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RE: Globular ClusterS
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Observations from the Hubble Space Telescope suggest that big galaxies steal globular star clusters from little ones, using gravity to pull off the heist.

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Globular star clusters, dense bunches of hundreds of thousands of stars, have some of the oldest surviving stars in the universe. A new study of globular clusters outside our Milky Way Galaxy has found evidence that these hardy pioneers are more likely to form in dense areas, where star birth occurs at a rapid rate, instead of uniformly from galaxy to galaxy. Astronomers used NASA's Hubble Space Telescope to identify over 11,000 globular clusters in the Virgo cluster of galaxies. Most are older than 5 billion years. The sharp vision of Hubble's Advanced Camera for Surveys resolved the star clusters in 100 galaxies of various sizes, shapes, and brightnesses, even in faint, dwarf galaxies. The images in this photo show four members of the Virgo cluster of galaxies. Comprised of over 2,000 galaxies, the Virgo cluster is the nearest large galaxy cluster to Earth, located about 54 million light-years away.

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NGC 2808
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Title: Radio observations of NGC 2808 and other globular clusters: constraints on intermediate mass black holes
Authors: Thomas J. Maccarone (Southampton), Mathieu Servillat (Toulouse)

We present the results of a deep radio observation of the globular cluster NGC 2808. We show that there are no sources detected within the core of the cluster, placing constraints on both the pulsar population of the cluster and the mass of a possible intermediate mass black hole in NGC 2808. We compare the results for this cluster with other constraints on intermediate mass black holes derived from accretion measures. With the exception of G1 in M~31 which has previously shown radio emission, even with considerably more conservative assumptions, only the clusters with the poorest of observational constraints are consistent with falling on the M_{BH}-\sigma relation. This result is interpreted in terms of the fundamental differences between galaxies and globular clusters.

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RE: Globular ClusterS
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gclust_420.jpg
Credit: NASA/CXC/Northwestern Univ/J.Fregau
JPEG (64.2 kb) Tiff (15.6 MB) PS (7.3 MB)

These are Chandra X-ray Observatory images of the centres of two globular clusters, NGC 6397 and NGC 6121, located in the Milky Way galaxy. They are part of a new study that shows globular clusters might be surprisingly lessmature in their development than previously thought.
Globular clusters are dense bunches of up to millions of stars that are among the oldest known objects in the Universe. Conventional wisdom is that they pass through three phases of evolution or development of their structure, corresponding to adolescence, middle age, and old age. These "ages" refer to the evolutionary state of the cluster, not the physical ages of the individual stars.

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Title: XMM-Newton observations of the Galactic globular clusters NGC 2808 and NGC 4372
Authors: M. Servillat, N. A. Webb, D. Barret

Galactic globular clusters harbour binary systems that are detected as faint X-ray sources. These close binaries are thought to play an important role in the stability of the clusters by liberating energy and delaying the inevitable core collapse of globular clusters. The inventory of close binaries and their identification is therefore essential. We present XMM-Newton observations of two Galactic globular clusters: NGC 2808 and NGC 4372. We use X-ray spectral and variability analysis combined with ultra-violet observations made with the XMM-Newton optical monitor and published data from the Hubble Space Telescope to identify sources associated with the clusters. We compare the results of our observations with estimates from population synthesis models. Five sources out of 96 are likely to be related to NGC 2808. Nine sources are found in the field of view of NGC 4372, none being located inside its half-mass radius. We find one quiescent neutron star low mass X-ray binary candidate in the core of NGC 2808, and propose that the majority of the central sources in NGC 2808 are cataclysmic variables. An estimation leads to ~20+/-10 cataclysmic variables with luminosity above 4.25 x 10^31 erg s^-1. Millisecond pulsars could also be present in the core of NGC 2808, and some sources outside of the half-mass radius could possibly be linked to the cluster.

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Title: Six New Galactic Orbits of Globular Clusters in a Milky-Way-Like Galaxy
Authors: Christine Allen (1), Edmundo Moreno (1), Barbara Pichardo (1,2) ((1) Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, México, (2) Institute for Theoretical Physics, University of Zurich, Switzerland.)

Absolute proper motions for six new globular clusters have recently been determined. This motivated us to obtain the Galactic orbits of these six clusters both in an axisymmetric Galactic potential and in a barred potential, such as the one of our Galaxy. Orbits are also obtained for a Galactic potential that includes spiral arms. The orbital characteristics are compared and discussed for these three cases. Tidal radii and destruction rates are also computed and discussed.

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After ten years of research, Korean astronomers have discovered scores of new densely packed collections of stars, called globular clusters, in the Andromeda galaxy, which is referred to as a sibling of our own galaxy.
These so-called astronomical fossils were made in the early stages of the galaxy's formation, so astronomers expect the discovery will be a milestone in the study of the formation and evolution of the universe.
The research team, led by Kim Sang-chul of the Korea Astronomy and Space Science Institute and Lee Myung-gyoon with the Department of Physics and Astronomy of Seoul National University, said it discovered 113 new GCs in Andromeda.

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Title: Evidence for Three Subpopulations of Globular Clusters in the Early-Type Post-Starburst Shell Galaxy AM 0139-655
Authors: A. Maybhate (STScI), P. Goudfrooij (STScI), F. Schweizer (OCIW), T. Puzia (HIA), D. Carter (LJMU)

We present deep HST ACS images of the post-starburt shell galaxy AM 0139-655. We find evidence for the presence of three distinct globular cluster subpopulations associated with this galaxy: a centrally concentrated young population (~ 0.4 Gyr), an intermediate age population (~ 1 Gyr) and an old, metal-poor population similar to that seen around normal galaxies. The g-I colour distribution of the clusters is bimodal with peaks at 0.85 and 1.35. The redder peak at g-I=1.35 is consistent with the predicted colour for an old metal-poor population. The clusters associated with the peak at g-I=0.85 are centrally concentrated and interpreted as a younger and more metal-rich population. We suggest that these clusters have an age of ~ 0.4 Gyr and solar metallicity based on a comparison with population synthesis models. The luminosity function of these "blue" clusters is well represented by a power law. Interestingly, the brightest shell associated with the galaxy harbours some of the youngest clusters observed. This seems to indicate that the same merger event was responsible for the formation of both the shells and the young clusters. The red part of the colour distribution contains several very bright clusters, which are not expected for an old, metal-poor population. Furthermore, the luminosity function of the "red" GCs cannot be fit well by either a single gaussian or a single power law. A composite (gaussian + power law) fit to the LF of the red clusters yields both a low rms and very plausible properties for an old population plus an intermediate-age population of GCs. Hence, we suggest that the red clusters in AM 0139-655 consist of two distinct GC subpopulations, one being an old, metal-poor population as seen in normal galaxies and one having formed during a recent dissipative galaxy merger.

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If we could view our Milky Way galaxy from afar, wed see a giant disk of stars. A big bar of stars would light up the centre, with lacy spiral arms wrapped around it. And scattered above, below, and through the disk, wed see cottonball-shaped puffs of stars.
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Koposov 1 and 2
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Title: The discovery of two extremely low luminosity Milky Way globular clusters
Authors: S. Koposov (1,2), J. T. A. de Jong (1), V. Belokurov (2), H.-W. Rix (1), D.B. Zucker (2), N.W. Evans (2), G. Gilmore (2), M.J. Irwin (2), E. F. Bell (1) ((1) Max Planck Institute for Astronomy, (2) Institute of Astronomy, University of Cambridge)

We report the discovery of two extremely low luminosity globular clusters in the Milky Way Halo. These objects were detected in the Sloan Digital Sky Survey Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory. The clusters, Koposov 1 and Koposov 2, are located at ~40-50 kpc and appear to have old stellar populations and luminosities of only M_V ~-1^m. Their observed sizes of ~3 pc are well within the expected tidal limit of ~10 pc at that distance. Together with Palomar 1, AM 4 and Whiting 1, these new clusters are the lowest luminosity globulars orbiting the Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to distant branch of the Sagittarius stream. The half-mass relaxation times of Koposov 1 and 2 are only ~70 Myrs and ~55 Myrs respectively (two orders of magnitude shorter than the age of the stellar populations), so it would seem that they have undergone drastic mass-segregation. Since they do not appear to be very concentrated, their evaporation timescales may be as low as ~0.1 t_{
m Hubble}. These discoveries show that the structural parameter space of globular clusters in the Milky Way halo is not yet fully explored. They also add, through their short remaining survival times, significant direct evidence for a once much larger population of globular clusters.

Koposov1and2
3 × 3 SDSS cutout images of Koposov 1 and 2. The bright star in the centre
of Koposov 1 is a foreground star with V ~ 14.5m and large proper motion (alpha, delta) ~ (32,12)arcsec/yr, according to the USNO-B1 catalogue (Monet et al. 2003). The bright extended object near the centre of Koposov 2 is a background galaxy.


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