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IAS, June 2008Caty 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
0
10
20
30
Num
ber o
f Clu
ster
s
0.2-0.2-0.6-1-1.4-1.8-2.2-2.6
[Fe/H]
Harris 1999
Omega Cen
Specs• 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
Right Ascension 13 : 26.8
Declination -47 : 29
Distance 17,000 LY
Visual Brightness
3.68 mag
Apparent Size 36 arc min
Color-Magnitud
e Diagrams
TypicalCluster 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
0
10
20
30
40
-2.2 -1.8 -1.4 -1 -0.6
[Fe/H]
Num
ber o
f Sta
rs
CTIO Hydra data, 180 stars, Johnson et al. 2008
Messier 12
0
20
40
60
80
-2.1 -1.7 -1.3 -0.9 -0.5
[Fe/H]
Nu
mb
er
of
Sta
rs
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-objectSpectroscopy 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 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 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!
