The Role of Images in Astronomical Discovery
Page 32
access to the Mount Wilson and Palomar observatories’ archives of superb photographic
5 D. F. Malin, Astronomical Photography, in Gérard and Antoinette de Vaucouleurs: A Life for Astronomy, M. Capaccioli and H. Corwin Jr. (editors), Singapore: World Scientific, 1989, pp. 53–64.
14:11:09, subject to the Cambridge Core
.012
10. Atlases of Galaxies, Picturing “Island-Universes”
207
plates, and encouraged Gérard to develop his own classification scheme.6 At that point in
his career, Sandage had obtained a relatively small percentage of the total number of plates
taken at these observatories. Then the de Vaucouleurs moved to the University of Texas
and McDonald Observatory in 1960 where they spent the rest of their careers. The couple
traveled together all the time. Wherever they were visiting, they always requested a quiet
office where they would work. They devoted their lives to astronomy, and spent little time
away from it. Antoinette was a keen observer herself. In 1957, she had noted that the bright-
ness of the central regions of several Seyfert galaxies (Chapter 7) varied perceptibly over a
period as short as a month. A skeptical Gérard told Antoinette at that time that if anything
in the universe did not vary, it was surely the galaxies. “That was the greatest error of my
life!” Gérard said.7 A rapid variability of the nuclear luminosity of Seyfert galaxies is now
commonly observed and understood as energetic events taking place in the accretion disks
around central black holes. Antoinette passed away in 1987. Gérard de Vaucouleurs, who
died in 1995, is considered a giant of twentieth-century astronomy, and Antoinette was a
pillar of the modern extragalactic field of astronomy.
The Ingredients of Galaxy Atlases
As seen in the previous chapter, the hundreds of images of galaxies that had been obtained,
especially in the first two decades of the twentieth century, provided a solid basis for artisans
of galaxy classification to develop discriminating and meaningful schemes. Despite some
limitations (for example, no two galaxies look exactly the same), the shapes of galaxies offer
a reliable, relatively non-arbitrary, epistemological basis for constructing atlases. Sorting
galaxies by morphology turned out to be possible, and meaningful, as several properties
correlate with shape. Many had proposed that morphology can be traced to fundamental
processes governing the formation, assembly and evolution of galaxies. By 1960, this auda-
cious and relatively simple idea, which was put forward by several during the nineteenth
century, had been vindicated.
Well-established and working at the world’s foremost observatory, Edwin Hubble ascer-
tained his overview of galaxy shapes concisely: “About 3 per cent are irregulars, but the
remaining nebulae fall into a sequence of type forms characterized by rotational symmetry
about dominating nuclei. The sequence is composed of two sections, the elliptical nebulae
and the spirals, which merge into each other.” In his 1936 The Realm of Nebulae, Hubble
published several fine sets of galaxy photographs of different morphology to illustrate his
proposed classification scheme.8 Although sometimes accused of ignoring others’ merits
and efforts to tackle the tricky subject, it is fair to say that Edwin Hubble has been a prime
mover in the contemporary classification of galaxies.
6 E. M. Burbidge, Gérard de Vaucouleurs 1918–1995, A Biographical Memoir, Washington: National Academy Press, 2002, Vol. 82, pp. 1–17.
7 E. M. Burbidge, Gérard de Vaucouleurs 1918–1995, A Biographical Memoir, Washington: National Academy Press, 2002, Vol. 82, p. 11.
8 E. P. Hubble, The Realm of Nebulae, New Haven: Yale University Press, 1936.
14:11:09, subject to the Cambridge Core
.012
208
Part III – Organizing the World of Galaxies
Hubble’s original scheme was revised and expanded by de Vaucouleurs, in 1959,9 and
by Allan Sandage in 1961.10 In both these refinements, the fundamental criterion remained
the shape of any given galaxy. Hubble’s “tuning-fork” model uses the progressive flattening
from ellipticals to spirals and highlights “form determined by rotational symmetry about
dominating nuclei.” An especially important add-on was the introduction of a transition
stage: the lenticulars, a “bin” fit between ellipticals and spirals. Later schemes, and that of
de Vaucouleurs in particular, included a better arrangement for the irregulars or Magellanic
systems, as originally proposed by Lundmark. It is no surprise that Hubble’s scheme has
been at the root of the majority of the galaxy atlases that were subsequently published.
There were some dissenters with regard to the simplified view put forward by Hubble.
“Too simple,” many said. As we will see in this chapter, Halton Arp with his Atlas of Pecu-
liar Galaxies, and to a lesser degree José Luis Sérsic with his Atlas de galaxias australes,
deviated from Hubble’s elegant scheme. Arp and Sérsic strongly emphasized “abnormal”
morphologies, most often caused by an interaction or merger. These contrarians showed
that galaxy atlases, as reference visual displays, were not only a way of identifying and
selecting archetypes, but also a tool for highlighting complexity or deviants.
The following sections detail the main galaxy atlases published over the last 60 years
(Fig. 10.2). The goal(s) or purpose(s) of each one are summarized, including the decision
that went into their design, and their contents are briefly described. I show how these atlases
of galaxies have been and continue to be “working objects” as defined in the Introduction.
Chapter 11 deals more specifically with the users and the impact of these atlases. The reader
just wishing for a quick overview of these atlases may concentrate on the first three and last
one below, and then go straight to the next chapter.
A Compendium of Galaxy Atlases
1961. The Hubble Atlas of Galaxies , Portraying Perfect Galaxies
The American astronomer Allan Sandage (1926–2010) put together the lavishly illustrated
mother of all atlases, The Hubble Atlas of Galaxies. Sandage had been an assistant of Hubble
and he became one of the most influential astronomers of the twentieth century (Fig. 10.3).
Sandage obtained his Ph.D. at Caltech in 1953, the year Hubble passed away. His supervisor
was Walter Baade (Chapter 6). Published in 1961, this first great atlas is the embodiment
and pillar of all galaxy atlases. Several generations of researchers and students have used
the atlas extensively since then. The American astronomer Ronald Buta, lead author of
The de Vaucouleurs Atlas of Galaxies, describes the impact of Sandage’s atlas on his own
professional trajectory: “The availability of this atlas at a price a high school student could
afford brought galaxy morphology to an audience well beyond professional astronomers.
Plus the quality of the reproductions and the general layout are so well done. The images
are fascinating, and there was so much mystique connected with the different galaxy types
9 G. de Vaucouleurs, Classification and Morphology of External Galaxies, Handbuch der Physik, 1959, Vol. 53, p. 275.
10 A. R. Sandage, The Hubble Atlas of Galaxies, Washington: Carnegie Institution of Washington Publications, 1961, No. 618.
14:
11:09, subject to the Cambridge Core
.012
10. Atlases of Galaxies, Picturing “Island-Universes”
209
Fig. 10.2 Atlases of galaxies, a montage by Zoltan Levay and the author. With permission of Carnegie
Science, University of Tokyo Press and Universidad de Córdoba, Observatorio Astronómico.
Fig. 10.3 Allan Sandage, 1973. Credit: Courtesy of the Archives, California Institute of Technology.
14:11:09, subject to the Cambridge Core
.012
210
Part III – Organizing the World of Galaxies
at the time. What the atlas showed captivated my imagination and directly contributed to
my interest in ringed galaxies, the subject of my PhD thesis.”11 There is more on the impact
of the atlas in the next chapter.
The atlas title carries the name of astronomer Hubble for a good reason. Edwin Hubble
himself had initiated the atlas project to systematize his morphological classification of
galaxies. As described in the previous chapter, Hubble presented his classification scheme
at the International Astronomical Union Meeting in 1925. Soon after, he published it as a
statistical investigation of 400 extragalactic nebulae in 1926.12 In this highly cited work,
Hubble used the inferred three-way morphology of galaxies to set the tuning-fork system,
where normal and barred spirals merge with each other into the elliptical class (see Plate
9.1). Hubble’s 1926 article influenced Harlow Shapley and Adelaide Ames when they put
together their 1932 catalogue of the brightest nearby galaxies (Chapter 9).
At the dedication of the McDonald Observatory (Texas) in 1936, Hubble declared: “The
structural forms of nebulae can be investigated with confidence because the necessary data
are already available in the collection of direct photographs made with large reflectors.”
With Hubble’s untimely death in September 1953, the classification project was left unfin-
ished. As Hubble’s intellectual heir, Allan Sandage inherited the unique collection of pho-
tographic plates of galaxies obtained by his mentor and other Mount Wilson observers. The
27-year-old Sandage did not delay and took up the reins of the extraordinary legacy. Using
Hubble’s notes, he re-inspected all the photographic plates that Hubble had used. Sandage
re-asserted the main classification groups of Sa, Sb, Sc and Sd. He also refined the clas-
sification of the intermediary group (lenticulars) between ellipticals and spirals by using
commentaries about E, S0, SB0 and Sa galaxy types that he gathered throughout Hubble’s
various notes and papers.
The photographic plates selected for the atlas came from the “magnificent set of plates”
of galaxies brighter than photographic magnitude 13.0, north of declination minus 30°.
These plates had been obtained with the Mount Wilson Observatory 60-inch and 100-inch
telescopes between 1919 and 1948. To complete the new atlas, Sandage inserted the material
from photographic plates just obtained with the new Mount Palomar 200-inch. A total of
176 separate galaxies were illustrated, accompanied by brief descriptions of observational
data and numerous side comments. Sandage added a “variety” dimension to the Hubble
system, and even included some “freak” cases.
As a working object, The Hubble Atlas of Galaxies, complete with its images, demon-
strated the soundness and usefulness of morphology for classifying and understanding
galaxies. It provided the visual forms or exemplars to apply the concept to other galax-
ies. The simple tuning-fork sketch was convivial and could be used effectively by a broad
community of students and researchers.
Sandage had a grand exploration scheme in mind. He designed the atlas to explore the
structural forms of galaxies as the initial step of a broad program of galaxy investigation.
11 E-mail notes to the author, October 2013.
12 E. P. Hubble, Extra-galactic Nebulae, The Astrophysical Journal, 1926, Vol. 64, pp. 321–369.
14:11:09, subject to the Cambridge Core
.012
10. Atlases of Galaxies, Picturing “Island-Universes”
211
“The Hubble Atlas is but one stage in a developing program of nebular research,” wrote
Sandage in the introduction to the atlas. Indeed over his career, Sandage would complete
and extend Hubble’s work in a grandiose way, producing three additional atlases of galaxies
published later. The best way to describe Sandage’s vision is to quote the revealing sub-title,
Breaking the Code of Cosmic Evolution, he gave to his superb book on the history of Mount
Wilson Observatory.13
As I will demonstrate in the next chapter in describing the scholarly impact of galaxy
atlases, The Hubble Atlas of Galaxies was an outstanding publication: it assembled the finest
pictures of what had been seen as the world of galaxies and provided a guide on how to look
at them in the proper way. Decades later, the value of Sandage’s atlas is solidly recognized.
MIT astrophysicist Alar Toomre emphasized “the extensive effort that Sandage made in
that volume to construct at least a modest sense of order within this zoo.”14 But a “zoo” it
remained for other researchers.
1966. Atlas of Peculiar Galaxies , Picturing Roguish Galaxies
The Hubble world of galaxies was too tranquil. The Swiss astronomer Fritz Zwicky (1898–
1974), Russian astronomer Boris Vorontsov-Velyaminov (1904–1994) and, almost contem-
poraneously, Argentine astronomer José Luis Sérsic (1933–1943) had already pointed out
strong morphological deviations from the elegant scheme initially proposed by Hubble.15
It was no surprise that at least one leading astronomer was taken aback by Hubble’s pol-
ished classification system. Every galaxy is “peculiar,” vigorously wrote the American
astronomer Halton Christian (“Chip”) Arp (1927–2013) (Fig. 10.4). Along with Sandage,
Arp had been a student of Walter Baade, at Caltech. He was also working at the Mount
Wilson and Palomar Observatories, the same institution as Hubble and Sandage. Arp
wanted to do something drastic to convey a different and more complete perspective of
the world of galaxies: he produced another atlas, a very different one.16,17 When published
in 1966, Arp’s Atlas of Peculiar Galaxies was received as a bombshell and even a frontal
attack on the “authoritative” work of Hubble and Sandage. Arp trumpeted that the world of
galaxies could and should be looked at in a very different way.
Several researchers had been uneasy with Sandage’s apparent ignorance of galaxies
showing signs of interaction. Even the “wrapped cigar” NGC 2685 or the obviously dis-
turbed Messier 81, Messier M82 and NGC 520, all shown together on the same page in The
Hubble Atlas of Galaxies, were considered “normal.” Sandage’s comment on NGC 520
is revealing as he wrote: “Inspection of the original plate suggests that NGC 520 is not a
13 A. R. Sandage, Centennial History of the Carnegie Institution of Washington, Volume 1: The Mount Wilson Observatory, Cambridge: Cambridge University Press, 2004.
14 A. Toomre, private e-mail to the author (16 October, 2014).
15 See F. Zwicky, Multiple Galaxies, Handbuch der Physik, 1959, Vol. 53, p. 373.
16 H. C. Arp, Atlas of Peculiar Galaxies, Astrophysical Journal Supplement Series, 1966, Vol. 14, pp. 1–20, and 57 plates
of photographic reproductions. As noted in the abstract, “The Atlas was also available in large size, 11 x 14-inch photographic reproduction, from the California Institute of Technology Bookstore for a price of about $60 bound.”
17 A very nice reproduction of the original atlas as well as an extensive commentary is found in J. Kanipe and D. Webb, The Arp Atlas of Peculiar Galaxies, A Chronicle and Observer’s Guide, Richmond: Willmann-Bell, Inc., 2006.
14:11:09, subject to the Cambridge Core
.012
212
Part III – Organizing the World of Galaxies
Fig. 10.4 Halton Arp at the Cassegrain focus of the Mount Palomar 200-inch telescope, preparing
for observations with an image intensifier, 1967. Credit: Image courtesy of the Observatories of the
Carnegie Institution for Science Collection at the Huntington Library, San Marino, California.
collision of two galaxies but rather a system of the M82 type.”18 Shrewdly, Arp character-
ized Hubble’s scheme as an “idealization.” Arp’s contrarian atlas immediately occupied a
18 A. R. Sandage, The Hubble Atlas of Galaxies, Washington: Carnegie Institution of Washington Publications, No. 618, 1961, pp. 41–42.
14:11:09, subject to the Cambridge Core
.012
10. Atlases of Galaxies, Picturing “Island-Universes”
213
special place on astronomical library bookshelves, because it highlighted very vividly and
with powerful images an obvious fact: that many galaxies in the present-day universe do not
fit well under any of the Hubble-based classification systems (Figure 6.7 and Plate 10.1).
Where Was Arp Coming From?
Arp’s perspective of the chaotic world of galaxies had already transpired in his review of
the proceedings of the 13th Solvay conference on physics at the University of Brussels
in 1964 that dealt with The Structure and Evolution of Galaxies. Arp felt that phenomena
displayed by peculiar galaxies might be indicative of unknown physics. He was particularly
impressed by the ideas of Armenian astronomer Victor Ambartsumian (1908–1996). Arp
wrote then: “ . . . Ambartsumian’s introductory report, on the nuclei of galaxies and their
activity, is outstanding. It courageously deals with the most important subject of all – the