Omega Centauri

ω Centauri
Omega Centauri by ESO.jpg
The globular cluster Omega Centauri. Credit ESO
Observation data (J2000 epoch)
Class Globular Cluster
Constellation Centaurus
Right ascension 13h 26m 45.89s[1]
Declination -47° 28′ 36.7″[1]
Distance 15.8 ± 1.1 kly[2] (4.85 ± 0.35 kpc)
Apparent magnitude (V) 3.7[3]
Apparent dimensions (V) 36′.3
Physical characteristics
Mass ~5 000 000[4] M☉ (~1•1037 kg)
Radius 86 ± 6 ly[5]
Estimated age ~12 Gyr[6]
Other designations NGC 5139,[1] GCl 24,[1] ω Centauri[2]
See also: Globular cluster, List of globular clusters

Coordinates: Sky map 13h 26m 45.89s, −47° 28′ 36.7″

Omega Centauri or NGC 5139 is a globular cluster[7] seen in the constellation of Centaurus, discovered by Edmond Halley in 1677 who listed it as a nebula. Omega Centauri had been listed in Ptolemy's catalog 2000 years ago as a star. Lacaille included it in his catalog as number I.5. It was first recognized as a globular cluster by the English astronomer John William Herschel in the 1830s.[8] Orbiting the Milky Way, it is both the brightest and the largest known globular cluster associated with our galaxy. Of all the globular clusters in the Local Group of galaxies, only Mayall II in the Andromeda Galaxy is brighter and more massive.[9] It is so different from other galactic globular clusters, it is thought to be of different origin.[10] Omega Centauri is located about 15,800 light-years (4,850 pc) from Earth and contains several million Population II stars. The stars in its center are so crowded that they are estimated to average only 0.1 light years away from each other. It is about 12 billion years old.

Omega Centauri is one of the few globular clusters visible to the naked eye and appears about as large as the full Moon.[11] Though it is not a star, Omega Centauri was given a Bayer designation. Kapteyn's star which is currently only 13 light years away is thought to originate from Omega Centauri.[12]

Contents

Dynamics

The internal dynamics of Omega Centauri have been analyzed using measurements of the radial velocities of 469 stars.[13] The cluster is strongly rotating, with a peak rotational velocity of 7.9 km s-1. The mass distribution inferred from the kinematics is slightly more extended than, though not strongly inconsistent with, the luminosity distribution.

Central black hole?

Small field near center of the cluster taken by HST with its new WFC3 sensor. July 2009

Reporting in the April 1, 2008 issue of The Astrophysical Journal, astronomers claimed to have found evidence of an intermediate-mass black hole at the center of Omega Centauri. The observations were made with NASA's Hubble Space Telescope and Gemini Observatory on Cerro Pachon in Chile.[14] Hubble's Advanced Camera for Surveys showed how the stars are bunching up near the center of Omega Centauri, as seen in the gradual increase in starlight near the center. Measuring the speed of the stars swirling near the cluster's center with the Gemini Observatory, the astronomers found that the stars closer to the core are moving faster than the stars farther away. The measurement implies that some unseen matter at the core has gravitational interactions with stars near it. By comparing these results with standard models, the astronomers determined that the most likely cause is the gravitational pull of a massive, dense object. They also used models to calculate the black hole's mass, ~4.0 x 104 solar masses.[14]

However, recent challenges to this work have been made, in particular disputing the previous work's assumed location of the center of the cluster.[15] [16] Using the newly determined center, the speeds of the stars were found to not vary with distance as would be expected if an IMBH were present. They also find that the starlight does not increase but instead remains relatively constant towards the center. From this work, the authors were able to rule out any intermediate mass black hole with mass greater than 1.2 x 104 solar masses.

Disrupted dwarf galaxy

It has been speculated that Omega Centauri may be the core of a dwarf galaxy which was disrupted and absorbed by our Milky Way galaxy.[17] Omega Centauri's chemistry and motion in the galaxy is also consistent with this picture. Like Mayall II, Omega Centauri has a range of metallicities and stellar ages which hints that it did not all form at once (as globular clusters are thought to form) and may in fact be the remainder of the core of a smaller galaxy long since incorporated into the Milky Way.[18]

See also

External links

References

  1. ↑ 1.0 1.1 1.2 1.3 "SIMBAD Astronomical Database". Results for NGC 5139. http://simbad.u-strasbg.fr/Simbad. Retrieved 2006-11-16. 
  2. ↑ 2.0 2.1 van de Ven, G.; van den Bosch, R. C. E.; Verolme, E. K.; de Zeeuw, P. T. (January II 2006). "The dynamical distance and intrinsic structure of the globular cluster ω Centauri". Astronomy and Astrophysics 445 (2): 513–543. doi:10.1051/0004-6361:20053061. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006A%26A...445..513V. "best-fit dynamical distance D=4.8±0.3 kpc ... consistent with the canonical value 5.0±0.2 kpc obtained by photometric methods". 
  3. ↑ Omega Centauri - Britannica Online Encyclopedia
  4. ↑ Globular cluster NGC 5139
  5. ↑ distance × sin( diameter_angle / 2 ), using distance of 5kpc and angle 36.3', = 86 ± 6 ly. radius
  6. ↑ "Peering into the Core of a Globular Cluster"[1]
  7. ↑ http://www.france-info.com/spip.php?article124990&theme=81&sous_theme=166
  8. ↑ Black Hole found in enigmatic Omega Centauri
  9. ↑ http://www.maa.clell.de/Messier/E/Xtra/NGC/n5139.html
  10. ↑ http://arxiv.org/abs/0801.2782
  11. ↑ "Black hole found in Omega Centauri". ESA. 2008-04-02. http://www.esa.int/esaSC/SEMPGM5QGEF_index_0.html. Retrieved 2009-11-06. 
  12. ↑ http://www.newscientist.com/article/mg20427334.100-backward-star-aint-from-round-here.html
  13. ↑ Merritt, David; Meylan, Georges; Mayor, Michel (September 1997). "The stellar dynamics of Omega Centauri". The Astrophysical Journal 114: 1074–1086. doi:10.1086/118538. http://adsabs.harvard.edu/abs/1997AJ....114.1074M. 
  14. ↑ 14.0 14.1 Noyola, E. and Gebhardt, K. and Bergmann, M. (apr 2008). "Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in ω Centauri". The Astrophysical Journal 676: 1008–1015. doi:10.1086/529002. arXiv:0801.2782. http://adsabs.harvard.edu/abs/2008ApJ...676.1008N. 
  15. ↑ Anderson, J. and van der Marel, R. P. (feb 2010). "New Limits on an Intermediate-Mass Black Hole in Omega Centauri. I. Hubble Space Telescope Photometry and Proper Motions". The Astrophysical Journal 710: 1032–1062. doi:10.1088/0004-637X/710/2/1032. arXiv:0905.0627. http://adsabs.harvard.edu/abs/2010ApJ...710.1032A. 
  16. ↑ van der Marel, R. P. and Anderson, J. (feb 2010). "New Limits on an Intermediate-Mass Black Hole in Omega Centauri. II. Dynamical Models". The Astrophysical Journal 710: 1063–1088. doi:10.1088/0004-637X/710/2/1063. arXiv:0905.0638. http://adsabs.harvard.edu/abs/2010ApJ...710.1063V. 
  17. ↑ "Astronomers Find Suspected Medium-Size Black Hole in Omega Centauri". Press release. 2008-04-02. http://hubblesite.org/newscenter/archive/releases/2008/14/full/. Retrieved 2009-11-06. 
  18. ↑ Hughes, 1999, "G1 in M31 - Giant Age and Metallicity Effects in Omega Centauri I: Stromgren Photometry"
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See also
Messier Catalogue · Catalogue of Nebulae · General Catalogue of Nebulae and Clusters · New General Catalogue · Index Catalogue · Revised New General Catalogue · Herschel 400 Catalogue
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