1 Galaxy Classification

Cambridge University Press 978-0-521-82048-6 - The de Vaucouleurs Atlas of Galaxies Ronald J. Buta, Harold G. Corwin and Stephen C. Odewahn Excerpt More information 1 Galaxy classification 1.1 Introduction and SBc, and the irregulars Irr. These he believed encom- passed nearly all the galaxies detected on photographic Galaxies are massive systems of stars, gas, dust, and other plates in his day. It was brilliant in the way the different forms of matter bound together gravitationally as a single types could be connected together in a tuning-fork-shaped physical unit. Based on deep imaging surveys, it can be sequence, and also in the way the classification did not estimated that the observable Universe includes more than depend too strongly on the orientation of a galaxy’s prin- 40 billion galaxies, spread around in complex large-scale cipal axis to the plane of the sky. The fact that measured structures, such as clusters, superclusters, and the bound- physical properties of galaxies correlated with position on aries of large, empty regions called voids. Our Sun is part the sequence gave the system a greater significance than of what we call the Milky Way Galaxy, which is a highly other classification schemes available at the time. flattened spiral galaxy containing perhaps as many as 400 Given the power of the original Hubble system, it billion stars. With large telescopes, we can see galaxies in was inevitable that it would be built upon in later years. most directions, populating even the remotest regions of Hubble himself worked on a revision to include the myste- the Universe. rious armless disk galaxies, which he labeled S0 systems, Understanding galaxies is an important part of modern and which he placed at the juncture of the tuning fork astronomy. Galaxies are fundamental units of matter in in his 1936 book, The Realm of the Nebulae. He knew space, and determining how they formed and achieved their these objects had to exist before he actually discovered current state is of critical importance to many issues in them, because the transition from E7 to Sa in his origi- astronomy. It is significant that much of what is known nal sequence seemed too “catastrophic” to be real. He had about galaxies began with a simple classification of their made notes on his revisions up until the time of his death appearance as seen on direct photographs taken with large in 1953. After that time, Hubble’s graduate student assis- observatory telescopes. Galaxies present a wide variety of tant, Allan Sandage (who was actually the Ph.D. student of forms, or morphologies, and can be naturally divided into Walter Baade), gathered up the notes and prepared a galaxy categories in much the same way as living organisms can be atlas in Hubble’s honor, called The Hubble Atlas of Galax- divided into genera and species. Although galaxy morphol- ies (Sandage 1961). The Hubble Atlas firmly cemented ogy presents problems for classification that would not be Hubble’s ideas on galaxy morphology into astronomy, and encountered in biological taxonomy, visual galaxy classifi- it stands as one of the finest galaxy atlases ever produced. It cation continues to be useful at a time when galaxies have was followed much later by The Carnegie Atlas of Galaxies never been better understood. Classification provides order (Sandage and Bedke 1994), which was based on the entire to the “impenetrable thicket of forms” – to quote Stephen Carnegie photographic plate collection and which further Jay Gould (1987), from an essay on biological taxonomy – shored up and improved the accuracy of the classification even if we do not yet fully understand how all the different system. forms came about. Classification also provides a frame- Not long after Hubble’s death, another astronomer was work for further studies and suggests a logical approach to gaining a reputation for extragalactic studies in the south- studying galaxies. ern hemisphere. This was Gérard Henri de Vaucouleurs In 1926, Edwin P. Hubble (1889–1953) published a (1918–1995), a native of Paris who in the 1940s had galaxy classification system that, with revision, has stood studied at the Sorbonne Physics Research Laboratory and the test of time. So useful has this system been to providing Institute of Astrophysics of the University of Paris. After physical insight into galaxies that thousands of undergrad- receiving his first doctorate (in physics) in 1949 from the uates learn about it every year. Hubble recognized a small University of Paris, de Vaucouleurs began a long-term number of broad classes: the elliptical galaxies E0–E7, the project at the Institute of Astrophysics to revise the famous normal spirals Sa, Sb, and Sc, the barred spirals SBa, SBb, 1932 Shapley–Ames catalogue of galaxies brighter than the 1 © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-82048-6 - The de Vaucouleurs Atlas of Galaxies Ronald J. Buta, Harold G. Corwin and Stephen C. Odewahn Excerpt More information 2 Galaxy classification 13th magnitude. He was eager to collect reliable, homo- This means that most astronomers use the de Vaucouleurs geneous information on these objects that could be used system without really understanding how much it actually for basic astrophysical studies of galaxies. The information differs from the Hubble–Sandage revision. was initially collected in the form of a card index. The de Vaucouleurs revision was published in 1959 De Vaucouleurs later moved to Australia and received a in the journal Handbuch der Physik, which in that year D.Sc. degree from Australian National University in 1957. was devoted entirely to astrophysics papers of a review It was here where he applied his knowledge of astropho- nature. The illustrations in de Vaucouleurs’s classification tography to the study of poorly known southern galaxies paper are partly based on plates de Vaucouleurs himself which were beyond the latitude reach of the large tele- took with the large reflectors at Mount Stromlo Observa- scopes of Mount Wilson and Palomar observatories. He tory, and partly on photographic mockups of Mount Wilson photographed the galaxies mostly with the 30-inch and and Palomar photographs from the pre-publication Hubble 74-inch reflectors atop Mount Stromlo, near Canberra at Atlas shared with him by Allan Sandage. However, being latitude −35 degrees, and became interested in providing printed on a small scale, mostly as negatives, and being classifications for the galaxies in the Hubble system. He relatively few in number, these illustrations are only barely encountered difficulty dealing with galaxies near the junc- adequate to illustrate the full range of morphologies and ture of the Hubble tuning fork, and in 1955 he visited notations possible from de Vaucouleurs’s point of view. Pasadena to discuss the problem with Allan Sandage, who The present volume is designed to illustrate de at the time was preparing the Hubble Atlas. This visit set the Vaucouleurs types for the first time in atlas form, with stage for de Vaucouleurs’s personal revision of the Hubble– numerous examples based on modern digital images taken Sandage scheme outlined in the Hubble Atlas. In 1958, with large reflectors. We will henceforth refer to the system Sandage invited de Vaucouleurs to return to Pasadena to as the de Vaucouleurs revised Hubble–Sandage, or VRHS, classify all the galaxies in the Mount Wilson and Palomar system, to acknowledge the role that Allan Sandage played plate collections. From his discussions with Sandage and in its early development. The atlas is not intended to his inspections, de Vaucouleurs developed the classifica- replace or even supercede the Hubble Atlas or the Carnegie tion system that is highlighted in this atlas. Eventually, de Atlas, because these excellent atlases illustrate the Hubble– Vaucouleurs inspected nearly all existing plate collections Sandage revision in a consistent and very useful manner. available in the late fifties and early sixties to shore up However, galaxy classification is somewhat subjective and his views on morphology, including plates from Harvard different observers may emphasize different features. This Boyden Station, the Lick Observatory Crossley reflector, is what the VRHS system does compared to the Hubble– and the Isaac Roberts collection stored at the Paris Obser- Sandage revision: it is a different point of view that places vatory. emphasis on details that are more broadly covered by the What de Vaucouleurs did was develop a revised Hubble–Sandage revision. Hubble–Sandage classification system that he felt provided We note also that what we present here as the VRHS a better description of what a galaxy looks like without system is actually a modified version of the 1959 Hand- being too unwieldy. His approach gave greater emphasis buch der Physik system, because we felt that there was and more accurate recognition to specific details, such as no reason why the classification system could not take rings, that have a bearing on the way galaxies evolve. into account more recent findings and suggestions by other He later published large catalogues summarizing classi- authors. None of these revisions changes the basic system; fications and other basic information for tens of thou- instead, they simply add to it in useful ways. Also, we sands of bright galaxies. These catalogues have made use underline notations following de Vaucouleurs (1963a) de Vaucouleurs’s revision the most used of the Hubble- to emphasize dominant characteristics in pseudorings and offshoot galaxy classification schemes today. However, a mixed bar morphologies. major disadvantage of his system is that it was published We have two principal goals with the present atlas. mainly as a research article in a relatively obscure jour- The first is to make the VRHS system more accessible nal. No galaxy atlas has ever been prepared that illus- and more understandable to modern astronomy researchers, trates his revised Hubble Atlas notation and point of view.

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