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Globular Clusters and Galaxy
Formation
Duncan A. Forbes
Centre for Astrophysics &
Supercomputing, Swinburne University
Collaborators - SAGES Project
Mike Beasley (Swinburne)
Jean Brodie (Lick Observatory)
John Huchra (Harvard-Smithsonian)
Markus Kissler-Patig (ESO)
Soeren Larsen (Lick Observatory)
Telescopes
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.
Two sub-populations
Most (perhaps all) large galaxies reveal two
distinct sub-populations of globular clusters.
[Fe/H]
V-I =
-1.5
0.95
-0.5
1.15
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:
http://astronomy.swin.edu.au/dforbes
Local Group GCs (D ~1 Mpc)
3-6m telescopes
Photometry
Spectra (V ~ 16)
• CMD Morphology
[0.1” ~ 0.4 pc]
• Metallicities
• Optical colours
• Infrared colours
• Abundances
• Relative Ages
• System Kinematics
• Sizes
Brodie & Huchra 1990, 1991
Virgo/Fornax GCs (D ~15 Mpc)
8-10m telescopes
Photometry
Spectra (V ~ 22)
• Optical colours (~50)
• Metallicities (~10)
• Infrared colours (~4)
• Abundance ratios (~6)
• Sizes (~20)
• Relative Ages (~6)
[0.1” ~ 7 pc]
• System Kinematics (~4)
Milky Way Globular Cluster System
Sub-populations:
Metal-rich ~50
Metal-poor ~100
Bulge
(RGC < 5 kpc)
Old Halo
(prograde)
Thick disk
(RGC > 5 kpc)
Young Halo
(retrograde)
Young halo + 4 Sgr dwarf GCs = Sandage noise
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
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
The Sombrero Galaxy
Metallicities
Number of metal-rich GCs scale with the bulge
Forbes, Brodie & Larsen 2001
Globular Clusters in M104
M104
M31
MW
Sa
Sb
Sbc
Metal-rich GCs
667
100
53
Bulge-to-total
0.80
0.25
0.19
Disk SN
4.2
0.21
0.19
Bulge SN
1.1
0.63
0.84
Hubble type
The metal-rich GCs in M104 must be associated
with the bulge not disk component.
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
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
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
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.
Luminosities
A Universal Globular Cluster Luminosity Function
MV

Ellipticals
–7.33 +/- 0.04
1.36 +/- 0.03
Spirals
–7.46 +/- 0.08
1.21 +/- 0.05
Ho = 74 +/- 7 km/s/Mpc
GCLF
Ho = 72 +/- 8 km/s/Mpc
HST Key Project
Harris 2000
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
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).
Metallicity vs
Galaxy Mass
Blue GCs <2.5
V–I ~ 0.95
Pregalactic ?
Red GCs ~4
Spirals fit the trend
Forbes, Larsen & Brodie 2001
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
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
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 ?
Ellipticals
Blue
Halo
Red
`Bulge’
Red
?
Spirals
Halo
Bulge
Disk
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
NGC 1399
NGC 1399
GC Ages
~3.5hrs on Keck 10m
blue
Hß error = +/- 0.2 to 0.3 A
1.6 Gyr
NGC 1399
2.3 Gyr
-2.2 < [Fe/H] < 0.3
ages ~ 12 Gyrs
red
12 Gyr
(some young GCs)
Caveat: BHBs
LRIS spectra, 2hrs
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 protoGCs in the ongoing merger NGC7252]
System Kinematics
M49
M49
red low  ~ galaxy
blue high  , rotate
M87
red ~ blue, rotate
No general trends
Zepf etal. 2000
GC Formation Scenarios
• Mergers of spirals
(Ashman & Zepf 1992)
• Two-phase collapse
(Forbes, Brodie & Grillmair 1997)
• In situ plus accretion
(Cote, Marzke & West 1998)
Merger Model
Ashman & Zepf 1992
Merger Model
Merger Model
Meanwhile, 10 billion years later...
Merger Model
Multi-phase collapse model
Pre-galactic
Phase
Galactic
DormantPhase
Phase
clumpy
collapse
of
additional
gas collapse
star formation
gas cloud
stopped
by SNII
formation of metal-rich
formation
of metalglobulars
and
bulge
gas
reheated
by
SNIa
poor globulars
stars and a
gas cools
few halo stars
T=0
T=2
Forbes, Brodie & Grillmair 1997
Multi-phase collapse model
Mike Beasley
Duncan Forbes
Ray Sharples
Carlton Baugh
• 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
Simply taking a fraction
of the stars formed in
the model produces a
skewed, unimodal
distribution in V-I.
[Fe/H]
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 bimodaldistribution.
V-I
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)
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.
A Massive Elliptical
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
What drives SN ?
The plot shows model
galaxies (blue), data
(red) and the spiralspiral merger model of
Bekki etal. (green).
High SN galaxies are
associated with large
numbers of blue GCs,
not red GCs as
expected in spiralspiral mergers.
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 = -22.0
MB = -20.4
SN of young ellipticals
Galaxy
Merger Age
SN
NGC3156
1 Gyr
0.3
NGC1700
2 Gyr
1.4 -> 2
NGC6702
2 Gyr
2.3 -> 4
NGC1316
3 Gyr
1.5 -> 2
NGC3610
3 Gyr
1.1
Old ellipticals have SN ~ 3 (field) to 6 (cluster)
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.
GC Age Predictions
Age
The mean age of
red GCs shows a
strong dependence
on dark matter halo
velocity (mass).
log 
The model predicts
that low-L and field
galaxies have
younger red GCs.
N
Age
NGC 1399
NGC 1399
GC Ages
~3.5hrs on Keck 10m
blue
Hß error = +/- 0.25 A
1.6 Gyr
NGC 1399
2.3 Gyr
-2.2 < [Fe/H] < 0.3
ages ~ 12 Gyrs
red
12 Gyr
(some young GCs)
LRIS spectra, 2hrs
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.
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.
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)
Formation Timeline
12
Gyrs
Blue GCs form in metal-poor gaseous
fragments with little or no knowledge of
potential well. Halo formation.
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
http://astronomy.swin.edu.au/dforbes
Number per unit
Starlight
McLaughlin (1999)
proposed a universal
GC formation
efficiency
 = MGC / Mgas + Mstars
= 0.26 %
Mgas = current
Xray gas mass
Ntot ~  L
2
 = 0.2%
NGC 5128
Reveals a mostly
metal-rich halo,
which has similar
metallicity to the
metal-rich globular
clusters.
Harris etal 1999