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Title: Relative distances of Omega Centauri and 47 Tucanae
Authors: G. Bono (INAF, Rome Univ.), P.B. Stetson (HIA-NRC), N. Sanna, A. Piersimoni (INAF), L.M. Freyhammer (Univ. Lancashire), Y. Bouzid (VUB, OBSS/WE), R. Buonanno (Rome Univ.), A. Calamida, F. Caputo, C.E. Corsi, A. Di Cecco, M. Dall'Ora, I. Ferraro, G. Iannicola (INAF), M. Monelli (IAC), M. Nonino, L. Pulone (INAF), C. Sterken (VUB, OBSS/WE), J. Storm (AIP), T. Tuvikene (VUB, OBBS/WE), A.R. Walker (CTIAO-NOAO)

We present precise optical and near-infrared ground-based photometry of two Globular Clusters (GCs): Omega Cen and 47 Tuc. These photometric catalogs are unbiased in the Red Giant Branch (RGB) region close to the tip. We provide new estimates of the RGB tip (TRGB) magnitudes--m_I(TRGB)=9.840.05, Omega Cen; m_I(TRGB)=9.460.06, 47 Tuc--and use these to determine the relative distances of the two GCs. We find that distance ratios based on different calibrations of the TRGB, the RR Lyrae stars and kinematic distances agree with each other within one sigma. Absolute TRGB and RR Lyrae distance moduli agree within 0.10--0.15 mag, while absolute kinematic distance moduli are 0.2--0.3 mag smaller. Absolute distances to 47 Tuc based on the Zero-Age-Horizontal-Branch and on the white dwarf fitting agree within 0.1 mag, but they are 0.1--0.3 mag smaller than TRGB and RR Lyrae distances.

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A cluster brimming with millions of stars glistens like an iridescent opal in this image from NASA's Spitzer Space Telescope. Called Omega Centauri, the sparkling orb of stars is like a miniature galaxy. It is the biggest and brightest of the 150 or so similar objects, called globular clusters, that orbit around the outside of our Milky Way galaxy. Stargazers at southern latitudes can spot the stellar gem with the naked eye in the constellation Centaurus.

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Millions of clustered stars glisten like an iridescent opal in a new image from NASA's Spitzer Space Telescope.
Called Omega Centauri, this sparkling orb of stars is like a miniature galaxy. It is the biggest and brightest of the more than 150 similar objects, called globular clusters, that orbit around the outside of our Milky Way galaxy. Stargazers at southern latitudes can spot the stellar gem with the naked eye in the constellation Centaurus.
While the visible-light observations highlight the cluster's millions of jam-packed stars, Spitzer's infrared eyes reveal the dustier, more evolved stars tossed throughout the region.

NGC5139
Credit: NASA/JPL-Caltech/NOAO/AURA/NSF

Screen-Resolution (360x450): JPEG (68 KB)
Medium-Resolution (720x900):
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JPEG (4.5 MB) | Mac TIFF (9.7 MB) | PC TIFF (9.7 MB)

Position (J2000): RA: 13h 26m 45.89 Dec: -47 28 36.7

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Title: Gemini and Hubble Space Telescope Evidence for an Intermediate Mass Black Hole in omega Centauri
Authors: Eva Noyola (1,2), Karl Gebhardt (2), Marcel Bergmann (3) ((1) MPE, (2) UT Austin, (3) Gemini)

The globular cluster omega Centauri is one of the largest and most massive members of the galactic system. However, its classification as a globular cluster has been challenged making it a candidate for being the stripped core of an accreted dwarf galaxy; this together with the fact that it has one of the largest velocity dispersions for star clusters in our galaxy makes it an interesting candidate for harbouring an intermediate mass black hole. We measure the surface brightness profile from integrated light on an HST}/ACS image of the centre, and find a central power-law cusp of logarithmic slope -0.08. We also analyse Gemini GMOS-IFU kinematic data for a 5x5 arcsec field centred on the nucleus of the cluster, as well as for a field 14 arcsecaway. We detect a clear rise in the velocity dispersion from 18.6 km/s at 14 arcsec to 23 km/s in the centre. A rise in the velocity dispersion could be due to a central black hole, a central concentration of stellar remnants, or a central orbital structure that is radially biased. We discuss each of these possibilities. An isotropic, spherical dynamical model implies a black hole mass of 4.0^{+0.75}_{-1.0} times 10^4 M_sun, and excludes the no black hole case at greater than 99% significance. We have also run flattened, orbit-based models and find similar results. While our preferred model is the existence of a central black hole, detailed numerical simulations are required to confidently rule out the other possibilities.

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Omega Centauri has been known to be an unusual globular cluster for a long time. A new result obtained by Hubble and the Gemini Observatory reveals that the globular cluster may have a rare intermediate-mass black hole hidden in its centre, implying that it is likely not a globular cluster at all, but a dwarf galaxy stripped of its outer stars.
Omega Centauri, the largest and brightest globular cluster in the sky, is visible from Earth with the naked eye and is one of the favourite celestial objects for stargazers from the southern hemisphere. Although 17 000 light-years away, it located just above the plane of the Milky Way and appears almost as large as the full Moon when seen from a dark, rural area.
Images obtained with the Hubble Space Telescopes Advanced Camera for Surveys and data from the GMOS spectrograph on the Gemini South telescope in Chile show that Omega Centauri appears to harbour an elusive and rare intermediate-mass black hole in its centre.

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NGC 5139
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The 4th magnitude globular cluster is located about 16,300 light years away in the constellation of Centaurus. It is easily visible without a telescope as a "star" . It contains about 2 millions stars, nearly twice as many as M13. The main region is about 20' in diameter.
The cluster of stars was discovered by Edmond Halley in 1677.

Position(2000): RA: 13h 26m 45.89s, Dec: -47 28 36.7

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Omega Centauri
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Title: A Spitzer Space Telescope Atlas of omega Centauri: The Stellar Population, Mass Loss, and the Intracluster Medium
Authors: Martha L. Boyer (1), Iain McDonald (2), Jacco Th. van Loon (2), Charles E. Woodward (1), Robert D. Gehrz (1), A. Evans (2), A. K. Dupree (3) ((1) Department of Astronomy, University of Minnesota, Minneapolis, MN (2) Astrophysics Group, School of Physical & Geographical Sciences, Keele University, UK (3) Harvard-Smithsonian Centre for Astrophysics, Cambridge, MA)

We present a Spitzer Space Telescope imaging survey of the most massive Galactic globular cluster, omega Centauri, and investigate stellar mass loss at low metallicity and the intracluster medium (ICM). The survey covers approximately 3.2x the cluster half-mass radius at 3.6, 4.5, 5.8, 8, and 24 microns, resulting in a catalogue of over 40,000 point-sources in the cluster. Approximately 140 cluster members ranging 1.5 dex in metallicity show a red excess at 24 microns, indicative of circumstellar dust. If all of the dusty sources are experiencing mass loss, the cumulative rate of loss is estimated at 2.9 - 4.2 x 10^(-7) solar masses per year, 63% -- 66% of which is supplied by three asymptotic giant branch stars at the tip of the Red Giant Branch (RGB). There is little evidence for strong mass loss lower on the RGB. If this material had remained in the cluster centre, its dust component (> 1 x 10^(-4) solar masses) would be detectable in our 24 and 70 micron images. While no dust cloud located at the centre of omega Cen is apparent, we do see four regions of very faint, diffuse emission beyond two half-mass radii at 24 microns. It is unclear whether these dust clouds are foreground emission or are associated with omega Cen. In the latter case, these clouds may be the ICM in the process of escaping from the cluster.

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