount Palomar

  in 1936 where Fritz Zwicky put it to work immediately (Fig. 3.11). Thereafter, several

  Schmidt-type telescopes were built at observatories around the world. Alongside these

  significant advances in telescope optics were developments in the cameras installed at

  telescopic foci to detect light together with light-sensitive materials that record images.

  The scientific requirements and evolving technologies driving these forward often acted in

  synergy.

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  Dewdrops in the Cosmic Web

  Astronomical imaging has undergone several transformations, as described in
the previ-

  ous chapters. Humans have used photosensitive materials for almost 200 hundred years.

  In the first half of the nineteenth century, we learned to impress the material and to reveal

  imprinted images through chemical processing. With the discovery of the electron and an

  understanding of its behavior, e.g. the photoelectric effect, cathode ray tubes (ancestors of

  the television) and image intensifiers of various sorts were put into use during the 1950s and

  1960s. For example, in the early 1960s, American physicist Robert Leighton (1919–1997)

  experimented with the vidicon, a sort of digital television tube, to be used on spacecraft

  (see Fig. 8.1). Although more sensitive, vidicons never provided the field of view of the

  photographic plate.

  By the late 1970s, semiconductors of all sorts were being designed and assembled. A

  profound change took place with the invention and fabrication of solid-state photosensi-

  tive devices, in particular charge-coupled devices (CCDs). A CCD is a solid-state device

  that converts light into an electrical signal with a very high efficiency (close to 100%). The

  invention triggered the era of digital electronic imaging. During the 1980s, CCDs were

  rapidly adopted and improved by astronomers in their drive for the higher sensitivity nec-

  essary for detecting low fluxes of photons. Nevertheless, much had been learned from using

  a good old photographic emulsion (Chapter 10), and it needs to be said that photographic

  plates had one huge advantage over CCDs: they were largely free of artefacts and had good

  cosmetics. However CCD have an enormous advantage: they are able to record photons

  ten times more efficiently than the most sensitive photographic emulsions. In the wave of

  technological spinoffs of the past two decades, CCDs have become ubiquitous in our daily

  life; they are part of the minuscule cameras of cell phones. In astronomy, CCDs have been

  hugely transformative and have opened up remarkable imaging opportunities (see Fig. 3.12

  and Fig. 3.13).

  Whether assembled in giant clusters or lined up in huge filamentary structures in an enor-

  mous spider’s-web pattern, galaxies weave the fabric of space in the universe. Each galaxy

  is a complex assembly of matter. Individual galaxies are giant stellar, gaseous and dust

  systems held together by the force of gravity. The gravitational potential that holds galax-

  ies and clusters of galaxies together is shaped by something subtle whose nature remains

  unknown. The descriptor we have for this “thing” is vague: it is called dark matter, or better

  still, invisible mass. Dark matter anchors everything we see and it seems to be ubiquitous,

  and dominant in large scales (Chapter 8). Although luminous or absorbing matter in galax-

  ies represents only a fraction of the total mass, we have learned from it almost all we know

  about galaxies, and several properties of dark matter itself.

  Fulfilling a century-old and once hopeless quest, twentieth-century large-aperture tele-

  scopes, operating under good seeing conditions, finally resolved individual stars into galax-

  ies (Fig. 6.2). By studying the images of galaxies obtained in both visible and infrared light,

  astronomers have been able to infer many important properties of galaxies and elucidate

  large-scale properties of the universe, such as those reviewed below.

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  Imaging Darkness and Magnetic Fields

  The gaseous state is one of the forms of interstellar matter. The other component of galaxies,

  hinted at in the previous sections, is dust, which is permeating interstellar space in the form

  of tiny particles of micron size or less, made of icy or mineral solids. Dust makes up about

  1% of the mass of the gas component. Although this sounds unimportant, dust holds several

  keys for chemical processing of elements and molecule formation. It also affects the way

  we view galaxies.

  The prevailing presence of dust was first inferred from its obscuring properties (Chapters

  3 and 4). Dispersed between stars, interstellar dust dims the light of distant stars; these

  appear fainter than they would be with no intervening material. Particularly noticeable are

  the dense dust clouds blocking starlight and producing dark nebulae. In the early twentieth

  century, American astronomer and astrophotographer Edward Emerson Barnard (1857–

  1923) highlighted the dramatic masking effect in his magnificent book, A Photographic

  Atlas of Selected Regions of the Milky Way.13 Like others, Barnard initially thought that the

  dark spots in the sky were holes in space, places with no stars.14

  It took some time for some astronomers to realize that the dark patches seen on pho-

  tographic plates were real and not photographic defects. The Italian astronomer Angelo

  Secchi (1818–78) talked about “canals,” while the Dutch amateur astronomer Cornelis Eas-

  ton (1864–1929) used more visual depictions, “great rift,” “chasms” or “dark streams.”15

  Despite strong evidence, astronomers of the day did not believe in the existence of inter-

  stellar obscuration.16 “Throughout the 1920s there was general disdain for the whole idea

  of an absorbing medium. It is one of the most astonishing examples of wishful thinking in

  the history of astronomy.”17 However, Heber Curtis and others agreed that certain spiral

  nebulae, seen edgewise showing a dark lane running down the length of the spirals, could

  be explained as due to a band of absorbing or occulting matter, and similarly for the dark

  patches masking the middle portions of the Milky Way (see Plate 6.1).18 It was Max Wolf

  who, in 1923, showed this obscuration to be caused by dust and not by lack of stars.19

  In a brilliant 1930 paper, Swiss–American astronomer Robert Julius Trumpler (1886–

  1956) demonstrated that dust of micron size also produces an artificial change of the colour

  of starlight we receive.20 This effect was called “reddening,” because dust absorbs more

  13 E. E. Barnard, A Photographic Atlas of Selected Regions of the Milky Way, Washington: Carnegie Institution of Washington, 1927.

  14 E. E. Barnard, On the Dark Markings of the Sky with a Catalogue of 182 Such Objects, The Astrophysical Journal, 1919, Vol. 49, pp. 1–23.

  15 C. Easton, A Photographic Chart of the Milky Way and the Spiral Theory of the Galactic System, The Astronomical Journal, Vol. 37, p. 109–110.

  16 I. Roberts had noticed and commented on the “broad dark band” that shuts out the light of the central condensation. See images of NGC 3628, 4565 and 4594 on Plate XX in Photographs of Stars, Star Clusters and Nebulae, London: Knowledge Office, 1899, Vol. II.

  17 J. D. Fernie, The Historical Quest for the Nature of the Spiral Nebulae, Publications of the Astronomical Society of the Pacific, 1970, Vol. 82, pp. 1209–1210.

  18 H. D. Curtis, A Study of Occulting Matter in the Spiral Nebulae, Publications of the Lick Observatory, 1918, Vol. 13, Part II, pp. 45–54, and seven plates.

  19 M. Wolf, Über den dunklen Nebel NGC 6960, Astronomische Nachritchten, 1923, Vol. 219, pp. 109–116.

  20 R. J. Trumpler, Absorption of Light in the Galactic System, Publications of the Astronomical Society of the Pacific, 1930, Vol. 42, pp. 214–227.

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  light of
shorter than of longer wavelength. Interstellar reddening is similar to the effect of

  the Sun or the Moon appearing redder before setting or after rising, because of dust in the

  lower layers of our atmosphere scattering the blue light.

  The magnificent luminous band crossing the night sky has filled our ancestors with won-

  der since prehistoric times. The obscuring dust can easily be seen when viewing the Milky

  Way from a dark site, especially from the southern hemisphere where the center of the Milky

  Way crosses close to the zenith. Extended patches can be seen where there appears to be

  fewer stars cutting across large sections of the Milky Way (Plate 6.1). Australian aborigines

  interconnected these dark patches and imagined a giant celestial emu stretching along the

  celestial vault. When observed in the more penetrating near-infrared light instead of visible

  light, these patches become transparent: a multitude of embedded stars, and many beyond

  the absorbing cloud, are revealed.21 Today, the dusty plane of the Milky Way is related to

  the “zone of avoidance” where galaxies appear absent, an apparent distribution that deeply

  puzzled observers of the nineteenth century (Chapter 5).

  Starlight heats interstellar dust. The tiny grains settle to a temperature of a few hundred

  kelvins, the equilibrium value depending on the composition of the grain and its energy loss

  rate. Hence, heated dust radiates and produces a thermal emission in the infrared domain.

  Dust clouds, obscuring at optical wavelength, become luminous at longer wavelengths,

  and are easily observable in the mid-infrared at wavelengths of 5 microns and more (Fig.

  0.4). This happens for dust near massive hot stars or close to the central active galaxy

  nuclei.

  Although dust represents only a small proportion of the mass of the galaxies, these tiny

  solids hold an unusual importance. Their affinity for certain elements is strong enough

  that the grains just suck up the available heavy elements. This chemical capture locks a

  significant fraction of the heavy elements dispersed in the interstellar medium, depleting

  the interstellar gas of elements such as deuterium, silicon and iron, which enter into the

  composition of the interstellar dust grains. While dust traps metals, it betrays a fundamental