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ScienceWeek
ASTROPHYSICS: GLOBULAR CLUSTERS AND GALACTIC HISTORY
The following points are made by M.J. West et al (Nature 2004 427:31):
1) One way to study the history of galaxies is by observing those that are far away. Because light travels at a finite speed, we see distant objects as they looked in the past. By observing galaxies over a wide range of distances, astronomers are able to look back in time to see how galaxies were at different epochs, with the goal of someday constructing a complete chronology of galaxy evolution. In practice, however, the same great distances that make remote galaxies interesting targets for study also make them difficult to observe(1-3). Additionally, the inherently statistical nature of this approach, although providing information about the evolution of galaxies as a population over cosmological timescales, can say little about the unique history of any particular galaxy(4,5).
2) An alternative way to learn about galaxy origins is to study those galaxies that are nearby. Because the properties of galaxies today reflect both the conditions at the time of their formation and the cumulative effects of billions of years of evolution, careful analysis of present-day galaxies can yield clues to their past. But the success of this approach hinges on the ability to identify some property of galaxies (or some "tracer" population within them) that can serve as a reliable gauge of their evolution over billions of years.
3) One of the most promising of tracer populations are globular clusters. Remarkable progress has occurred over the past decade in our understanding of the globular cluster systems of galaxies, in particular the discovery that many galaxies possess two or more distinct subpopulations of globular clusters. Astronomers are on the verge of reconstructing the individual formation histories of large numbers of galaxies from careful analysis of their globular cluster populations.
4) Globular clusters are dense, gravitationally bound collections of hundreds of thousands to millions of stars that share a common age and chemical composition. Most galaxies are surrounded by systems of tens, hundreds or even thousands of globular clusters that swarm about them like bees around a hive. In general, the number of globular clusters that a galaxy possesses is roughly proportional to its luminosity.
5) In summary: Nearly a century after the true nature of galaxies as distant "island universes" was established, their origin and evolution remain great unsolved problems of modern astrophysics. One of the most promising ways to investigate galaxy formation is to study the ubiquitous globular star clusters that surround most galaxies. Globular clusters are compact groups of up to a few million stars. They generally formed early in the history of the Universe, but have survived the interactions and mergers that substantially alter their parent galaxies. Recent advances in our understanding of the globular cluster systems of the Milky Way and other galaxies point to a complex picture of galaxy genesis driven by cannibalism, collisions, bursts of star formation and other tumultuous events.
References (abridged):
1. Barger, A. J. et al. Submillimeter-wavelength detection of dusty star-forming galaxies at high redshift. Nature 394, 248-251 (1998)
2. Blain, A. W., Smail, I., Ivison, R. J. & Kneib, J.-P. The history of star formation in dusty galaxies. Mon. Not. R. Astron. Soc. 392, 632-648 (1999)
3. Abraham, R. G. & van den Bergh, S. The morphological evolution of galaxies. Science 293, 1273-1278 (2001)
4. Cowie, L. L., Songaila, A., Hu, E. & Cohen, J. G. New insight on galaxy formation and evolution from Keck spectroscopy of the Hawaii Deep Fields. Astron. J. 112, 839-864 (1996)
5. Lilly, S. J., Le Fevre, O., Hammer, F. & Crampton, D. The Canada-France Redshift Survey: The luminosity density and star formation history of the universe to z approximately 1. Astrophys. J. 460, L1-L4 (1996)
Nature http://www.nature.com/nature
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ON THE MYSTERY OF GALAXY FORMATION
The following points are made by Marco Scodeggio (Science 2001 294:537):
1) There are two basic models for galaxy formation. In the monolithic collapse scenario, all galaxies were formed in a single event, through the gravitational collapse of a cloud of primordial gas, very early in the history of the universe (1,2). In the hierarchical merging scenario, galaxies are gradually assembled through multiple mergers of smaller subgalactic units, a process that continues from the early universe to the current epoch (3,4).
2) These differences extend to ideas about galaxy evolution. In the monolithic collapse scenario, galaxies of different morphological types (spirals and ellipticals) are born intrinsically different, whereas in the hierarchical merging scenario, galaxies end up as spirals or ellipticals depending on the details of their merger history. As a result, the first model predicts that the number of galaxies of a given type should be approximately constant at all redshifts (that is, throughout the history of the universe), whereas the second predicts that there number should decrease with increasing redshift (that is, decreasing age).
3) Attempts to discriminate between the two models focus mostly on elliptical galaxies, which are easier to study than spiral ones. Present-epoch ellipticals form a very homogeneous family with very similar intrinsic properties. Compared with the heterogeneous family of spiral galaxies, ellipticals in the local universe have little or no dust, gas, and star formation activity (5). Furthermore, they are mostly if not exclusively composed of an old stellar population, about as old as the universe, with very similar relative ages. This fact is responsible for the most distinctive property of ellipticals: their color. Ellipticals are the reddest galaxies in the local universe (5).
4) Neither galaxy formation model can be discarded convincingly, although, until recently, the monolithic collapse scenario had to contend with one important, albeit indirect, piece of evidence against it. If elliptical galaxies all formed at high redshift in a single event, for a short period they must have had very strong star formation activity. Simple model calculations indicate that galaxies with so many young and bright stars should be luminous enough to be observable with current telescopes, despite their large distances. But they were never observed.
References (abridged):
1. O. J. Eggen, D. Lynden-Bell, A. Sandage, Astrophys. J. 136, 748 (1962)
2. R. B. Larson, Mon. Not. R. Astron. Soc. 166, 585 (1974)
3. S. D. M. White, M. J. Rees, Mon. Not. R. Astron. Soc. 183, 341 (1978)
4. S. Cole et al., Mon. Not. R. Astron. Soc. 271, 781 (1994)
5. M. Roberts, M. P. Haynes, Annu. Rev. Astron. Astrophys. 32, 115 (1994)
Science http://www.sciencemag.org
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ON THE EVOLUTION OF GALAXIES
The following points are made by Alan Dressler (Sky & Telescope 1998 October):
1) When the century opened, most astronomers assumed the Universe was eternal and basically changeless, its general structure immutable. In 1915, with the publication of Einstein's general theory of relativity, there were implications of the cosmic role of gravity, but these implications were for the most part ignored. Indeed, even after 1924 and the proof by *Edwin Hubble (1889-1953) that the spiral nebulae are other galaxies at vast distances, astronomers were slow to recognize the implications of the new observations.
2) The first big step in changing the view of the cosmos was the construction by *George Ellery Hale (1868-1938) of the 100-inch Hooker reflector on Mount Wilson (US), a project completed in 1918. This was the telescope used by Hubble and his colleagues to reveal the large-scale organization of the Universe into galaxies, the vast size of the Universe, and the expansion of the Universe. *George Lemaitre (1894-1966) soon proposed that the expansion of the Universe implied a dense explosive birth of the Universe at a specific finite time in the past (the event that came to be called the Big Bang).
3) An even greater telescope was needed, and again George Hale led the way in the building of the 200-inch reflector on Palomar Mountain (US), that instrument finally completed in 1948. The 200-inch telescope produced the first observations of galaxy evolution -- the first evidence that galaxies observed at high redshift are unlike galaxies closer to us in time. An even greater accomplishment of the 200-inch telescope was a series of observations concerning the spectra of *quasars, and the evidence for their immense distances and luminosities. Theorists eventually proposed that quasars were *black holes of 100 million solar-masses or more. It is now clear that most galaxies with a central bulge, including our own Galaxy, harbor massive black holes at their cores.
4) In 1961, Allan Sandage published a landmark paper outlining the possibility of testing cosmological models with the 200-inch telescope, and over the next two decades Sandage devoted himself to this project. Unfortunately, the underlying premise of the project -- that the brightest galaxy in every galactic cluster has about the same true luminosity -- was demonstrated by *Beatrice Tinsley (1941-1981) to be untenable.
5) In the fall of 1977, at a Yale University (US) conference on the evolution of galaxies, Harvey Butcher and Augustus Oemler presented their evidence for relatively young star-forming galaxies. This evidence, which implied strong galaxy evolution during relatively recent cosmic time, met with controversy and skepticism.
6) In the 1980s, observations by various groups proved that Butcher and Oemler were correct, and it was now understood that these relatively young galaxies were often producing new stars in huge bursts. These bursting galaxies are evidently spirals with a more disheveled appearance than is common today, and in their twisted and distorted disks huge numbers of stars were recently born.
7) During the past 2 years, among the most interesting results of various observations with various instruments is the formulation of the so-called Madau diagram (popularized by Piero Madau) that plots the Universe-wide rate of star formation from early times to today, spanning almost the whole history of the cosmos. The rate of star formation apparently rose rapidly in the first few billion years, the peak rate at about 5 or 6 billion years later at redshifts of 1 to 2. (Our Sun apparently formed at a time corresponding approximately to redshift = 0.5.) The author concludes: "Our generations are fortunate to live to see one of the great mysteries of where we came from in process of being solved."
Sky and Telescope http://www.skyandtelescope.com
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Notes:
Edwin Hubble (1889-1953): Hubble first studied law before switching to astronomy at the age of 25. He began his work at the Mount Wilson Observatory with the 100-inch telescope at the age of 30. In 1941, at the age of 52, he tried to join the US Army to fight the Nazis, but he was persuaded that he could do more in war-related research.
George Ellery Hale (1868-1938): Hale is best known for his work building large-telescopes (and for obtaining the funds for the Yerkes Observatory, named after the street-car magnate Charles Tyson Yerkes), but already at the age of 21 he invented the spectroheliograph, a device that made it possible to photograph the light of a single spectral line of the sun, and he made several ground-breaking observations with this instrument.
George Lemaitre (1894-1966): Lemaitre began his professional life as a civil engineer, then at 21 he switched to physics and mathematics. He also became a Roman Catholic priest at the age of 22. After obtaining his PhD at the Massachusetts Institute of Technology in 1927, he settled in Belgium as a professor of astrophysics at the University of Louvain. At the time of his death, he was president of the Pontifical Academy of Sciences at Rome. Lemaitre's theoretical ideas concerning the origin of the Universe were published in 1927, when he was 31, but the paper was largely unnoticed until the astrophysicist Arthur Eddington (1882-1944) called attention to it much later.
quasars: (quasi-stellar objects). Extremely luminous sources radiating energy over the entire spectrum from x-rays to radio waves, and which are apparently the oldest and most distant objects in the universe.
black holes: If the terminal stages of star death leave a remnant star mass greater than 3 solar-masses, the ultimate gravitational collapse will produce a black hole, a relativistic singularity. A black hole is a localized region of space from which neither matter nor radiation can escape. The "trapping" occurs because the requisite escape velocity, which can be calculated from the relevant equations, exceeds the velocity of light and is therefore unattainable. Another view of a black hole is that it is a mass that has collapsed to such a small volume that its gravity prevents the escape of all radiation. Space and time essentially have no meaning in a black hole. The boundary of the black hole is called the "event horizon", because any event within the boundary is invisible outside, the invisibility resulting from the fact that no radiation can escape to be detected. The radius of the black hole depends upon how much matter has fallen into the region; it is called the "Schwarzchild radius", and it is usually a few kilometers. However, massive black holes are possible and are thought to be the source of quasars. If quasars indeed involve black holes, the radiation is from material just outside the black hole, and not from anything within it. Nothing inside a black hole can get out of it.
Beatrice Tinsley (1941-1981): During her short life, Tinsley managed to be a force in astronomy from her first entry into the field. At the age of 25, an unknown graduate student at the University of Texas, she rose before an audience about to hear Allan Sandage and publicly challenged his idea that giant elliptical galaxies exhibited luminosities constant enough to be used as "standard candles" to estimate distances. She proved her point by the age of 36, and the variability of galaxy luminosities became the consensus view. It was Tinsley who co-hosted the 1977 Yale conference that set the course of galaxy-evolution studies. She died 4 years later of cancer. Near the end, she wrote the following: "Let me be like Bach, creating fugues; till suddenly the pen will move no more."
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