The Role of Images in Astronomical Discovery
Page 40
“little cloud” in Andromeda in AD 964. Let us recall that the pioneering observers of the
eighteenth and nineteenth centuries had no idea what “nebulae” were: a sidereal fluid, some
interstellar fog, mysterious clouds, other solar systems in formation, unresolved clusters of
stars, or distant stellar systems? While we now know and understand the basics of “island-
universes,” it is humbling to realize that this epiphany occurred less than 100 years ago
and that the disconcerting puzzles of dark matter and dark energy have emerged as the
new frontiers of sidereal exploration. Let us close with English philosopher Francis Bacon
(1561–1626) whose wisdom may outlast galaxies. “We may, therefore, well hope that many
excellent and useful matters are yet treasured up in the bosom of nature, bearing no relation
or analogy to our actual discoveries, but out of the common track of our imagination, and
still undiscovered; and which will doubtless be brought to light in the course and lapse of
years, as the others have been before them; but in the way we now point out, they may
rapidly and at once be both represented and anticipated.”11 Far from being powerless, our
minds make us capable of exploring the “bosom” of nature. In employing an amazing arse-
nal of imaging tools and techniques, humans continue to dream of exploring the universe
in its entirety, with a deep desire to decipher its significance and their own place within it.
9 See www.eso.org/public/images/archive/category/galaxies/ and www.spacetelescope.org/images/archive/category/galaxies/
where thousands of galaxy images are available.
10 See AstroPix at astropix.ipac.caltech.edu
11 Francis Bacon, Aphorism 109, Novum Organum, 1620.
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Appendix
Atlases for Special Usages
This appendix is a complement to Chapter 10 where the main galaxy atlases published
since 1961 are presented. These atlases were based on photographic material of images of
galaxies at optical wavelengths. In addition to these, there have been a number of specialized
atlases, which are targeted at experts in the field. These works were based on a selected
number of galaxies. Often they were published in professional journals.1 There have been
very few atlases of galaxies at wavelengths other than optical and infrared, some of which
are reviewed below.
Sidney van den Bergh’s monograph on Galaxy Morphology and Classification occupies
a special place.2 It is not an atlas in itself but it addresses the fundamental questions about
morphology being the most common tool to classify galaxies. It is also a fine introduction to
the galaxy classification systems of Hubble and Sandage, de Vaucouleurs, Morgan (Yerkes
System), Elmegreen and Elmegreen (the arm classification system) and that of the author
himself – the David Dunlap Observatory (DDO) system. The book is concise and deals very
effectively with the challenges of galaxy classification based on optical imaging. It provides
a critical overview of the successes, limitations and failures of the various schemes.
William Morgan wrote in 1958, “the value of a system of classification depends on its
usefulness.” Let us go over a few of those specialized atlases to give a flavour of their scope
and purpose, and ultimate usefulness.
Radio Mapping of Galaxies
Arnold Rots (1979) put together the Atlas of 21 cm HI line Profiles for 61 Galaxies of
Large Angular Size, based on radio observations of galaxies conducted with the 300-ft
(91-m) National Radio Astronomy Observatory telescope at Green Bank, West Virginia.3
1 A fine example is B. T. Lynds, An Atlas of Dust and HII Regions in Galaxies, Astrophysical Journal Supplement Series, 1974, Vol. 28, pp. 391–306. The plates of 41 galaxies include photographs accompanied by sketches where HII regions and the dust band are shown.
2 S. van den Bergh, Galaxy Morphology and Classification, Cambridge: Cambridge University Press, 1998.
3 A. H. Rots, An Atlas of 21 cm HI Line Profiles of 61 Galaxies of Large Angular Size, Green Bank: National Radio Astronomy Observatory, 1979.
253
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Appendix
The sample included all spirals and irregulars north of declination –19˚ with a large angular
diameter. The galaxies chosen by Rots had Holmberg radii between 9 and 36 arcmin; this is
the radius where the galaxy has a surface brightness of 26.5 blue magnitude per square arc-
second. The observations were presented in the form of global radial profiles and integrated
neutral hydrogen contour maps, published separately.4 The purpose of the atlas, which
had no photographs, was to display the large-scale coherent structure of neutral hydrogen.
The contour maps were printed at the same scale as the Palomar Sky Survey (67.4”/mm)
so that transparencies made from these figures could be directly overlaid on Sky Survey
prints.
A Census of Galaxy Families
Galaxies rarely come isolated. Large assemblies of several tens or even thousands of galax-
ies over volumes of a few megaparsecs have been known for some time. Fritz Zwicky had
systematically observed clusters of galaxies; he produced a massive catalogue of 9,134
galaxy clusters in several volumes. Later, the American astronomer George O. Abell (1923–
1983) used the Palomar Sky Survey to catalogue 2,712 clusters (the Northern Survey in
1958) and a further 1,361 clusters (the Southern Sky Survey in 1989) from the SRC–ESO
Sky Survey.
In producing his Atlas of Compact Groups of Galaxies, the Canadian astronomer Paul
Hickson focused on the set of small, relatively isolated, systems of galaxies with projected
separations comparable to the diameters of the galaxies themselves.5 Here is how the author
explains the purpose of his work. “Consideration of the short apparent dynamical times of
compact groups, as well as lingering questions concerning the frequency of galaxies with
discordant redshifts in known groups prompted the author, in 1981, to begin a systematic
program of study of these objects.”
The atlas assembled a homogeneous sample of 100 compact groups of galaxies from a
systematic search of the Palomar Sky Survey prints. Images of the groups were obtained
at the Canada–France–Hawaii Telescope in blue and red band-passes, employing a CCD
camera built by the author. The atlas included a catalogue, compiling information on pho-
tometric and spectroscopic parameters, along with other optical, infrared and radio data.
Because several of Hickson’s groups display signs of interaction, they had already been
included in the collection of interacting galaxies by Arp and Vorontsov-Velyaminov.6 Some
images included objects not physically associated with the group, most likely due to chance
alignments, as indicated by their discordant redshifts.
4 A. H. Rots, A Neutral Hydrogen Mapping Survey of Large Galaxies, I – Observations, Astronomy & Astrophysics Supplement Series, 1980, Vol. 41, pp. 189–209.
5 P. Hickson, Atlas of Compact Groups of Galaxies, Amsterdam: Gordon and Breach Science Publishers, 1994. It was also published in Astro. Letters and Communications, 1993, Vol. 29, pp. 1–207.
6 B. Vorontsov-Velyaminov, Atlas and Catalog of Interacting Galaxies, Moscow: The Sternberg Astronomical Institute, Moscow University,
1959.
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Appendix
255
Surveying Our Neighbors
Another specialized atlas of galaxies is An Atlas of Local Group Galaxies by Paul Hodge,
Brooke Skelton and Joy Ashizawa.7 The focus of the atlas is the three dozen galaxies of
the nearby group of galaxies to which our Milky Way belongs; it is called the Local Group.
The atlas shows and describes 32 members of the Local Group; it excludes our Milky Way
Galaxy, Messier 31, its companions and the nearby Magellanic Clouds (for which separate
atlases had been published by the main author). Because the Local Group of galaxies are
close to us, they can be imaged with a fine spatial scale, providing an enormous amount
of detail, including resolved individual luminous stars and variable stars. It is indeed a
remarkable feature of this atlas to be able to identify a large number of objects, such as
emission regions (HII regions) and star clusters in each galaxy. The purpose of the atlas
is stated on the back cover: “It is unique in its coverage and format and provides a source
of these fundamental data that will be used for many years.” Most unfortunately, the atlas
suffered from poor reproduction by the publisher, which affected the quality of the work
and most likely diminished its use, interest and impact.
Displaying Galaxy Fireworks
The Hubble Space Telescope has provided a number of spectacular images of galaxies
in interaction or in the final process of merging. Lars Christensen, Davide de Martin and
Raquel Shida created a magnificent teaching and pedagogical work, Cosmic Collisions:
The Hubble Atlas of Merging Galaxies.8 It is based on optical images with the fine spatial
resolution dreamt of by Sandage and Bedke when they put together their ambitious Atlas
of Galaxies Useful for Measuring the Cosmological Distance Scale in 1988. The images
obtained by the many researchers using the Hubble Space Telescope are accessible through
the Mikulski Archive of Space Telescopes (MAST) at the Space Telescope Science Insti-
tute. The MAST archive has been named in honor of the United States Senator Barbara
Ann Mikulski from Maryland, who has been a staunch supporter of space sciences, science
education and literacy.
The highly perturbed objects or weird galaxies illustrate vividly the cosmic collisions
between galaxies, either merging or in highly disturbed states from a recent close passage.
Zwicky, Ambartsumian and Sérsic would have greatly relished seeing so many galaxies in
their “excited” states displayed in such a spectacular way. The authors have also written
a fine introduction to the world of galaxies, including a general description of the Hubble
scheme, stating that merging galaxies hold the clues for the history and future of galaxies.
Imaging Dust
The Spitzer Space Observatory provided researchers with an exceptional instrument to
study galaxy morphology in the mid-infrared. It provided a tool to observe galaxies at longer
7 P. W. Hodge, B. P. Skelton and J. Ashizawa, An Atlas of Local Group Galaxies, Dordrecht: Springer Netherlands, 2002.
8 L. L. Christensen, D. de Martin and R. Y. Shida, Cosmic Collisions: The Hubble Atlas of Merging Galaxies, New York: Springer, 2009.
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Appendix
wavelengths where, with the appropriate use of filters, it becomes possible to disentangle
stellar populations and the dust component very clearly. There have been several morpho-
logical studies in the infrared, but most have been selective and none very extensive. The
sampling gap has now been filled by Ronald Buta and his colleagues, who published a large
study of galaxies based on Spitzer imaging.9
Their article includes a full review of previous classification works and is supported by
an on-line atlas with images of 207 galaxies at 3.6 microns.10 The authors have chosen
this wavelength because, seeing through the dust, it provides reddening-free sensitivity,
mainly to older stars. Several key features of the de Vaucouleurs classification are partic-
ularly distinctive when galaxies are viewed at infrared wavelengths. The article explores
the imprint of the de Vaucouleurs classification volume scheme. As the authors write, “3.6
μ m classifications are well correlated with blue-light classifications, to the point where the
essential features of many galaxies look very similar in the two very different wavelength
regimes. Drastic differences are found only for the most dusty galaxies.” A great advantage
of the Spitzer 3.5-micron imaging is that it reaches deeper in brightness than other near-
infrared imaging from the ground. This atlas, and the corresponding on-line material, gives
the flavour of what the digital age adds to the power of scientific atlases.
A fine display of Spitzer infrared images of galaxies can be found on-line with the Spitzer
Infrared Nearby Galaxies Survey (SINGS), which matches the Hubble tuning-fork classi-
fication.
An Atlas for Training
In order to fill a training need, the Australian astronomer Glen Mackie produced The Mul-
tiwavelength Atlas of Galaxies.11 Its purpose is to depict how the best observed galaxies
appear at different wavelengths, from gamma rays, X-rays, ultraviolet, optical, infrared to
long radio wavelengths. It is clearly a pedagogical atlas for students and young researchers,
showing well-known archetypal galaxies. What the small atlas lacks in homogeneity is com-
pensated for in the display of diversity. The image gallery is supplemented with insights on
the complex physical processes at play during various epochs of galaxy evolution.
Galaxies are illustrated over a range of dynamic states: normal, interacting, merg-
ing, starburst and active galaxies. The number of galaxies shown is small (34), but the
objects have been observed over wavelengths spanning the whole electromagnetic spec-
trum. The atlas is a striking demonstration of the varying morphologies and shapes of galax-
ies between wavelengths, e.g. Cygnus A. In other cases, they differ less dramatically from
one wavelength to another, e.g. Messier 101.
9 R. Buta, et al., Mid-Infrared Galaxy Morphology from the Spitzer Survey of Stellar Structure in Galaxies (S G): The Imprint 4
of the de Vaucouleurs Revised Hubble–Sandage Classification System at 3.6 micron, Astrophysical Journal Supplement Series, 2010, Vol. 190, pp. 147–165.
10 http://kudzu.astr.ua.edu/s4gatlas/images.html
11 G. Mackie, The Multiwavelength Atlas of Galaxies, Cambridge: Cambridge University Press, 2011.
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Cambridge University Press
978-1-108-41701-3 — Unveiling Galaxies
Jean-René Roy
Index
More Information
Index
A Monograph of the Trochilidae, or Family of
Atlas de galaxias australes, 208, 215–218, 235, 238
Humming-Birds,36
Atlas of 21 cm HI line Proiles for 61 Galaxies of
A Photographic Atlas of Selected Regions of the Milky
Large Angular Size, 253
Way, 136
Atlas of Compact Groups of Galaxies, 230, 254
A Survey of the External Galaxies Brighter Than the
Atlas of Galaxies Usef
ul for Measuring the
Thirteenth Magnitude, 198, 230
Cosmological Distance Scale, 222–224, 238
A Theory of the Universe, 110
Atlas of Peculiar Galaxies, 208, 211–215, 235, 238,
Abell, George O., 254
239
Academia Naturae Curiosorum,20
citations, 235
active galactic nuclei, 137, 165, 245
Cosmic Collisions: The Hubble Atlas of Merging
Adams, Walter Sydney, 84, 164
Galaxies, 234, 255
adaptive optics, 173
drivers of, 191, 244–245
Agassiz, Louis, 35, 38
evolution of visual versus textual content, 231
Alexander, Stephen
evolving role of, 250
on classiication of “nebulae”, 190
ingredients of, 207–208
on nebular forms, 193
inspiring scientiic programs, 237–239
Alfvén, Hannes, 158
Photometric Atlas of Northern Bright Galaxies, 225
al-Sui, Abd al-Rahman ibn Umar, 21, 109, 252
photometric data in, 192, 217, 221, 225
amateur astronomers, 88, 233
repositories of images, 233
Ambartsumian, Victor, 213, 255
role of, 239, 245
Ames, Adelaide, 176, 187, 198, 230
The Arp Atlas of Peculiar Galaxies: A Chronicle
Andromeda (Galaxy), Messier 31. See galaxies,
and Observer’s Guide, 234
individual: Andromeda
The Carnegie Atlas of Galaxies, 199, 227–228, 230,
aperture synthesis, 160–164
238, 239
Apollonius of Perga, 170
The Color Atlas of Galaxies, 224–225, 234
Arago, François, 62
The de Vaucouleurs Atlas of Galaxies, 228–229
Aristarchus of Samos, 5
The Hubble Atlas of Galaxies, xi, 132, 191, 205,
Aristotle, 189
208–211, 214, 228, 234, 235, 237, 238
Arnold Rots, 253
The Multiwavelength Atlas of Galaxies, 234, 256
Arp, Halton Christian, 131, 208, 218, 232, 248, 254
atlases of galaxies, book reviews, 240–244
Atlas of Peculiar Galaxies, 211–215
Atlas of Compact Groups of Galaxies, 243
artists of nature, 35–39
Atlas of Galaxies Useful For Measuring the
Ashizawa, Joy, 255
Cosmological Distance Scale, 241
Astrographic Catalogue, 64, 65, 93
Atlas de galaxias australes, 243