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!