Globular Clusters: HST Breathes New Life

into Old Fossils

Hubble Science Briefing

Jay Anderson

STScI

June 3, 2010

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Outline

• Globular Clusters

• Omega Cen Early release images– Colors– Simulations

• Stellar Populations

• The cutting edge with HST

2

Globular Clusters• “Textbook” simple stellar populations

– Formed stars early– Single cloud, single metallicity, single age– Not large enough to self-enrich– Continue orbiting in spheroid of Galaxy

• Perfect laboratories to evaluate stellar evolution

3

OMEGA CEN EARLY-RELEASE IMAGE

History of this image1) Why taken?2) Why ERO?3) Why “saturated” ?

4

Familiar digital cameras: all at once

Hubble: one Filter at a time

JamesWebb ST:two at a

time

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10x10 ACS

Early Release

Field

CentralField

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The central field…

RGB

7.1

The central field…

Blue

7.2

The central field…

Red

7.3

The central field…

RGB

7.4

HUBBLE IMAX:MAYA was used to render

the scene in 3-D

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Decomposing the image… “a point source has no hair” Realclose up wide view

The PSF 9.1

Decomposing the image… “a point source has no hair” Simulatedclose up wide view

The PSF 9.2

What Astronomers see… 10.1

What Astronomers see… 10.2

What Astronomers see… 10.3

What Astronomers see… 10.4

What Astronomers see… 10.5

What Astronomers see… 10.6

What Astronomers see… 10.7

What Astronomers see… 10.8

What Astronomers see… 10.9

What Astronomers see… 10.10

What Astronomers see… 10.11

10.12What Astronomers see…

What Astronomers see… 10.13

What Astronomers see…

1) Main Sequence 2) SubGiant

Branch

3) Red Giant Branch

4) Horizontal Branch

5) White Dwarf Sequence

10.14

Easy to identify stars…RGB

WDs

SGB

HB

MSTO

RedDwarfs

BSs

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More metals

More Helium

Age

Red GiantBranch

A

B

C Globular clusters have traditionallybeen defined as textbook “simple” stellar populations: bound clusters of stars at the same distance, withthe same age, and the same metallicity.

same small cloud

Stellar Populations

“Isochrone” 12

Omega N6397 47T

Omega Cen NGC6397 47 Tuc

Extrasequences

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Globular Cluster or Dwarf Spheroidal?

Cambridge, UK 2001

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Stellar Populations

More metals

More Helium

Age

Inversion!

More metals

metalpoor

intermediate

metal rich

Red GiantBranch

Similar togalaxies…

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Could the textbook globular cluster not be one?

47Tuc

N2808

N6656N6388

OmCen ®

Pluto

Is Omega Cen a GC?16

Is Omega Cen a globular?Are there any globular clusters?

Questions to answer:1) How do clusters enrich themselves?2) Why are they all so different?3) Is it all clusters, or just heavy ones?4) What connection is there between clusters and galaxies?5) Any relevance for star formation going on today?

NGC2808

NGC6652

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More HST with Clusters…• HST’s advantages

1) Resolution: separate stars2) Lower background: concentrate star, not sky3) Stability: no atmosphere, same PSF4) UV and IR coverage

• Proper motions: 1) Cluster-field separation 2) Search for black holes

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The first full CMD

• Richer et al. 2005 observed NGC6397 with 126 orbits

– Discoveries• End of WD cooling seq• Blue hook at bottom!• End of MS?

– Limitations: field stars

Blue

Faint

Bright

Red

HBL?WDCS

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Proper-Motion Cleaning PI-Rich, UCLA20.1

Proper-Motion Cleaning PI-Rich, UCLA20.2

Proper-Motion Cleaning PI-Rich, UCLA20.3

Proper-Motion Cleaning PI-Rich, UCLA20.4

Intermediate-Mass Black Holes (IMBHs) in GCs

• BHs known to exist with

– ~10 MSUN : HMXBs

– ~106 MSUN: SMBHs at centers of galaxies

– ~104 MSUN: IMBHs?

• Could help explain how SMBHs grow

• ULXs in other galaxies• M relation…

10,000 MSUN

~20 km/sGreene & Ho

(2006)

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IMBHs in Globular Clusters II• Several ways to find (PMs or RVs)

– Fast moving star in orbit (smoking gun)– Rise in velocities at center– Also pulsars, accretion signatures, certain cusps

• Several ways to not find– Not enough stars/gas to sample its environs– Nearby stars all dark, ejecting binaries

• Easiest places to look:– Clusters with cusps & slow velocities

• Zone of influence rBH ~ MBH/2

• Omega Cen is not the easiest place to look…

Yet…

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• Detections/limits in the literature– M15: back and forth; currently not required…

– NGC6752: negative P pulsars: 1000 MSUN

– 47 Tuc: upper limit of 1500 MSUN

– G1 in M31? X/Radio emission and velocities

Cen circumstantial evidence:– Noyola, Gebhardt & Bergmann 2008 (NGB08)

1) Detected a brightness cusp at center in ACS images

2) Used an IFU at Gemini and saw a velocity spike there

Could be explained by a 40,000 MSUN IMBH

• This dispersion increase should be detectable with HST PMs…

Current evidence?23

MeasuringProper

Motions

Epoch1: 2002 Epoch2: 2006

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Stars with good measured motions 25

126.

226.

326.

426.

526.

626.

726.

826.

926.

1026.

PM Profile

Total motions ofthe 1000 stars in

the inner 10

100 stars/bin

Motions at the center are consistent with

those at 10

(we also checked the NGB08 center…)

Proper Motions indicate nointeresting mass concentrationat the center.

Certainly not 40,000 MSUN;at most 12,000 MSUN

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Future Directions…• Omega Cen

– More stars? Better models?

• Other clusters coming!NGC 362

M15

NGC 6624

NGC 7099

NGC 6681

AND MORE!

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THANK YOU!

• Watch for– Movies of Omega Cen motions– Movies of the CMD-morph

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