1. Observational Properties of Extragalactic Globular Cluster Systems
Duncan A. Forbes
Centre for Astrophysics & Supercomputing, Swinburne University

2. Globular Clusters and Galaxy Formation
Duncan A. Forbes
Centre for Astrophysics & Supercomputing, Swinburne University

3. Collaborators - SAGES Project
Mike Beasley (Swinburne)
Jean Brodie (Lick Observatory)
John Huchra (Harvard-Smithsonian)
Markus Kissler-Patig (ESO)
Soeren Larsen (Lick Observatory)

4. Telescopes

5. Globular Clusters Bound homogeneous collections of ~105 M
All galaxies with MV < -15 have at least one globular, some have over 10,000
They have a universal luminosity function which gives the Hubble constant accurate to 10%
Share the same star formation and chemical enrichment history as their host galaxy.
Provide a powerful and unique probe of galaxy formation.

6. Two sub-populations
Most (perhaps all) large galaxies reveal two distinct sub-populations of globular clusters.
[Fe/H] =
V-I =

7. Why study extragalactic GCs ?
The MW has only 140 known GCs, M31 has ~400, M87 has ~10,000
The Local Group has no giant ellipticals
Probe to higher metallicity GCs
Same distance and reddening
No foreground stars in the GC spectra
Discover something new !
Extragalactic Globular Cluster Database:

8. Local Group GCs (D ~1 Mpc)
3-6m telescopes
Photometry
CMD Morphology [0.1” ~ 0.4 pc]
Optical colours
Infrared colours
Sizes
Spectra (V ~ 16)
Metallicities
Abundances
Relative Ages
System Kinematics
Brodie & Huchra 1990, 1991

9. Virgo/Fornax GCs (D ~15 Mpc)
8-10m telescopes
Photometry
Optical colours (~50)
Infrared colours (~4)
Sizes (~20)
[0.1” ~ 7 pc]
Spectra (V ~ 22)
Metallicities (~10)
Abundance ratios (~6)
Relative Ages (~6)
System Kinematics (~4)

10. Milky Way Globular Cluster System
Sub-populations:
Metal-rich ~50
Bulge (RGC < 5 kpc)
Thick disk (RGC > 5 kpc)
Metal-poor ~100
Old Halo (prograde)
Young Halo (retrograde)
Young halo + 4 Sgr dwarf GCs = Sandage noise

11. Milky Way Bulge Clusters
The inner metal-rich GCs are:
spherically distributed
similar metallicity to bulge stars
similar velocity dispersion to bulge
follow the bulge rotation
=> associated with the bulge
Minniti 1995

12. Bulge Clusters in M31 and M81
The inner metal-rich GCs are:
spherically distributed M31 M81
similar metallicity to bulge stars M31 M81?
similar velocity dispersion to bulge M31 M81
follow the bulge rotation M31 M81

13. The Sombrero Galaxy

14. Metallicities Number of metal-rich GCs scale with the bulge
Forbes, Brodie & Larsen 2001

15. Globular Clusters in M104 M104 M31 MW Sa Sb Sbc 667 100 53 Hubble type
Hubble type
Metal-rich GCs
Bulge-to-total
Disk SN
Bulge SN
The metal-rich GCs in M104 must be associated with the bulge not disk component.

16. GCs in Spirals
Inner metal-rich GCs in spirals are associated with the bulge not the disk.
The number of bulge GCs scales with bulge luminosity (bulge SN ~1).
Forbes, Brodie & Larsen 2001

17. The Elliptical Galaxy Formally Known as The Local Group
M31+MW+M33+LMC+SMC+...
gE with MV = – 22.0
and N = 700 +/- 125
Universal luminosity function

18. Local Group Elliptical
Metallicity peaks at
[Fe/H] = –1.55, –0.64
Ratio 2.5:1
SN = 1.1 -> 2.5
S + S -> gE with low SN
Forbes, Masters, Minniti & Barmby 2000

19. Numbers, Specific Frequency
The bulge SN for spirals is ~ 1
The total SN for field ellipticals is 1-3 (Harris 1991)
The fraction of red GCs in ellipticals is about 0.5
The bulge SN for field ellipticals is ~1
=> Spirals and field ellipticals have a similar number of metal-rich GCs per unit starlight.

20. A Universal Globular Cluster Luminosity Function
Luminosities
A Universal Globular Cluster Luminosity Function
MV 
Ellipticals –7.33 +/ /- 0.03
Spirals –7.46 +/ /- 0.05
Ho = 74 +/- 7 km/s/Mpc GCLF
Ho = 72 +/- 8 km/s/Mpc HST Key Project
Harris 2000

21. Sizes
For Sp  S0 E  cD the GCs reveal a size–colour trend. The blue GCs are larger by ~20%.
This trend exists for a range of galaxy types and galactocentric radii.
Larsen et al. 2001

22. Metallicities
All large (bulge) galaxies reveal a similar GC metallicity distribution.
All ( MV < –15 ) galaxies, reveal a population of GCs with [Fe/H] ~ –1.5.
The WLM galaxy has one GC, [Fe/H] = –1.52 age = 14.8 Gyrs (Hodge et al. 1999).

23. Metallicity vs Galaxy Mass
Blue GCs <2.5
V–I ~ 0.95
Pregalactic ?
Red GCs ~4
Spirals fit the trend
Forbes, Larsen & Brodie 2001

24. Metallicity vs Galaxy Mass
Red GC relation has similar slope to galaxy colour relation.
Red GCs and galaxy stars formed in the same star formation event.
Forbes, Larsen & Brodie 2001

25. Colour - Colour Galaxy and GC colours from the same observation.
The red GCs and field stars have a very similar metallicity and age.
Also NGC 5128 (Harris et al. 1999)
Bulge C-T1 colour
Forbes & Forte 2001

26. Spatial Distribution
Red GCs are centrally concentrated, have similar azimuthal and density profile to the `bulge’ light.
Blue GCs are more extended. Does the blue GC density profile follow the X-ray/halo profile ?
Blue Red Red Ellipticals Halo `Bulge’ ?
Spirals Halo Bulge Disk

27. Summary from Photometry
Blue GCs = halo
common to all galaxies
[Fe/H] ~ –1.5
constant colour  Pregalactic ?
Red GCs = bulge
[Fe/H] ~ –0.5
colour varies with galaxy mass
formed in same event as bulge stars

28. GC Ages NGC 1399 NGC 1399 blue ~3.5hrs on Keck 10m
Hß error = +/- 0.2 to 0.3 A
NGC 1399
-2.2 < [Fe/H] < 0.3
ages ~ 12 Gyrs
(some young GCs)
blue
1.6 Gyr
2.3 Gyr
red
12 Gyr
LRIS spectra, 2hrs
Caveat: BHBs

29. Abundance Ratios
High S/N spectra of GCs in ellipticals suggests that all GCs have supersolar alpha abundance ratios, eg [Mg/Fe] ~ +0.3 (similar to the MW).
Supersolar ratios indicate the dominance of SN II vs SN Ia products in the GC-forming gas.
This is due to:
short time formation timescale
IMF skewed to high mass stars
=> important chemical evolution clues
[Note: Maraston etal 2001 found solar ratio proto-GCs in the ongoing merger NGC7252]

30. System Kinematics M49 M49 red low  ~ galaxy blue high  , rotate M87
red ~ blue, rotate
No general trends
Zepf etal. 2000

31. GC Formation Scenarios
Mergers of spirals (Ashman & Zepf 1992)
Two-phase collapse (Forbes, Brodie & Grillmair 1997)
In situ plus accretion (Cote, Marzke & West 1998)

32. Merger Model
Ashman & Zepf 1992

33. Merger Model

34. Meanwhile, 10 billion years later...
Merger Model
Meanwhile, 10 billion years later...

35. Merger Model

36. Multi-phase collapse model
Galactic Phase
additional gas collapse
formation of metal-rich globulars and bulge stars
Pre-galactic Phase
clumpy collapse of gas cloud
formation of metal-poor globulars and a few halo stars
Dormant Phase
star formation stopped by SNII
gas reheated by SNIa
gas cools
T = 0
T = 2
Forbes, Brodie & Grillmair 1997

37. Multi-phase collapse model

38. Mike Beasley
Duncan Forbes
Ray Sharples
Carlton Baugh

39. Merging of Dark Matter halos using Monte-Carlo method yielding ‘Merger Trees’.
Gaseous fragments form some metal-poor stars and GCs before the main galaxy.
Gaseous pre-galactic fragments may merge/collapse forming a burst of stars that results in an elliptical galaxy. The bulk of stars are formed in this burst.
Hot gas cools onto the elliptical galaxy, forming a cold gas disk over time and hence a spiral galaxy.
Merger Tree

40. Simply taking a fraction of the stars formed in
the model produces a skewed, unimodal distribution in V-I.
[Fe/H]
V-I
Assuming that the
efficiency is dependent
upon the SFR
(e.g. Larsen 2000)
produces a sharper unimodal peak.
Truncating the blue GC
formation at high-redshift
produces a bimodal-
distribution.

41. GC Colour Diversity
After correcting for magnitude incompleteness and limited areal coverage, the model can reproduce the diversity of GC colour distributions seen with HST (eg Larsen etal. 2001)

42. GC Number vs Host Galaxy Luminosity
The model galaxies (small circles) are well matched to the observations (red symbols).
The purple line shows constant GC formation efficiency.

43. A Massive Elliptical Model Age ‘peak’ of blue GCs is a
result of truncation at high
redshift.
Ages of red GCs show a
greater range and appear
‘bursty’.
Old ages and particularly bimodal metallicity distributions lead
to bimodal colour
distributions.
Model

44. What drives SN ?
The plot shows model galaxies (blue), data (red) and the spiral-spiral merger model of Bekki etal. (green).
High SN galaxies are associated with large numbers of blue GCs, not red GCs as expected in spiral-spiral mergers.

45. Evolution in SN
For most ellipticals there is very little evolution in SN in the last ~10 Gyrs.
In the low luminosity elliptical, SN increases from 4.5 to 4.7 as the result of a major merger 4 Gyrs ago.
MB = MB = -20.4

46. SN of young ellipticals
Galaxy Merger Age SN
NGC Gyr 0.3
NGC Gyr > 2
NGC Gyr > 4
NGC Gyr > 2
NGC Gyr 1.1
Old ellipticals have SN ~ 3 (field) to 6 (cluster)

47. GC Metallicity vs Mass The red model GCs, are generally
consistent with
the data, but do not have the same slope.
The blue model GCs behave
similarly to the red GCs, but with less scatter.

48. The model predicts that low-L and field galaxies have younger red GCs.
GC Age Predictions
Age
The mean age of red GCs shows a strong dependence on dark matter halo velocity (mass).
The model predicts that low-L and field galaxies have younger red GCs.
log  N
Age

49. GC Ages NGC 1399 NGC 1399 blue ~3.5hrs on Keck 10m
Hß error = +/ A
NGC 1399
-2.2 < [Fe/H] < 0.3
ages ~ 12 Gyrs
(some young GCs)
blue
1.6 Gyr
2.3 Gyr
red
12 Gyr
LRIS spectra, 2hrs

50. Observational Summary
The inner metal-rich GCs in the Milky Way and other spirals have a bulge (not disk) origin.
The GC systems of spirals and ellipticals show remarkable similarities.
Blue and red GCs have similar ages ~12 Gyrs (but red GCs could be younger by 2-4 Gyrs)
Some very young (~2 Gyrs) GCs have been found in `old’ ellipticals.
Blue and red GCs have [Mg/Fe] ~ +0.3.
Red GCs trace elliptical galaxy star formation.

51. Model Predictions
Our model (Beasley etal. 2002) of GC formation in a Hierarchical Universe assumes truncation of blue GCs at z = 5, and predicts:
SN is determined at early epochs; late stage mergers have little effect on SN
Blue GCs formed ~12 Gyrs ago in all ellipticals.
Red GCs have a mean age of ~8-10 Gyrs in field and low luminosity ellipticals.
In general: spirals and ellipticals have similar GC systems.

52. Future Developments
HST+ACS U-band and 8m wide-field K-band imaging studies (eg Puzia etal)
Age structure within the red GCs (eg Beasley etal)
Nature of the new large (Reff ~ 10 pc) red low luminosity (MV ~ -6) clusters (eg Brodie & Larsen)
Importance of stripped nucleated dE (eg Bekki etal)
HST+ACS CMDs for Local Group GCs (eg Rich etal)
Halo mass estimates: GC kinematics vs X-rays
Better SSP grids (eg BHBs, AGB, supersolar)

53. Late epoch mergers of Sp + Sp  (low SN) E
Formation Timeline
Blue GCs form in metal-poor gaseous fragments with little or no knowledge of potential well. Halo formation.
12 Gyrs
8-11 Gyrs
Clumpy collapse/merger of gaseous fragments form metal-rich red GCs and `bulge’ stars.
Late epoch mergers of Sp + Sp  (low SN) E
Time

55. Number per unit Starlight
 = 0.2%
McLaughlin (1999) proposed a universal GC formation efficiency
 = MGC / Mgas + Mstars
= 0.26 %
Mgas = current Xray gas mass 2
Ntot ~  L

56. NGC 5128
Reveals a mostly metal-rich halo, which has similar metallicity to the metal-rich globular clusters.
Harris etal 1999