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Freedman in the 1990s. The result: the Hubble constant is 70 km/s per megaparsec (or
about 20 cm/s per light-year).
The Harvard Astronomer Margaret Geller, writing about the monumental The Carnegie
Atlas of Galaxies (1994), was extraordinarily eloquent about the two-volume atlas, the final
and greatest work by Sandage and Bedke: “This atlas is a work of art. Nowhere are the
grandeur and aesthetic appeal of the cosmos more elegantly displayed. . . . The Carnegie
Atlas should be in art museum shops along with other collections of famous photographs.”26
The British astronomer Gerry Gilmore wrote: “I have not previously reviewed a scientific
or technical book using remotely as many superlatives as I have here. . . . It is a truly out-
standing contribution to astronomy.” Then qualifying his enthusiasm in more sober terms:
“The clear and objective exposition transforms this atlas and classification scheme from a
transient, personal, and possibly somewhat subjective phenomenon to a basic and enduring
tool for scientific research. It ensures its continuing utility for many years to come, and
makes this atlas a major milestone on the pathway to enhanced scientific knowledge.”27
With his educationalist hat, Gilmore gets close to preaching: “Those young scientists need
this atlas. So do we old ones. . . . The young researchers to whom the atlas is addressed
are being offered a helping hand up, to stand on the shoulders of a giant.” But the atlas
might have been a big step for the newcomer. Geller suggested that a separate informal
guide for young people and their teachers would have been a valuable companion to the
atlas.
In the popular French Ciel et Terre, Belgian astronomer René Dejaiffe notes that The de
Vaucouleurs Atlas of Galaxies (2007) is “ . . . the first atlas based on digital detectors instead
of photographic plates. The images of the 523 galaxies of the atlas also come from a combi-
nation of ground-based and space-based telescopes. The extensive discussion and interpre-
tations presented make this atlas a new reference for galaxy work.”28 Indeed, Buta et al.’s
recent atlas had finally done justice to the very fine and precise classification scheme pro-
posed by de Vaucouleurs more than half a century previously (Chapter 10). Stewart Moore,
in a review for the Journal of the British Astronomical Association, recalls the limitations
of the Hubble tuning-fork-shaped classification system and its challenge to deal with the
junction between the ellipticals and the spirals, again the headache of the lenticular (S0)
galaxies, as noted by Sidney van den Bergh in his review of The Hubble Atlas of Galaxies
of 45 years earlier.29 There have been many indications that things in galaxy morphology
were subtle and complicated. Early in the 1950s, de Vaucouleurs had introduced a three-
dimensional diagram creating a classification “volume” that dealt more systematically and
rigorously with the secondary features of galaxy structure (Chapter 10, see discussion on
the The de Vaucouleurs Atlas of Galaxies). Moore went on to discuss the many indications
that things in the galaxy world were subtle. “The long axis contained the main classes of
26 M. Geller, Book Review, Science, 1995, Vol. 268, p. 1214.
27 G. Gilmore, Book Review, The Observatory, 1995, Vol. 115, No. 1128, p. 278.
28 R. Dejaiffe, Book Review, Ciel et Terre, 2007, Vol. 123, p. 121.
29 S. L. Moore, Book Review: The de Vaucouleurs Atlas of Galaxies, Journal of the British Astronomical Association, 2007, Vol. 117, no. 4, p. 211.
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galaxies while a cross-section at each class described families within that class.” He noted
that the authors of the atlas had modified and improved de Vaucouleurs’ system to include
new findings with space-based observatories and infrared imaging.
Authors of atlases and architects of the classification systems themselves commented
on other atlases. We read in the preceding chapter praise by Allan Sandage for Wray’s The
Color Atlas of Galaxies. Sidney van den Bergh also liked the atlas and recommended it
as follows: “For an excellent introduction to the beauty and variety of galaxy morphology,
the reader is referred to The Color Atlas of Galaxies”.30 The blunt and critical opinions
of Halton Arp on Sandage’s The Hubble Atlas of Galaxies have already been discussed
(Chapter 10). Gérard de Vaucouleurs wrote a detailed review, in Spanish, of José Luis Sér-
sic’s Atlas de galaxias australes, emphasizing the importance of the photometric maps and
data, which the atlas was the first to include.31 Vera Rubin found the photographs of the
RSA “exquisite,” and being a very practical researcher, added: “In among all the goodies is
Appendix A: a table of additional galaxies,” which expanded on the original Shapley–Ames
catalogue.32
Often written by well-known astronomers, book reviews added an aura of authority that
sent the appropriate signals to librarians to make a safe purchase, or to the individual keen to
acquire the work, especially graduate students who wanted a guaranteed intellectual return
for their investment. As mentioned before, astronomy is a discipline that enjoys a world-
wide community of amateurs. They form a strong group of readers and users of atlases,
as demonstrated by the numerous reviews of atlases that were written by both professional
and amateur astronomers and published in astronomy magazines. More recently published,
The Arp Atlas of Peculiar Galaxies has amateurs as its prime target audience.33
A few atlases received criticisms: Wray’s The Color Atlas of Galaxies, for the very poor
quality of reproduction of the images (the criticism was aimed at the publisher), or Hick-
son’s Atlas of Compact Groups of Galaxies for which Irish astronomer David Andrews
questioned the physical relevance of the groups and the selection of objects, and was hoping
for an improved revised edition.34 The British astronomer Jeremy Allington-Smith wrote
that Hickson had assembled “an objective catalogue,” but “as a coffee-table book, it must be
judged a failure since its black-and-white pictures are too humdrum, despite the inclusion
of every astronomer’s favorite pinup – the original Stephan’s quintet.”35 While not being
a bedtime reading book, Hickson’s atlas – the first to employ CCD imaging – enjoyed the
largest number of book reviews.
The recent The de Vaucouleurs Atlas of Galaxies has also received a mixed set of reviews
for its “dull presentation” aimed at the specialist: “no pretty colour images,” and “not for
beginners.” In my opinion, these statements are rather unfair, as the reviewers overlooked
30 S. van den Bergh, Galaxy Morphology and Classification, Cambridge: Cambridge University Press, 1998.
31 G. de Vaucouleurs, Algunos comentarios sobre fotometria de galaxias y el “Atlas de galaxies australes” de Jose Lui Sérsic, Revista Astronomica, 1969, Vol. XLI, pp. 27–34.
32 V. Rubin, Book Review: A Revised Shapley–Ames Catalogue of Bright Galaxies, Sky and Telescope, 1982, Vol. 63, p. 478.
33 J. Kanipe and D. Webb, The Arp Atlas of Peculiar Galaxies, Richmond: Willmann
-Bell, 2007.
34 A. D. Andrew, Book Review: Atlas of Compact Groups of Galaxies, Irish Astronomical Journal, 1995, Vol. 22, p. 226.
35 J. R. Allington-Smith, Book Review: Atlas of Compact Groups of Galaxies, The Observatory, 1995, Vol. 115, p. 347.
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the advantages of the logarithmic, sky-subtracted digital images, such as revealing details of
both the inner and outer structure and also the homogeneity of the illustrations. Furthermore,
the images in The de Vaucouleurs Atlas of Galaxies are no less appealing than those of The
Hubble Atlas of Galaxies or of The Carnegie Atlas of Galaxies. Nevertheless, from these
reviewers it is clear that there is an expectation for atlases to be well produced and visually
attractive, almost like art books.
Overall, the majority of galaxy atlases garnered overwhelmingly positive and fair
reviews, albeit with a few cases of overenthusiastic pronouncements on the potential usage.
The reviews most certainly helped to stimulate the market for the atlases. As opposed to the
great scientific atlases of the nineteenth century, which had to be sold through advanced sub-
scriptions to sustain their outlandish production costs, atlases of the galaxies went straight
to the market, like normal books.
Finally, it is worth noting that the number of citations to the atlas (i.e. its apparent use)
does not necessarily correlate with the number of book reviews received. For example,
Atlas de galaxias australes had only a single review by de Vaucouleurs, as part of a longer
commentary published in an obscure Argentine journal on the Sérsic formula describing
galaxy structure. And yet it is the most cited atlas, alongside Sandage’s The Hubble Atlas
of Galaxies. Similarly Arp’s atlas, published in a very specialized research journal, did not
receive many reviews, perhaps on account of the emphasis by the controversial author on
“rogue” galaxies and his strong contrarian approach to galaxy classification.
The Scholarly Driving Forces for Galaxy Atlases
Lorraine Daston and Peter Galison have written in eloquent and illuminating ways about
the many purposes and driving forces of scientific atlases and of their role in the research
and learning process. “Atlases are systematic compilations of working objects. They are
the dictionaries of the sciences of the eye. For initiates and neophytes alike, the atlas trains
the eye to pick out certain kinds of objects as exemplary and to regard them in a certain
way. . . . The atlases drill the eye of the beginner and refresh the eye of the old hand.”36
Atlases of galaxies fit this broad description in their design, production and usage.
The authors of atlases and their publishers carefully put together selections of “stan-
dardized objects” for researchers and students. Hence, as shown through examples in
this chapter, atlases have been tools for researchers (by assembling archetypes) and for
learners/students (by showing well-identified exemplars) to view, explore and understand
the morphological diversity of galaxies. They have been “working objects” in the full
sense.
Thus equipped, it becomes possible for the researcher or the student to relate shapes and
morphology to various cosmic phenomena; for example:
r environmental density, which affects the interaction between galaxies and their merger
history;
36 L. Daston and P. Galison, Objectivity, New York: Zone Books, 2007, pp. 22–23.
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r internal perturbations (bars) and secular evolution, which determine the strength of the
spiral arms and bars: or the presence of other resonances, such as ovals, rings and lenses;
r gas accretion, which may sustain the nuclear activity and correlate with the presence of
a supermassive black hole at the galaxy center;
r the mix of structures, which indicates the instabilities or past accretion events: disks with
an exponential radial profile, bulge and nucleus, and dust lanes; and
r processes leading to massive star formation and differences between objects, even of
similar classes.
Galaxy atlases have been used beyond observing programs. They have provided sets of
”working objects” for theoreticians and “simulators” as highlighted in the early and pio-
neering work of Alar and Juri Toomre. Galaxy morphology gives clues on formation and
evolution – there is order, as multiple chaotic processes do not dominate galaxy shapes as is
the case for nebulae. Deviations such as “tails” or outer shells can in the end be understood
as the result of tidal effects, and bars as dynamical instabilities in a large assembly of stars,
orbiting with a slightly asymmetric gravitational potential. Sorting galaxies by morphology
has been possible and meaningful, just as it is possible to sequence stars using their spectra.
While schemes of galaxy classification have been driven by optical observations, the arrival
and recent use of infrared imaging has reinforced the robustness of the scheme proposed
by Hubble as early as 1927 (and improved by Sandage in 1961), refined by de Vaucouleurs
in 1959, and reorganized by Kormendy and Bender in 2012.
Galaxy classification is undergoing a revival.37 It has become a vibrant area of citizen
science as the digital age gives access to huge samples of galaxies. Astronomy is entering
a new era.38 Furthermore, the HST, looking backward in time, is pushing galaxy classi-
fication to higher redshifts and challenging our local universe classification schemes (see
Plate 11.2). Galaxies look different, based on their star formation rate and active interac-
tion history. “Zooites” is the name given to the people who participate in the Galaxy Zoo,
a vibrant citizen science project. More than 50 million galaxy classifications were received
in the first year from almost 150,000 people. The Zooites of Galaxy Zoo 2, who work with
ultra-deep HST deep images, have noticed a greater number of irregular galaxies in Hubble
deep fields compared to previous studies. A large number of galaxies do not fit the tradi-
tional profile. They have come up with new classes, for example “pea galaxies,” which are
compact galaxies undergoing very intense bursts of star formation (Fig. 0.9). But this push
to include weirder objects will allow the scientific tradition of atlases to continue to be a
repository of knowledge and a vehicle for identifying new classes of objects.
Some Conclusions about Galaxy Atlases
What conclusions can we draw from this survey and analysis of galaxy atlases? The main
points may be summarized as follows:
37 See the mid-infrared atlas based on the Spitzer Survey of Stellar Structure in Galaxies by Ronald Buta et al., accessible at http://
kudzu.astr.ua.edu/s4g-morphology/s4g-morphology.html and discussed in the appendix.
38 See K. W. Willett, C. J. Lintott, S. P. Bamford et al., Galaxy Zoo 2: Detailed Morphological Classification for 304,122 Galaxies from the Sloan Digital Sky Survey, Monthly Notices of the Royal Astronomical Society, 2013, Vol. 435, pp. 2835–2860.
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1. Images of galaxies have been a powerful discovery tool for understanding the nature of
galaxies and for inferring the processes of their formation and evolution.
2. Morphology has been, and continues to be, a most powerful parameter for sorting the
categories of galaxies.
3. When classification schemes appear to be in opposition – even contradictory, such as
those of Hubble–Sandage versus Arp’s – they are actually describing galaxies at different
stages.
4. Atlases based on optical imaging have been powerful tools to understand galaxies at all
wavelengths; imaging in the infrared (3.6 micron) appears promising for classification.
5. The Shapley–Ames catalogue of 1932 (and its revision by Sandage and Tammann – the
RSA) played a key role (by defining samples of objects) in galaxy research programs of
the 1980s and 1990s.
6. The number of citations of atlases are uncorrelated with the number of reviews at the
time of publication.
7. The quantitative (and unifying) approach of José Luis Sérsic contributed to his less-well-
known atlas being one of the most cited galaxy atlases.
8. With digital archives and powerful tools of enquiry, galaxy atlases are bound to undergo
significant changes.
Finally, galaxy classification has been at the root of many cosmological studies, for
example the fraction of spirals as a function of redshifts, or the presence of a bar in relation
to the age of the universe. Studies of distant and young galaxies are rooted in the simplest
elements of galaxy classification from nearby objects. “Remarkably galaxy evolution mod-
els across the history of the universe are often based first and foremost on morphological
signatures first identified 150 years ago!”39
39 P. Martin in an e-mail note to the author (February 2015).
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Conclusion
This was the astronomical observing experience at its best – a dark, quiet
dome, a silently moving monster telescope, and mastery of the dangerous
Newtonian platform, all in the interest of collecting data on a problem of
transcendental significance.
Allan Sandage 1
What is the Significance of Galaxy Images?
Allan Sandage (see Fig. 10.3) was a giant of twentieth-century astronomy. He spearheaded