by Rene Roy
de galaxias australes and The Carnegie Atlas of Galaxies, it has served legions of students
and researchers trying to understand and explain morphology or to account for the “tuning-
fork” classification: How does one interpret the observed silhouette of a galaxy? How do
galaxies form and evolve? What gives shape to galaxies? This questioning is epistemolog-
ically fruitful because several observed properties of galaxies correlate with shape. Shapes
of galaxies matter as they tell us about the origin, formation process and dynamic states of
galaxies. Thus, galaxy atlases, in presenting collections of shapes, are revealing compendia
of key information.
The Hubble Atlas of Galaxies and others that followed became engines for defining
new research programs. Sandage himself kept expanding his survey of galaxies beyond the
original project of Hubble. For example, by exploiting the wide field of view of the Irénée
du Pont telescope in Chile, he conducted a survey of the Virgo cluster, obtaining 67 large
50 × 50-cm plates to cover an area of about 140 square degrees of the sky.9 The survey
yielded a catalogue of 2,096 galaxies, most of which are Virgo cluster members. It was
used in particular to refine the classification of dwarf galaxies.10 Below are given a few
more examples of such research programs.
The work of the American astronomer Sandra Faber on elliptical galaxies is a fine exam-
ple. Because of their differing colours and gas content, one may think that spirals are rel-
atively young and ellipticals, old. In disentangling their mass-to-light ratios, stellar popu-
lations, age and metallicity, Faber demonstrated that stellar populations in many ellipticals
are surprisingly young.11 Or the work of the French astronomer Françoise Combes together
with the Swiss theoreticians Daniel Pfenniger and Louis Martinet on the secular evolution
of spiral galaxies, where disk galaxies may go through successive episodes of bar formation
followed by destruction, the central bulge of the galaxies growing each time through scatter-
ing the orbits of stars. Disk galaxies would swing from one branch of the tuning fork to the
other: normal to barred and barred to normal, growing a bigger bulge. On a grander scale,
some spirals would evolve dynamically from Sm to Sa through bar formation and destruc-
tion, bulge growth and mergers.12 These works aimed at explaining the Hubble sequence,
as illustrated in atlases of galaxies.
9 The Irénée du Pont telescope was the result of a gift in 1970 from Mr. and Mrs. Crawford H. Greenewalt to the Carnegie Institution of Washington. Mrs. Crawford H. Greenewalt was the daughter of Irénée du Pont, the founder of the Dupont Company.
10 Allan Sandage published the work on the Virgo cluster in six papers between 1984 and 1987 as collaborative works with Gustav Tammann and Bruno Binggeli.
11 The Faber–Jackson relation between the luminosity and the central stellar velocity dispersion of elliptical galaxies is a fundamental empirical relation. It can be used for determining distances to external galaxies.
12 D. Pfenniger, L. Martinet and F. Combes, Secular Evolution of Galaxy Morphologies, 1996, arXiv:astro-ph/9602139v2
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Part III – Organizing the World of Galaxies
For the Australian astronomer and cosmologist Kenneth Freeman, The Hubble Atlas of
Galaxies was the main atlas available early in his career and it certainly influenced his work.
With the images covering most types of galaxies and the texts full of Sandage’s wisdom on
the subject, much of Freeman’s views about the nature of disk galaxies came from looking
at the atlas images of edge-on disks. Freeman was later intrigued by Arp’s Atlas of Peculiar
Galaxies, and spent much time looking at its images. Piqued by the idea of galaxies inter-
acting with the intergalactic medium, Freeman saw a set of galaxies of the atlas (Arp 118,
142, 144, 147) as if they were being stripped while ploughing through the intracluster hot
gas (Plate 7.7). Sérsic’s Atlas de galaxias australes served a different function. “The pic-
tures are not so great, but the atlas includes photographic surface photometry (and I think
it may have introduced the Sérsic law). Maybe that is why it is so well cited. I used the
Sérsic atlas a lot when I was working on the properties of the exponential disk, because
there was not so much quantitative surface photometry available at the time.”13 The Sérsic
profile describes mathematically how the intensity of light of a galaxy varies as a function
of distance from its center.
For the Canadian astronomer Wendy Freedman, a principal investigator of a large project
on the Hubble Space Telescope (HST), Sandage and Bedke’s Atlas of Galaxies Useful for
Measuring the Cosmological Distance Scale was “enormously helpful in preparing for the
Key Project and many of us spent many, many hours pouring over those images to choose
our target fields.”14 For quite a few other researchers, the most immediate use of the galaxy
atlas images has been to compare against other images of the same galaxy to check for
suspected supernovae, by making sure that the suspected supernova is not a nearby star
hiding in the galaxy background light.
The select classification of barred galaxies by de Vaucouleurs was the inspiration, for
Pierre Martin, to distinguish and characterize bars using quantitative parameters, such as
bar length, ellipticity and torque. The Carnegie Atlas of Galaxies, on the other hand, was
an essential tool during his postdoctoral years. The superb images helped in identifying
objects of choice to observe at the telescope, in order to reveal the impact of bars in galaxy
evolution.
Arp’s Atlas of Peculiar Galaxies had an enormous influence on the learning and under-
standing of the structure and evolution of galaxies. It inspired the early work of the
Estonian–American astronomers Alar and Jüri Toomre, who were among the very first
researchers to carry out successful computer simulations of galaxy interactions and tidal
tails. Arp’s work and the Toomres’ simulations gave fuel to the debate about the respective
role of nature and nurture in galaxy evolution.15 Arp and the Toomres showed us that our
telescopes were catching galaxies in the act of transformation.
Indeed, the Toomres’ computer simulations opened up a tremendously productive
approach; they invited galaxy researchers to look at familiar but oddly shaped galaxies with
new eyes. It became obvious that galaxies did not always evolve passively nor isolated from
13 Private e-mail note to the author (November 2014).
14 Private e-mail note to the author (October 2014).
15 A. Toomre and J. Toomre, Galactic Bridges and Tails, The Astrophysical Journal, 1972, Vol. 178, pp. 623–666.
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239
each other. There were deviants from Hubble’s ideal shapes shown even in Sandage’s The
Hubble Atlas of Galaxies (p. 41). Alar Toomre described the change of paradigm, writing
that for many researchers of the 1970s, “NGC 520 has instead seemed a pairwise galaxy
merger well underway . . . whereas M82 has instead been exhibiting serious signs of indi-
gestion brought on by eating too much interstella
r gas from the outer disk of M81 during a
fairly recent (and direct-sense) close passage that seems also to have caused the unusually
wide-open pair of beautiful outer spiral arms in M81 itself!”16 (Plates 7.1 and 11.1).
An interesting story is that of the Carnegie Observatories astronomer François
Schweizer: “In the middle of the summer of 1972, Ivan King came back from a meet-
ing and handed me a preprint of the Toomre & Toomre (1972) paper on Galactic Bridges
and Tails. In reading this preprint, I instantly understood that they were opening a first win-
dow on galaxy evolution, and I immediately turned much more to Arp’s Atlas of Peculiar
Galaxies to study the subject and prepare a research proposal for my job applications in
1973. It is this proposal that got me a Carnegie Fellowship (1974–75) and launched me into
researching whether ellipticals could originate from mergers of spirals, as TT72 suggested.
The rest is history.”17,18 Arp’s atlas inspired the work of François Schweizer and the Space
Telescope Science Institute astronomer Brad Whitmore’s long research program on inter-
acting galaxies. They demonstrated that collisions and mergers have shaped galaxies and
determined their stellar and gaseous content in ways Arp would not have expected.
The digital age has brought with it a massive number of images, which has changed the
course of the impact of atlases. The latest and most unusual galaxy project is the Galaxy
Zoo, a wonderful example of the emergence of “big data” science and a new way of involv-
ing a very broad community in science projects. Huge samples are used to nail down bet-
ter the transformative processes that galaxies undergo during their lifecycle. The Indian
astronomer Preethi Nair of the University of Alabama worked at classifying several thou-
sands of galaxies from the Sloan Digital Sky Survey. She went through many of the atlases
and wrote: “The version I used as a template for my classification was The Carnegie Atlas
of Galaxies. The book sat on my desk for two years. One reason I used it more often was
because it was available on-line so I could refer to it from any location.”19
These examples of reactions and inspirations for programs of researchers are only a
few among many others. In summary, atlases of galaxies with carefully organized images
and sequences of images are a sort of “shopping window” on what the universe is offer-
ing as products of complex physical processes with long evolutionary histories. The atlases
described in Chapter 10 differed enough from each other to provide contrasting and incre-
mental views on the extremely rich world of galaxy shapes. The diversity of programs and
projects that emerged from looking at and working with atlases is a powerful testimony to
the fact that images, especially in their systematic presentation within these atlases, trig-
gered the generation of new knowledge.
16 A. Toomre, private e-mail to the author (October 2014).
17 Private e-mail note to the author (September 2014).
18 A. Toomre and J. Toomre, Galactic Bridges and Tails, The Astrophysical Journal, 1972, Vol. 178, pp. 623–666.
19 Preethi Nair, private e-mail to the author (October 2014).
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Part III – Organizing the World of Galaxies
Fig. 11.1 Revised S0 branch. From The Astrophysical Journal Supplement. C
AAS. Reproduced
with permission.
And now for a a trickier question: Can one predict shapes/objects using the classifica-
tion schemes? A fascinating recent development in the physical morphology of galaxies
is that the so-called lenticulars, S0, once considered to be transitional (Chapters 6 and 9)
between ellipticals and spirals (Sa) may not be transitional at all. Sidney van den Bergh had
already suspected this in 1998.20 The American astronomer John Kormendy and the Ger-
man, Ralf Bender, have proposed that they should be added as a new separate branch parallel
to spirals. This challenge to the Hubble tuning fork has received solid support from recent
observations and numerical simulations.21 The new sequence would be a natural home of
an important class of galaxies that has been a “troublesome kid” for decades. “The revision
to the tuning fork represents the coming to fruition of quantitative and interpretative galaxy
morphology.”22 This is also a dramatic demonstration that classification schemes of galax-
ies, as illustrated by atlases, can lead to completely new perspectives. In 2012, Kormendy
and Bender proposed a major revision of the parallel-sequence morphological classification
of galaxies. They added a parallel sequence with lenticulars having their own S0a, S0b, S0c
and Sph suite to mark the bulge-to-total ratio diminishing from left to right (Fig. 11.1).
Book Reviews of the Atlases: What do they Say?
Book reviews offer interesting windows into the contemporary research and academic con-
text of a given discipline at the time of publication. All atlases were objects of book reviews,
but in varying numbers; these reviews were generally published in well-known astronomy
or general science magazines (Table 11.1). Reviews that appeared shortly after the publica-
tion of the various atlases are also revealing of their reception and of their expected impact.
20 S. van den Bergh, Galaxy Morphology and Classification, Cambridge: Cambridge University Press, 1998.
21 J. Kormendy and R. Bender, A Revised Parallel-Sequence Morphological Classification of Galaxies: Structure and Formation of S0 and Spheroidal Galaxies, Astrophysical Journal Supplement Series, 2012, Vol. 198.
22 R. J. Buta, Galaxy Morphology, In Planets, Stars and Stellar Systems, Vol. 6: Extragalactic Astronomy and Cosmology, Dordrecht: Springer, 2013, p. 83.
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Today, they make fascinating and revealing reading. According to their general remit, these
book reviews highlighted the innovative features of the atlases, described their format and
presentation, and estimated their potential impact on the discipline. The emphasis by the
reviewer was often about the quality of the publication and format, as expected for major
reference books such as atlases. The comments on the aesthetic value of the atlases were
often surprising and entertaining. Let us pick a few of the most noticeable perspectives.
On The Hubble Atlas of Galaxies (1961), American astrophysicist George Field wrote
in glowing terms in The American Scientist: “ . . . at $10.00, it is a bargain you are not likely
to find again in your lifetime.” Highlighting the breathtaking illustrations, he added: “An
artistic achievement, this volume is also a scholarly work of major proportions.”23 In the
Journal of the Royal Astronomical Society of Canada, Sidney van den Bergh praised “the
finest collection of photographs of external galaxies” ever published, but he did not refrain
from a minor criticism: for disk galaxies, the S0 and Sa types may not correspond to a
“physically distinct type of object” and instead appear to have become “depository for sys-
tems having widely differing physical characteristics.”24 There appeared to be a missing
link between ellipticals and disk galaxies: S0 were thought to be the intermediary step. V
an
den Bergh felt the class was a “hotchpotch” box. Considering the more recent proposal of
Kormendy and Bender, just mentioned above, that lenticulars were a fully separate mor-
phological class, this early comment from van den Bergh was visionary.
Writing an extensive review for the prestigious scientific weekly Science, the American
astronomer Frank Edmondson quoted generously from Sandage’s introductory text to the
atlas:25 “The Hubble Atlas of Galaxies is a major contribution to the observational study
of cosmology.” He even carried out a detailed account of the observers who had obtained
the photographs used in the atlas. The 186 photographs in all of the 176 different galaxies
contained in the atlas were distributed as follows: Hubble took 72, Sandage, 63, Milton
Humason, 24; and seven other astronomers took the rest. The use of halftones and the very
high quality of the material was praised as stimulating an “esthetic appreciation for fine
astronomical photographs.”
Commenting in Astronomy magazine on the gigantic size of the Atlas of Galaxies Useful
For Measuring the Cosmological Distance Scale (1988), Jeff Kanipe wrote in a humorous
quote that the new giant atlas, “the biggest galaxy book ever,” could have legs added and
become a coffee table! Walter S. Houston of the magazine Sky and Telescope highlighted
in particular the high quality of the images and re-stated the purpose set out by the authors,
Sandage and Bedke: “The programmers of the HST can use these prints to pick out regions
where Cepheids should be easiest to find.” The Cepheids of dozens of nearby galaxies were
indeed used to nail down the value of the Hubble constant, i.e. the rate of the expansion of
the universe, with extreme accuracy. The goal was achieved following a multi-year observ-
ing program with a key project using the HST led by the Canadian astronomer Wendy
23 G. Field, Book Review: The Hubble Atlas of Galaxies, American Scientist, 1962, Vol. 50, p. 212A.
24 S. van den Bergh, Review of Publications: The Hubble Atlas of Galaxies, Journal of the Royal Astronomical Society of Canada, 1962, Vol. 56, pp. 29–30.
25 F. Edmondson, Reconnaissance of Outer Space, Science, 1961, Vol. 134, p. 464.
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