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!