The Role of Images in Astronomical Discovery Read online

  universal force permeating interstellar space. The small grain can become magnetized.

  A large-scale and low-intensity magnetic field pervades the interstellar space of galax-

  ies. Its strength is weak, a million times weaker than Earth’s magnetic field, but it spreads

  on a scale of light-years. As clouds of interstellar gas and dust move around, they collide

  with each other. Like invisible springs, the embedded magnetic fields become compressed.

  Expanding bubbles, driven by the stellar winds and supernovae explosions of massive stars,

  also compress the gas and dust and its magnetic field. As this happens, electrically charged

  particles, trapped in the magnetic field lines, are accelerated to higher energies, like peb-

  bles spun with a slingshot. It was the Italian physicist Enrico Fermi (1901–1954) who

  showed that moving magnetic fields have the net effect of accelerating charged particles

  to extremely high energies.22 When the accelerated particles reach high enough energies,

  they escape their cloud and propagate through space as “cosmic rays.” These are electrically

  21 The near-infrared domain corresponds to wavelengths of 1 to 5 microns, and the mid-infrared to wavelengths of 5 to 25 microns.

  22 E. Fermi, Galactic Magnetic Fields and the Origin of Cosmic Radiation, The Astrophysical Journal, 1954, Vol. 119, pp. 1–6.

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  Fig. 6.3 Map of starlight polarization for 7,000 stars across the sky. The short lines indicate the strength and direction of the polarization (E-vector), indicative of the projected magnetic field permeating the interstellar medium. The galactic latitude is shown as the y-axis, and galactic longitude as the 14:05:49

  x-axis, with 0° corresponding to the direction of the galactic center. From Mathewson and Ford (1970), Memoirs of the Royal Astronomical Society.23

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  23 D. S. Mathewson and V. L. Ford, Polarization Observations of 1800 stars, Memoirs of the Royal Astronomical Society, 1970, Vol. 74, pp. 139–182.

  6. Galaxies in Focus

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  charged particles, mainly protons and electrons, microscopic bullets traveling at velocities

  close to the speed of light. They are a natural source of radioactivity, and when they collide

  with living cells, they can trigger mutations. Hence cosmic rays may have played a signifi-

  cant role in the evolution of living species. Details of how energetic particles also produce

  electromagnetic waves in the radio domain of the electromagnetic spectrum are given in

  Chapter 7.

  A special imaging technique, polarimetry, gives us the means to measure and map the

  magnetic fields. Here is how it works. The small dust grains are weakly magnetized. As tiny

  magnets, they line up with the general magnetic field, like the needle of the compass lines

  up with the Earth’s magnetic field. Light going through or scattered by the dust varies in

  intensity with orientation as viewed against the plane of the sky: light is polarized, i.e. it is

  more intense at a certain angle as it passes more easily through dust grains aligned in a given

  direction. The degree of polarization is a measure of strength of the magnetic field that lines

  up the grains. By analyzing the polarization of starlight over many directions in the sky, a

  map of the Milky Way’s magnetic field can be made, an informative non-homomorphic

  representation (Fig. 6.3; see also Plate 7.2).

  Furthermore, an active surface chemistry makes the interstellar dust grains micro-

  factories of complex molecules. The interstellar molecular products go from rather simple

  radicals, such as OH or CH+, or molecular hydrogen, to a whole range of molecules like

  carbon monoxide, water or more complex molecules made up of as many as 17 atoms,

  and even amino acids. The interstellar medium is particularly efficient at making water ice.

  For example, water on Earth was produced in interstellar clouds prior to the formation of

  our solar system. Molecules emit mostly in the infrared and radio domain of the electro-

  magnetic spectrum, and astronomers are able to make images of molecular clouds at those

  wavelengths.

  Shaping the Milky Way

  If the nearest star is the Sun, the nearest galaxy is the Milky Way, and we are embedded

  in it. In one of the great breakthroughs of early twentieth-century astronomy, the Amer-

  ican astronomer Harlow Shapley proved, in 1918, that we were not at the center of this

  giant system of stars as many believed until then.24 Shapley assumed zero dust and perfect

  transparency. He also mistook the short-period variable stars, which he used for determin-

  ing the distance, for brighter ones, and hence grossly overestimated the size of our Milky

  Way by a factor of three. However, his basic approach and his conclusion were correct. He

  derived that our Sun and solar system are in orbital motion around the galactic center that

  lies in the direction of the constellation of Sagittarius. It is now well established that the

  Sun and its planetary system is located at about 27,000 light-years from the center of the

  24 H. Shapley, Studies Based on the Colors and Magnitudes in Stellar Clusters – Seventh Paper: The Distances, Distribution in Space and Dimensions of 69 Globular Clusters, The Astrophysical Journal, 1918, Vol. 48, pp. 154–181.

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  Fig. 6.4 Projection of 334 open star clusters (within 1,000 parsecs of the Sun) projected on a plane perpendicular to the galactic plane. The dotted line marks the plane of symmetry of the open clusters. From Trumpler (1930), Lick Observatory Bulletins.

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  6. Galaxies in Focus

  141

  Milky Way. At the average speed of 828,000 km/h (230 km/s), it takes us about 225 to 250

  million terrestrial years to complete a full galactic revolution.

  Knowing our Milky Way and its constituents has been an essential step in understanding

  other galaxies, including those of different histories, shapes and masses. But the epistemic

  process also works in reverse. It became easier to chart our own galaxy and describe its

  shape once the extragalactic nature of “nebulae” was established. The Lick Observatory

  astronomer Robert Trumpler found inspiration in images of other spirals. He figured out

  the overall shape of the Milky Way system by mapping the distribution of star clusters. In

  his second seminal paper, also published in 1930, Trumpler presented the results of a study

  of 334 open star clusters, groups of stars that formed coevally from the same molecular

  cloud. By mapping the distribution of these clusters, Trumpler showed that the Milky Way

  had a flattened disk shape (Fig. 6.4). “The hypothesis supports the view that our Milky Way

  system is a highly resolved spiral nebula, a right-handed spiral as seen from the galactic

  north pole, of dimensions similar to those of the Andromeda nebula.”25

  Crystallizing Galaxy Shapes

  As has been shown in the previous chapters, eye observations, drawings and photographs

  helped to find order in the span of galaxy shapes. In his 1811 article, William Herschel drew

  forms of “nebulae” where basic galaxy silhouettes are recognizable (Chapters 1 and 2). Half

  a century later, the Birr Castle observers sketched most of the key shapes that twentieth-

  century observers used in designing classific
ation schemes (Chapter 9). With the ability to

  measure distances and the understanding of how dust affects our viewing, exploring the

  shapes of the various galaxies became an important area of research. Ellipticals and disks

  were the two main categories of galaxies quickly identified, both presenting varying degrees

  of flattening for ellipticals and central concentration for spirals or disk systems.

  Contrary to Curtis’ early insight, there were more than just spirals in the extragalactic

  world. As discussed briefly above, the contents and shapes of galaxies are determined by

  nature, inherited conditions at birth, and by nurture, later events from interaction with their

  environment. Galaxies can be rich or poor in interstellar gas. Their present gaseous and

  dust content reflects the conditions that have prevailed throughout galactic history. These

  are driven, first, by initial states such as the mass and angular momentum of the proto-

  galaxy, and second, by the environment, factors such as intergalactic density, or interaction

  with neighbors at birth or later.

  The New Zealander–American astronomer Beatrice Tinsley (1941–1981) produced fun-

  damental work that linked stellar evolution, gas consumption and the integrated properties

  of galaxies as they evolved. She found that “while these calculations cannot prove that the

  sequence irregular–spiral–elliptical is not an evolutionary order, they show that all the prin-

  cipal galactic types may have originated at the same time, but with some differences in

  25 R. J. Trumpler, Preliminary Results on the Distances, Dimensions and Space Distribution of Open Star Clusters, Lick Observatory Bulletin, 1930, No. 42, pp. 154–188.

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  Part II – Images as Galaxy Discovery Engines

  physical conditions that led to different stellar birth rates.”26 Modern astrophysicists have

  been able to disentangle these various factors using a range of techniques. Images of galax-

  ies have helped provide insight into the process of proto-galaxies transforming into the

  galaxies seen today.

  Galaxies as Interstellar Gas Processors

  Although their morphology and mass may have changed considerably over aeons, most

  galaxies appear to have been in place for billions of years. In addition to billions, and

  sometimes trillions of stars, galaxies hold variable amounts of gas and dust, current con-

  tents being mostly dictated by how galaxies initially came together. Primordial collapse of

  giant clouds took place at rates that depended on the large-scale properties of the primor-

  dial units and on the environment. These materials have been recycled in and out of stars

  to form successive generations of stars. This lifecycle rhythm has determined the morpho-

  logical properties of the galaxies we observe today, as inferred by Tinsley.

  The process continues today at various rates. With each generation of stars, the heavy-

  element content of stars and gas clouds is enhanced. Adding to this “closed-box” process-

  ing, the merging of galaxies, cannibalizing each other, has affected their evolution to a

  degree that we are still trying to understand. In today’s galaxies, a large fraction of the pri-

  mordial gas has been used up, having condensed into stars and planets, or lost to intergalac-

  tic space. As this process has happened at different rates, interstellar gas and dust represent

  a varying fraction of the visible mass of galaxies of the present day. In some galaxies, ellip-

  ticals for example, the gas fraction is almost zero. In the flatter spirals, the collapse of the

  proto-galaxy was slower; the gas content is still relatively high, several percent of the total

  mass.

  “Starburst galaxies” or galaxies currently undergoing intense star-formation episodes

  make for spectacular images (see Plate 6.2 and Plate 11.1). We find them at both ends of

  the scale of mass, among small irregulars and giant ultraluminous galaxies. They harbor a

  high proportion of young, massive stars that sculpt the interstellar medium of these galax-

  ies. The irregular and filamentary appearance of these galaxies is indicative of a highly

  turbulent interstellar medium ploughed by shocks and expanding super-bubbles, phenom-

  ena associated with the evolution of giant clusters of massive stars.

  Order from Chaos

  If a proto-galaxy originated from an initially slowly rotating cloud, its collapse was fast

  and led to an intense firework with a huge number of stars formed in rapid sequences of a

  few hundred million years long: most of the initial gas condensed into stars of all masses.

  This scenario can be viewed as a grand scale version of what Immanuel Kant and Pierre

  26 B. J. Tinsley, Evolution of the Stars and Gas in Galaxies, The Astrophysical Journal, 1968, Vol. 151, p. 558.

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  Simon Laplace imagined for the formation of the solar system. Then, the large number of

  very massive stars evolved rapidly into supernovae and blew away the unused gas into the

  circum-galactic and intergalactic environment. The dried-up descendants of these rapidly

  forming stellar fireworks are observed today as elliptical galaxies, or elliptical as per their

  silhouettes. Physically, they are spheroidal or oblate systems (see Plate 6.3).

  In ellipticals, stellar orbits occupy no preferential plane; they spread out in all directions

  and the multitude of stars move either in prograde or retrograde directions. One can imagine

  such systems as being somewhat analogous to gigantic swarms of bees collecting around a

  beehive, or for the stars, around a common center of mass.

  Selected directions or planes may host a denser traffic due to dynamic resonances, or

  gravitational potential asymmetry induced by the capture of a smaller galaxy. Internal reso-

  nant modes determine a variety of three-dimensional shapes, from the perfect spheroidal to

  ellipsoidal or even prolate. The whole galaxy slowly spins on itself, especially in the cen-

  tral parts where rapidly rotating cores have been mapped. There are even cases of counter-

  rotating cores, the inner core rotating in reverse from the main body of the galaxy.

  Ellipticals encompass the whole range of galaxy masses, including the most massive in

  the universe. The mammoth elliptical Messier 87, near the center of the Virgo cluster of

  galaxies, 53 million light-years away, has more than one trillion stars and is surrounded by

  30,000 globular clusters (Fig. 6.5). The least massive galaxies are also ellipticals: the dwarf

  spheroidal Leo I galaxy, 820,000 light-years away, is a tiny member of the Local Group of

  galaxies. It has a few tens of millions of stars and three known globular clusters. Likewise,

  the Sculptor dwarf galaxy harbors only a few million stars and it can hardly be distinguished

  from a blown-up globular cluster.

  Disks from Orderly Traffic

  Proto-galaxies with significant initial angular momentum collapsed more slowly than those

  that produce ellipticals. This led to a slower assembly of stars, with most of the gas assem-

  bling into a flat disk. The accumulation of gas continued over a much longer period than

  for rapidly collapsing spheroidal systems. Despite mass loss, disk galaxies have been able

  to retain a generous
gaseous reservoir. Hence, star formation is still continuing in gas-rich

  galaxies such as ours, particularly in the spiral arms, regions of higher gas density (see

  Plate 6.4).

  Consequently disk galaxies are much flatter than ellipticals and most of their stars are

  revolving in the same direction around the center of mass. All spirals and many large irregu-

  lars are disk galaxies. Like pizzas, some are flatter, some bulkier.27 One of the most striking

  features of disk galaxies is their spiral arms as discovered in Messier 51 by William Parsons

  in 1845. The spiral pattern is not due to stars or gas flowing along the arms, inward or out-

  ward. The “spirality” is a resonance pattern (Fig. 6.6). The spiral shape arises as a vibration

  27 In spirals, the diameter-to-thickness ratio of the disks varies from about 100 to 1 for the most flattened disks to 10 to 1 for the bulkier disks.

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  Part II – Images as Galaxy Discovery Engines

  Fig. 6.5 Messier 87 in the Virgo cluster about 54 million light-years away. It is the most massive

  galaxy in the local universe. This image by the Hubble Space Telescope shows the jet of energetic

  charged particles coming out of the center. Credit: NASA, ESA.

  mode of the whole disk system, which induces a concentration of mass at certain loci, which

  lines up; this leads to the beautiful spiral pattern. It is like a traffic jam, with a pile-up of gas

  at specific locations along the galactic orbit. The resonances last a few million years and

  are accompanied by an increase of density of the interstellar gas by a factor of at least two

  or three, enough to locally enhance dust and star formation. With the loci of overdense gas

  and dust, spiral arms are cradles of young stars and nebulae, which enhance their visibility.

  While spiral arms correspond to transient regions of stellar formation, the underlying older

  stellar population remains more smoothly distributed. Infrared images, which highlight the

  older stellar population, do not show the spiral pattern as strikingly.

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  Fig. 6.6 The formation of spiral arms. Graphic illustrating how slightly elliptical orbits pile up to