The Role of Images in Astronomical Discovery
Page 28
fore, galaxies behave just like atoms in the deep potential created by the mass of the dark
matter. Neither could escape. And the more massive the cluster was, the hotter the gas was
found to be.
Even today, the origin of the gas remains a mystery, especially because it exists in such
huge amounts. The suspicion is that it was produced by the explosions of past generations
of massive stars located within the member galaxies of the cluster: the expelled gas from
these galaxies dispersed into the surrounding medium, distributing itself evenly around a
central condensation. The X-ray-emitting gas shows more spherical symmetry than the dis-
tribution of the individual galaxies. It maps the dark matter more closely than the galaxies.
X-ray imaging from space has become the way to image indirectly the invisible mass in the
universe. It is safe to say that imaging the X-ray emissions of galaxy clusters is imaging
the dark matter distribution. It is one of the most direct ways to “see” dark matter, at least
visualize its presence, and the extent to which it carves the deep gravitational potentials of
clusters into the fabric of spacetime.
Rotational Curves of Disk Galaxies
Not too dissimilarly from Jansky’s discovery of cosmic radio emission, “Zwicky’s findings
and arguments were not much noticed. It took about forty more years until observational
evidence clearly showed that large amounts of invisible matter, gravitating matter was lodg-
ing in the disks of rotating galaxies. Without it, the galaxies would fly apart because of a
too weak gravitational attraction.”14
However, another prescient young astronomer had opened Pandora’s box of dark matter.
In his 1939 Ph.D. thesis work on the rotational velocity of the Andromeda “nebula,” Horace
Babcock (1912–2003) had found abnormally high rotational speeds in the outer parts of
the Andromeda Galaxy (Fig. 8.3).15 While Babcock refrained from making a link with the
13 Plasma physicists express temperature in electronvolts (eV) or kilo-electronvolt (keV), where 1 eV = 11,605 K.
14 H. S. Kragh, Conceptions of Cosmos – From Myths to the Accelerating Universe: A History of Cosmology, Oxford: Oxford University Press, 2007, p. 214.
15 H. W. Babcock, The Rotation of the Andromeda Nebula, Lick Observatory Bulletin, 1939, Vol. XIX, No. 498, pp. 41–51.
Plate III shows the image of M31, the points where spectroscopic measurements were obtained and a plot of the galactocentric velocities. Babcock concluded his extraordinary article comparing the kinematics of Messier 31 with that of the Milky Way.
14:05:47, subject to the Cambridge Core
.010
180
Part II – Images as Galaxy Discovery Engines
Fig. 8.3 Horace Babcock’s rotation curve of the Andromeda Galaxy from his Ph.D. thesis, based
on spectroscopic measurements with the Crossley 36-inch telescope. The most distant galactocentric
points were derived from gaseous nebulae. From Babcock (1939), Lick Observatory Bulletin.
invisible mass inferred by Zwicky in the Coma cluster, he warned in the same way about the
uncertainties of deriving masses of “nebulae” from spectroscopic rotational curves. Proba-
bly less bold than Zwicky, the young Babcock invoked absorption of light increasing out-
ward from the nucleus, or some “internal gravitational viscosity” to be possibly greater than
supposed and to produce the unexpected high orbital velocities of stars in Messier 31. A
few decades later, radio observations of neutral hydrogen at 21 cm in M31 indicated that
rotational speeds remained constant instead of falling in a Keplerian fashion as one gets
further and further away from the center of the galaxy. This finding, 36 years after Bab-
cock’s noteworthy discovery, came from the American astronomers Morton S. Roberts and
Robert N. Whitehurst who published their radio work in 1975.16
“These observations indicated that the mass in the outer regions of the Andromeda
galaxy increased with galactocentric distance, even though the optical luminosity of M31
did not.”17 Sidney van den Bergh noted that neither Babcock nor Roberts and Whitehurst
“A new discrepancy is now directly apparent when the rotations of the two systems are compared, for the nearly constant angular velocity of the outer parts of M31 is the opposite of the ‘planetary type’ of rotation believed to obtain in the outer parts of the Galaxy.”
16 M. S. Roberts and R. N. Whitehurst, The Rotation Curve and Geometry of M31 at Large Galactocentric Distances, The Astrophysical Journal, 1975, Vol. 201, pp. 327–346.
17 S. van den Bergh, The Early History of Dark Matter, Publications of the Astronomical Society of the Pacific, 1999, Vol. 111, p. 660.
14:05:47, subject to the Cambridge Core
.010
8. Imaging the Invisible
181
Fig. 8.4 The rotational curve of the spiral Messier 33. Credit: Stefania Deluca.
cited the 1933 or 1937 papers by Zwicky and his inference of dark matter.18 Instead, as
Babcock had hypothesized, Roberts and Whitehurst attributed the hidden mass to an abun-
dance of dwarf M stars, the most common type of stars in the solar vicinity. These pioneers
remained oblivious to the elephant in the room despite their highly reliable measurements.
In 1970, the Australian astronomer Kenneth Freeman suggested “undetected” and “addi-
tional” matter beyond the optical extent of several nearby galaxies.19 A young American
female astronomer and her colleagues picked up the thread three decades after Babcock’s
early hint. Studying the behavior of the rotational curves (the velocity of rotation as a func-
tion of distance from the galaxy center) in the 1970s, American astronomer Vera Rubin
(1928–2016) and her colleagues showed, for several spiral galaxies, that their masses were
many times that of their visible stars and gas content (Fig. 8.4). The team established com-
pelling evidence for the presence of a large amount of dark matter in individual galaxies.
They also found that the invisible mass was much less concentrated than visible light. “By
the late 1970s dark matter had been discovered, not just hypothesized, and in amounts much
larger than visible matter.”20
James Binney and Michael Merrifield warned, cautiously, “the emergence of a tenuous
distribution of utterly dark material cannot be ascertained by studying circular-speed curves
and photometric profiles alone.”21 Dark matter was indeed difficult to swallow, as most
astronomers and, at first, physicists had ignored it. In an interview, Vera Rubin stated: “I
think many people initially wished that you didn’t need dark matter. It was not a concept that
people embraced enthusiastically. But I think that the observations were undeniable enough
18 S. van den Bergh, The Early History of Dark Matter, Publications of the Astronomical Society of the Pacific, 1999, Vol. 111, p. 660.
19 K. C. Freeman, On the Disks of Spiral and S0 Galaxies, The Astrophysical Journal, Vol. 160, pp. 811–830.
20 H. S. Kragh, Conceptions of Cosmos – From Myths to the Accelerating Universe: A History of Cosmology, Oxford: Oxford University Press, 2007, p. 214.
21 J. Binney and M. Merrifield, Galactic Astronomy, Princeton: Princeton University Press, 1998, p. 510.
14:05:47, subject to the Cambridge Core
.010
182
Part II – Images as Galaxy Discovery Engines
Fig. 8.5 Weak gravitational lensing inducing the distortion of galaxy
shapes, assumed here to be
ellipses. The effect shown is exaggerated relative to real systems. Credit: Wikipedia Commons/
TallJimbo.
so that most people just unenthusiastically adopted it.”22 Moreover, theoretical astrophysi-
cists needed it to stabilize galaxy disks. Without dark matter, spiral galaxies would just fly
apart like broken flywheels.
Imaging with Gravitational Lenses
As predicted by general relativity, concentrations of mass between us, as cosmic observers,
and distant sources curve the fabric of spacetime. This curvature is capable of bending light
paths. If there is a distant light source beyond a large mass like a cluster of galaxies, the
interfering mass acts like a lens: it amplifies the apparent brightness of the distant source
and distorts its shape. This weird phenomenon is called lensing, or mirage (Plate 8.2).
When the lensing effect is important, we have strong lensing. In its more subtle manifes-
tation, it introduces small distortions in the shape of the object and is called weak lensing
(Fig. 8.5). Although posited by our visionary Fritz Zwicky in 1937, the large-scale lensing
22 Interview with Dr. Vera C. Rubin by Alan Lightman, Washington, April 3, 1989, Niels Bohr Library Archives, Center for History of Physics.
14:05:47, subject to the Cambridge Core
.010
8. Imaging the Invisible
183
by spacetime curvature was first observed only in 1979.23 With today’s telescopes and their
fine imaging capabilities, gravitational lensing is commonly observed. The Hubble Space
Telescope and large ground-based telescopes have produced spectacular images of many
such cases. As predicted by Zwicky, the most dramatic lensing effects are those created by
large clusters of galaxies.
Gravitational lensing is an independent and powerful way of mapping dark matter in
galaxies and clusters of galaxies and for viewing the most distant galaxies. Again, it was
Zwicky who suggested in his seminal 1937 paper that gravitational lensing might be used
to “weigh” galaxies. Distant galaxies viewed through the field of a midway galaxy cluster
will have their shape distorted into elongated arcs. The amount of distortion is a measure
of the mass of the cluster.24
Strong lensing is active through the inner part of the cluster and weak lensing in its outer
parts. In gravitational optics, mathematical models are applied to reconstruct the shape of
the distant galaxies and to estimate the amount of brightness magnification. As an inverse
mathematical solution, the reconstruction spills out the total mass of the cluster. Relativistic
physics applied to images of lensing clusters offers an elaborate and powerful way to image
dark matter. By carefully mapping the distorted arcs, astronomers have been able to esti-
mate not only the mass of the lens, i.e. the mass of the intervening cluster of galaxies, but
also its geometry and spatial distribution. Lensing measurements and gravitational-potential
reconstructions have confirmed that the major part of the mass of galaxy clusters is dark
matter. The amounts derived are consistent with those inferred from the virial and X-ray
measurements.
Moreover, large concentrations of dark matter in clusters can be employed as gravita-
tional telescopes. Their light-amplifying power is exploited to probe the distant universe.
Consequently, the lensing of clusters of galaxies allows us to look beyond and see the most
distant galaxies in the universe, reaching even the youngest objects, born soon after the Big
Bang. It is amazing that we use pockets of invisible matter as telescopes to look and image
further in space and time. So, we know where dark matter is, how much there is, but we do
not know what it is.25
This chapter on dark matter concludes the overview of modern ideas about galaxies and of
how they developed using a whole range of imaging techniques and tools. Let us shift to
a more targeted topic, the use of images in the pursuit of the world of galaxies: atlases of
galaxies, their history as books of discovery and as tools for unveiling galaxies and learning
about them.
23 D. Walsh, R. F. Carswell and R. J. Weynmann, 0957+561 A, B: Twin Quasi-Stellar Objects or Gravitational Lens?, Nature, 1979, Vol. 279, pp. 381–384.
24 The angle of deflection θ is given by θ = 4 GM/( rc 2) toward the mass M at a distance r from the affected radiation; G is the gravitational constant and c the velocity of light.
25 There are several excellent books on dark matter. For example, K. Freese, The Cosmic Cocktail, Three Parts of Dark Matter, Princeton: Princeton University Press, 2014; L. Randall, Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe, New York: HarperCollins Publishers, 2015.
14:05:47, subject to the Cambridge Core
.010
14:05:47, subject to the Cambridge Core
.010
Part III
Organizing the World of Galaxies
14:12:49, subject to the Cambridge Core
14:12:49, subject to the Cambridge Core
9
The Galaxy Classification Play-Off
The establishment of an ordering of galaxies into taxonomic categories is
clearly only one step towards understanding the physical nature of galaxy
structure.
Ronald J. Buta 1
It is of supreme importance to the creation of our operational morpholo-
gies that we use the “things themselves” as our picture-language for stel-
lar spectra, and direct photographs for galaxies and clusters of galaxies.
W. W. Morgan 2
If the original Hubble scheme were to be called a Mark I model, the
revised but unpublished Hubble–Sandage system might be regarded as
Mark II, and the writer’s present system as Mark III.
Gérard de Vaucouleurs 3
Is the World of Galaxies too Chaotic for a Meaningful Classification?
Adelaide Ames (1900–1932), the Pickering Fellow in 1923, was the first woman graduate
student in astronomy at Harvard University (Fig. 9.1). She worked on “bright spiral neb-
ulae” with the ebullient Harvard College Observatory director Harlow Shapley (Fig. 5.2).
Ames is seen on some of the photographs picturing life at the observatory in the 1920s and
1930s. She died tragically in a canoe accident at Squam Lake, New Hampshire, on June 26,
1932, the year of the publication of the Shapley–Ames catalogue of bright galaxies of which
she was co-author. She was probably one of the first astronomers to have had a full, though
too short, career devoted entirely to the study of galaxies. She worked on the properties of
galaxies and of clusters of galaxies and found deviations from isotropy in the general distri-
bution of galaxies. Ames had been appointed to the Commission of Nebulae and Clusters at
the International Astronomical Union Assembly of Leiden, the Netherlands, in 1928. One
wonders if Ames foresaw how explosively the field of galaxy research would develop just
1 Ronald J. Buta, http://ned.ipac.caltech.edu/level5/March01/Buta/Buta4.html
2 W. W. Morgan, A Morphological Life, Annual Review of Astronomy and Astrophysics, 1988, Vol. 26, pp. 1–10.
3 Gérard de Vaucouleurs, Classification, Dimensions, and Distances of Bright Southern Galaxies, Sky and Telescope, 1957, Vol. 16, p. 582.
187
14:11:09, subject to the Cambridge Core
.011
188
Part III – Organizing the World of Galaxies
Fig. 9.1 Adelaide Ames. Credit: Schlesinger Library, Radcliffe Institute, Harvard University.
in the few years following her untimely death. Classifying galaxies was a key feature of the
new catalogue she and her supervisor had put together.4 This catalogue would become the
roadmap and guide of research on galaxies for the remainder of the twentieth century.
Atlases as Reference Visual Displays
On removing oneself a little from the multitude of objects and physical components, it
is possible to find order in the jungle of galaxies. In contrast to gaseous nebulae, chaotic
processes are present but rarely dominate the morphology of a galaxy. As seen in previous
chapters, galaxies, like butterflies, come in a few basic shapes: disk-shaped and spheroidal
systems that define rather continuous sequences of silhouettes. The challenge has been to
build a meaningful and accepted sequence of forms. The story of establishing a significant
classification system is instructive and fascinating to reconstruct. Atlases of galaxies will
help us (Chapter 10).
Galaxy atlases, being packed with images, are essential and advantageous vehicles of
any galaxy classification scheme. Using representative cases, astronomers set themselves
the task of designing and producing these atlases, of which about 18 in total emerged from
various inspirations; they followed their own convoluted paths, including controversial out-
comes in the ways galaxies are viewed. The atlases have been a significant part of the story
of understanding galaxies by classifying them.
Galaxy atlases are reference visual displays (Fig. 0.6); thus the creation of an atlas is an
epistemological undertaking. Atlases are also heuristic tools for identifying and selecting
4 H. Shapley and A. Ames, A Survey of the External Galaxies Brighter Than the Thirteenth Magnitude, Annals of the Astronomical Observatory of Harvard College, 1932, Vol. 88, pp. 41–76.
14:11:09, subject to the Cambridge Core
.011
9. The Galaxy Classification Play-Off
189
archetypes, for discovering hints of more complexity, and for making use of qualitative, then
quantitative, patterns in a reproducible way. As Lorraine Daston stated, discerningly, atlases