IAS, June 2008 Caty Pilachowski

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

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?

The Milky Way

Globular Clusters 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

Omega Cen Specs NGC 5139 The brightest GC in the Galaxy The most massive: 5 x 10 6 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 Right Ascension13 : 26.8 Declination-47 : 29 Distance17,000 LY Visual Brightness3.68 mag Apparent Size36 arc min

Color- Magnitude Diagrams

Typical Cluster CMDs

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

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

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

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

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????

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

Multi-object Spectroscopy with Gemini South Noyola & Gebhardt 2007 Measuring the velocity dispersion at the center of Omega Cen

Does Omega Cen host a black hole?? It seems so… Mass = 4 x 10 4 suns The mass of the black hole is consistent with BHs in the nuclei of other galaxies

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

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

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

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 Sagittarius Tidal Stream

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)

Tidal Streams from CMa Wrap around the Milky Way

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!