Transcript Slide 1
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 Number of Clusters 30 Harris 1999 20 10 0 -2.6 -2.2 -1.8 -1.4 -1 [Fe/H] -0.6 -0.2 0.2 • 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 • • • • Right Ascension 13 : 26.8 Declination -47 : 29 Distance 17,000 LY Visual Brightness 3.68 mag Apparent Size 36 arc min 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 ColorMagnitude Diagrams Typical Cluster CMDs Omega Cen’s CMD Why so different??? Rey et al. AJ 2004 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 Johnson et al. 2008 The Giants of Omega Centauri Stars observed in Omega Cen CTIO multi-fiber spectrograph Used to determine composition Omega Cen Metallicity Distribution Number of Stars 80 Number of Stars 40 Messier 12 60 40 20 0 30 -2.1 -1.7 Caretta et al. -1.3 -0.9 [Fe/H] 20 10 0 -2.2 -1.8 -1.4 [Fe/H] CTIO Hydra data, 180 stars, Johnson et al. 2008 -1 -0.6 -0.5 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 Measuring the velocity dispersion at the center of Omega Cen Noyola & Gebhardt 2007 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 The Special Case of Omega Centauri: The Milky Way’s most massive star cluster…. a globular cluster, …or something else? The Origin of w Cen • 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 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 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 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!