globular clusters: hst breathes new life into old fossils stsci public lecture series jay anderson...

Globular Clusters: HST Breathes New Life

into Old Fossils

STScI Public

Lecture SeriesJay Anderson

August 3, 2010

Plan for the Talk• Overview of Globular Clusters (GCs)• Omega Cen and IMAX• Stellar populations GCs• Black holes in GCs?

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 study stellar evolution

OMEGA CEN EARLY-RELEASE IMAGE

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

Familiar digital cameras: three at once

Hubble: one Filter at a time

JamesWebb ST:two at a

time!

10x10

Early ReleaseField

CentralField

The central field…

Blue

Red

HUBBLE IMAX:MAYA was used to render

the scene in 3-D

8

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

The PSF

What Astronomers see…

1) Main Sequence 2) SubGiant

Branch

3) Red Giant Branch

4) Horizontal Branch

5) White Dwarf Sequence

Easy to identify stars…RGB

WDs

SGB

HB

MSTO

RedDwarfs

BSs

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” all at the sameage

Omega N6397 47T

Omega Cen NGC6397 47 Tuc

Extrasequences

Omega Centauri:Globular Cluster or Dwarf Spheroidal?

Cambridge, UK 2001

ME!

Stellar Populations

More metals

More Helium

Age

Inversion!

More metals

metalpoor

intermediate

metal rich

Red GiantBranch

Similar togalaxies…

More metals

Is Omega Cen a GC?Could the textbook globular cluster not even be one?

47Tuc

N2808

N6656N6388

OmCen®

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 we see going on today?

Is Omega Cen a globular?

NGC2808

NGC6652

HST is a GC machine…HST’s advantages

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

Cluster topics: 1) Cluster-field separation 2) Search for black holes

The first full CMD ever

• 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

Proper-Motion Cleaning PI-Rich, UCLA

CLUSTER IN A CLUSTER

Intermediate-Mass Black Holes (IMBHs) in GCs

• BHs known to exist with

– ~10 MSUN : HMXBs

– ~106 MSUN: SMBHs at galaxy centers

High-MassX-ray Binaries

Super-MassiveBlack Holes

Ghez Group at UCLA

See: http://www.astro.ucla.edu/~ghezgroup/gc/pictures/orbitsMovie.shtml

Intermediate-Mass Black Holes (IMBHs) in GCs

• BHs known to exist with

– ~10 MSUN : HMXBs

– ~106 MSUN: SMBHs at galaxy centers

why not?

– ~104 MSUN: IMBHs

• Could explain a lot!

10,000 MSUN

~20 km/s

Greene & Ho (2006)

Higher Velocities

Big

ger

Bla

ck

Ho

le

High-MassX-ray Binaries

Super-MassiveBlack Holes

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

– Fast moving star in orbit (smoking gun)– Rise in velocities at center– Also ways without PMs or RVs…

• Pulsars, accretion signatures– Indirect: 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…

• 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:– IFU from ground saw fast stars at center

• 40,000 MSUN IMBH?

• HST should see this…

Previous Observations…

Omega Cen: a Ground-Based Image (Lehman)

HST-vs-Ground

MeasuringProper

Motions

Epoch1: 2002 Epoch2: 2006

Stars with good measured motions

3 Zone of Influence

PM Profile

Total motions ofthe 1000 stars in

the inner 10

100 stars/bin

Proper Motions indicate no obvious mass at the center.

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

Simulation on Following Page:

One frame = 100 years

Total time = 1000 years

Moving stars

12765349108

AT OUR SPEED…

With a BH…

See:

http://www.stsci.edu/~jayander/omcen_sim_pms_2d_wbh.mpg

Future Directions…• Omega Cen

– More time? More stars? Better models?

• Other clusters coming!NGC 362

M15

NGC 6624

NGC 7099

NGC 6681

AND MORE!