The Amazing Omega Centauri!

1. The Amazing Omega Centauri! IAS, June 2008 Caty Pilachowski

2. Visible in the Southern Sky • Listed in Ptolemy's catalog • Discovered by Edmond Halley in 1677 • non-stellar • "luminous spot or patch in Centaurus"

6. Outline • What are globular clusters? • The Milky Way GC system • What’s special about Omega Centauri? • Specs • Color-magnitude diagram • Composition • Black hole? • Where did Omega Cen come from? • Is Omega Cen unique?

7. The Milky Way

11. Typically 100,000 – 1,000,000 stars that formed together Still held together by gravity Orbit the center of the Milky Way Old (12-14 Gyr) – formed early in MW history Typically SINGLE metal abundance 2 subpopulations, distinguished by orbit and color Globular Clusters

12. Omega CenSpecs • NGC 5139 • The brightest GC in the Galaxy • The most massive: 5 x 106 solar masses • Galactic Coordinates: • longitude 309 • latitude +14 • Distance from the Galactic Center: 20,500 LY • Ellipticity: 0.17 (= 1-b/a) • Orbit highly retrograde, nearly in Galactic plane

13. Color-Magnitude Diagrams

14. TypicalCluster CMDs

15. Omega Cen’s CMD Rey et al. AJ 2004 Why so different???

16. Omega Cen contains stars with a range of metal abundance • Formation of stars was episodic, extended over ~4 Gyr • Must have occurred away from disk Rey et al. AJ 2004

17. Johnson et al. 2008 The Giants of Omega Centauri • Stars observed in Omega Cen • CTIO multi-fiber spectrograph • Used to determine composition

18. Caretta et al. Omega Cen Metallicity Distribution CTIO Hydra data, 180 stars, Johnson et al. 2008

19. Another surprise: Omega Cen’s Main Sequence • Omega Cen has TWO main sequences! • The bluer stars are twice as "metal-rich" as the redder ones • Do the two populations of stars have a different abundance of helium? • The red stars have a normal helium abundance • The bluer stars must be enriched in helium by more than 50% • The most helium-rich stars ever found????

20. And Another Surprise! Spectroscopic observations from the Gemini 8-m telescope suggest that Omega Cen may host a black hole! Artist’s conception – Lynette Cook

21. Multi-objectSpectroscopy with Gemini South Measuring the velocity dispersion at the center of Omega Cen Noyola & Gebhardt 2007

22. Does Omega Cen host a black hole?? It seems so… Mass = 4 x 104 suns The mass of the black hole is consistent with BHs in the nuclei of other galaxies

23. The Milky Way’s most massive star cluster…. a globular cluster, …or something else? The Special Case of Omega Centauri:

24. Both supernovae and giant stars added to the chemical enrichment of w Cen Enrichment occurred over 2-3 Gyr The timescale and chemical enrichment suggests that w Cen formed outside the Milky Way The Origin of w Cen Is Omega Cen the nucleus of a captured galaxy?

25. The Milky Way Is Still Growing • Nearby dwarf galaxy discovered in 1994 in the direction of Sagittarius • Discovered by radial velocity • Distance about 88,000 light years • Merging with the Milky Way

26. Sagittarius Tidal Stream • Orbits the Milky Way • Orbital period about a billion years • “Tidal stream” of stars from Sagittarius circles the Milky Way • Sagittarius may contain significant dark matter

27. Yet Another New Galaxy! • Canis Major Dwarf • Nearest galaxy to the Milky Way (yet discovered…) • 25,000 light years from the Sun • 44,000 light years from the center of the Milky Way • Discovered with IR light (hidden behind dust in the MW’s disk)

28. Tidal Streams from CMa Wrap around the Milky Way

29. A Globular Cluster – NOT! • Modern evidence suggests that Omega Cen is not a globular cluster, but the former nucleus of a small galaxy • Similar tidal captures are occurring today in the Milky Way • A handful of “globular clusters” share similar properties with Omega Cen (e.g. M54 in Sagittarius) • A new class of objects!