ScienceWeek

ASTRONOMY: ON GLOBULAR CLUSTERS

The following points are made by Kenneth C. Freeman (Science 2006 311:1105):

1) Elliptical galaxies are believed to form from the mergers of a few smaller galaxies. Most ellipticals are surrounded by a cloud of very old globular star clusters. As a feature of this merger origin, Ashman and Zepf [1] predicted that elliptical galaxies would have two distinct populations of clusters that appeared at different phases in the process of formation and chemical evolution of the parent galaxies. Over the last decade, the color distribution of the clusters in most ellipticals was indeed found to be bimodal. We know that the color of old stellar systems correlates with their metallicity [2], so the bimodal colors were interpreted as a bimodal metallicity distribution, and therefore as evidence for the predicted two populations of clusters. This bimodal color distribution was an exciting confirmation of the current ideas about galaxy formation, but this confirmation is now less secure. New work[3] shows that a single cluster population with a broad monomodal metallicity distribution can have a bimodal distribution of color, because the relation between color and metallicity is not linear.

2) Most galaxies contain some globular star clusters, which are dense, nearly spherical collections of typically 10^5 to 10^6 stars. The Milky Way has about 150 globular clusters, whereas some of the largest elliptical galaxies have more than 20,000. The globular clusters in the Milky Way are all very old, with ages up to about 13 billion years. Their metallicities range from -2.5 for clusters in the halo of our Galaxy to near zero for the most metal-rich clusters of the galactic bulge. The low metallicities of the halo clusters show that they formed very early in the life of the Galaxy, before much of the chemical-element building had taken place.

3) We do not yet understand how these dense and massive clusters form. What was so special about the conditions early in the life of the Galaxy that led to their formation at that early time? Although globular clusters are not forming in the Milky Way at the present time, they are forming in violently interacting galaxy pairs like the Antennae system, in which two spirals are in the late stages of merging. The interaction has compressed the gas and stimulated much star formation, and the conditions in the interstellar gas are clearly right for the formation of young globular clusters.

4) For astronomers interested in galaxy formation, one of the most exciting discoveries of the last decade was the bimodal color distribution of the clusters in the bright elliptical galaxies [4]. The metallicities of old clusters were already known to correlate with their colors (although the precise form of the color-metallicity relation was not well established observationally). From the time of its discovery, the bimodal color distribution of the clusters in elliptical galaxies was interpreted as a bimodal metallicity distribution. Typically, one mode has a metallicity around -1.5 and the other mode, a metallicity around 0. Both modes of clusters are old (>10 billion years). Astronomers found this exciting because the presence of two cluster subsystems of different metallicities was consistent with the picture of elliptical galaxies forming from mergers. The metal-poor subsystem would be associated with the formation of the individual galaxies that later merged to form the elliptical. The metal-rich mode would be produced in the phase of rapid star formation during the merger process, from gas that had been already chemically enriched in the individual galaxies (as in the Antennae and other present-day merging spirals) [5]. But there is a catch. The interpretation of the bimodal colors depends on the true shape of the color-metallicity relation.

References:

1. K. Ashman, S. Zepf, Astrophys. J. 384 50 (1992). [ADS]

2. Metallicity is the ratio of "metals" to hydrogen, where metals include all elements heavier than helium. It is usually expressed logarithmically relative to the Sun, so metallicities of 0 and -2 represent (1.00 and 0.01) the solar metallicity.

3. S.-J. Yoon, S. K. Yi, Y.-W. Lee, Science 311, 1129 (2006)

4. S. Zepf, K. Ashman, D. Geisler, Astrophys. J. 443, 570 (1995)

5. Alternatively, some authors have argued that the metal-rich red mode of clusters are the original clusters of the underlying parent elliptical, whereas the metal-poor blue-mode clusters have been accreted from smaller in-falling galaxies.

Science http://www.sciencemag.org

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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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COSMOLOGY: ON EARLY STAR AND GALAXY FORMATION

The following points are made by S. George Djorgovski (Nature 2004 427:790):

1) The appearance of the first sources of light in the Universe ended the so-called cosmic Dark Ages, which had lasted a few hundred million years. The ultraviolet light from these sources also changed the physical state of the gas (hydrogen and helium) that fills the Universe, from a neutral state to a nearly fully ionized one. This was the era of reionization. Quasars -- the brightest and most distant objects known -- offer a window on the reionization era, because neutral hydrogen gas absorbs their ultraviolet light. A recent analysis by Wyithe and Loeb(1) adds to a growing body of evidence that the early history of galaxy formation was more complex than was supposed even a couple of years ago.

2) When the Universe was about 380,000 years old, it underwent a phase transition, changing from an incandescent plasma containing the heat of the Big Bang to a space filled with dark matter, energy, and neutral gas. The glow of the primordial plasma is what we now observe as the cosmic microwave background (CMB). The Universe then entered the Dark Ages, with embryonic structures growing from the seeds of dark-matter fluctuations (fluctuations that are observed today as ripples in the CMB). After a few hundred million years, these condensations became dense enough for the first stars to form, leading to the appearance of the first galaxies and the growth of the massive black holes that are believed to power quasars.

3) As these first objects lit up, they also modified the gas between them, ionizing the hydrogen and making it transparent to ultraviolet light. Effectively, the Universe underwent another phase transition, from a neutral to an ionized state. Each of the primordial sources of light -- most of them powered by young, massive stars, but some powered by the accretion of matter into growing black holes (the early quasars) -- excavated a bubble of ionized gas, called a "Stroemgren sphere", in the otherwise neutral surrounding medium. When these bubbles began to overlap, reionization was complete. The reionization era is thus a cosmological milestone, marking the appearance of the first stars, galaxies and quasars.

4) The light reaching us now from distant quasars was emitted just before reionization was complete. Any neutral hydrogen remaining in the Universe at that point would have been a very effective absorber of this radiation at a particular set of wavelengths (extending up to 91.2 nanometers) known as the "Lyman series". Even minuscule quantities of neutral hydrogen -- as little as 10^(-4) or 10^(-5) by mass fraction in intergalactic clouds -- can leave a tell-tale absorption signature in the spectra of quasars; this is the "Lyman forest" of absorption lines in quasar spectra. As the fraction of the neutral gas increases, the forest thickens; as the fraction approaches 100%, essentially all of the observed flux at wavelengths below the so-called Lyman alpha line (at a wavelength of 121.6 nanometers, in the restframe) is absorbed.

References (abridged):

1. Wyithe, J. S. B. & Loeb, A. Nature 427, 815-817 (2004)

2. Gunn, J. E. & Peterson, B. Astrophys. J. 142, 1633-1642 (1965)

3. Becker, R. et al. Astron. J. 122, 2850-2857 (2001)

4. Fan, X. et al. Astron. J. 125, 1649-1659 (2003)

5. Fan, X. et al. Astron. J. 123, 1247-1257 (2002)

Nature http://www.nature.com/nature

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