Globular Clusters and Globular Clusters and Galaxy FormationGalaxy Formation

Duncan A. ForbesDuncan A. Forbes

Centre for Astrophysics & Centre for Astrophysics & Supercomputing, Swinburne Supercomputing, Swinburne

UniversityUniversity

Collaborators - SAGES ProjectCollaborators - SAGES Project

Mike Beasley (Swinburne)

Jean Brodie (Lick Observatory)

John Huchra (Harvard-Smithsonian)

Markus Kissler-Patig (ESO)

Soeren Larsen (Lick Observatory)

TelescopesTelescopes

Globular ClustersGlobular 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-populationsTwo sub-populationsMost (perhaps all) large galaxies reveal two distinct sub-populations of globular clusters.

V-I = 0.95 1.15 [Fe/H] = -1.5 -0.5

Why study extragalactic GCs ?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)Local Group GCs (D ~1 Mpc)

Photometry

• CMD Morphology [0.1” ~ 0.4 pc]

• Optical colours

• Infrared colours

• Sizes

Spectra (V ~ 16)

• Metallicities

• Abundances

• Relative Ages

• System Kinematics

3-6m telescopes

Brodie & Huchra 1990, 1991

Virgo/Fornax GCs (D ~15 Mpc)Virgo/Fornax GCs (D ~15 Mpc)

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)

8-10m telescopes

Milky Way Globular Cluster System

Sub-populations:Sub-populations:

Metal-rich ~50Metal-rich ~50

BulgeBulge (R (RGCGC < 5 < 5 kpc)kpc)

Thick disk Thick disk (R (RGCGC > 5 > 5 kpc)kpc)

Metal-poor ~100Metal-poor ~100

Old Halo Old Halo (prograde)(prograde)

Young Halo Young Halo (retrograde)(retrograde)

Young halo + 4 Sgr dwarf GCs = Sandage noiseYoung halo + 4 Sgr dwarf GCs = Sandage noise

Milky Way Bulge Clusters

The inner The inner metal-richmetal-rich GCs are: GCs are:

• spherically distributedspherically distributed

• similar metallicity to bulge starssimilar metallicity to bulge stars

• similar velocity dispersion to bulgesimilar velocity dispersion to bulge

• follow the bulge rotationfollow the bulge rotation

=> associated with the bulge

Minniti 1995

Bulge Clusters in Bulge Clusters in M31M31 and and M81M81

The inner metal-rich GCs are:The inner metal-rich GCs are:

• spherically distributed spherically distributed M31 M31 M81M81

• similar metallicity to bulge stars similar metallicity to bulge stars M31 M31 M81?M81?

• similar velocity dispersion to bulge similar velocity dispersion to bulge M31 M31 M81M81

• follow the bulge rotation follow the bulge rotation M31 M31 M81M81

The Sombrero GalaxyThe Sombrero Galaxy

MetallicitiesMetallicitiesNumber of metal-rich GCs scale with the bulgeNumber of metal-rich GCs scale with the bulge

Forbes, Brodie & Larsen 2001Forbes, Brodie & Larsen 2001

Globular Clusters in M104Globular Clusters in M104

M104 M31 MW

Sa Sb Sbc

667 100 53

0.80 0.25 0.19

4.2 0.21 0.19

1.1 0.63 0.84

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.

GCs in SpiralsGCs 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 The Elliptical GalaxyElliptical Galaxy Formally Known as Formally Known as The Local GroupThe Local Group

M31+MW+M33+LMC+SMC+...

gE with MV = – 22.0

and N = 700 +/- 125

Universal luminosity function

Local Group EllipticalLocal Group Elliptical

Metallicity peaks at

[Fe/H] = –1.55, –0.64

Ratio 2.5:1

Forbes, Masters, Minniti & Barmby 2000Forbes, Masters, Minniti & Barmby 2000

SN = 1.1 -> 2.5

S + S -> gE with low SN

Numbers, Specific FrequencyNumbers, 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.

LuminositiesLuminositiesA Universal Globular Cluster Luminosity FunctionA 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 2000Harris 2000

Sizes

Larsen et al. 2001Larsen et al. 2001

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.

Metallicities

All ( MV < –15 ) galaxies, reveal a population of GCs with [Fe/H] ~ –1.5.

All large (bulge) galaxies reveal a similar GC metallicity distribution.

The WLM galaxy has one GC, [Fe/H] = –1.52 age = 14.8 Gyrs (Hodge et al. 1999).

Metallicity vs Metallicity vs Galaxy MassGalaxy Mass

Forbes, Larsen & Brodie 2001Forbes, Larsen & Brodie 2001

Blue GCs <2.5

V–I ~ 0.95

Pregalactic ?

Red GCs ~4

Spirals fit the trend

Metallicity vs Metallicity vs Galaxy MassGalaxy Mass

Forbes, Larsen & Brodie 2001Forbes, Larsen & Brodie 2001

Red GC relation has similar slope to galaxy colour relation.

Red GCs and galaxy stars formed in the same star formation event.

Colour - ColourColour - Colour

Forbes & Forte 2001Forbes & Forte 2001

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

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

Summary from Photometry 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

GC Ages

blue

red

LRIS spectra, 2hrs

~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)

NGC 1399NGC 1399

1.6 Gyr

2.3 Gyr

12 Gyr

Caveat: BHBs

Abundance RatiosHigh 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]

System KinematicsM49M49

red low ~ galaxy

blue high , rotate

M87

red ~ blue, rotate

No general trends

Zepf etal. 2000

GC Formation ScenariosGC Formation Scenarios

• Mergers of spirals (Ashman & Zepf 1992)

• Two-phase collapse (Forbes, Brodie & Grillmair 1997)

• In situ plus accretion (Cote, Marzke & West 1998)

Merger ModelMerger Model

Ashman & Zepf 1992

Merger ModelMerger Model

Merger ModelMerger Model

Meanwhile, 10 billion years later...

Merger ModelMerger Model

Multi-phase collapse modelMulti-phase collapse model

Forbes, Brodie & Grillmair 1997

Pre-galactic Phase

clumpy collapse of gas cloud

formation of metal-poor globulars and a

few halo stars

T = 0 Dormant Phase

star formation stopped by SNII

gas reheated by SNIa

gas cools

Galactic Phase

additional gas collapse

formation of metal-rich globulars and bulge

stars

T = 2

Multi-phase collapse modelMulti-phase collapse model

Mike BeasleyDuncan Forbes

Ray SharplesCarlton 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 fractionof the stars formed inthe model produces a skewed, unimodal distribution in V-I.

Assuming that theefficiency is dependentupon the SFR(e.g. Larsen 2000)produces a sharper unimodal peak.

Truncating the blue GCformation at high-redshiftproduces a bimodal-distribution.

[Fe/H] V-I

GC Colour DiversityAfter 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 highredshift.

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 SWhat drives SNN ? ?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.

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

SSNN of young ellipticals 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)

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 Metallicity vs Mass

GC Age Predictions

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.

Age

N

Age

log

GC AgesGC Ages

blue

red

LRIS spectra, 2hrs

~3.5hrs on Keck 10m

Hß error = +/- 0.25 A

NGC 1399

-2.2 < [Fe/H] < 0.3

ages ~ 12 Gyrs

(some young GCs)

NGC 1399NGC 1399

1.6 Gyr

2.3 Gyr

12 Gyr

Observational SummaryObservational 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 PredictionsModel PredictionsOur 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 DevelopmentsFuture 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 TimelineFormation TimelineBlue GCs form in metal-poor gaseous fragments with little or no knowledge of potential well. Halo formation.

8-11 8-11 GyrsGyrs

12 12 GyrsGyrs

TimeTime

Late epoch mergers of Sp + Sp (low SN) E

Clumpy collapse/merger of gaseous fragments form metal-rich red GCs and `bulge’ stars.

http://astronomy.swin.edu.au/dforbes

Number per unit Number per unit StarlightStarlight

McLaughlin (1999) proposed a universal

GC formation efficiency

= MGC / Mgas + Mstars

= 0.26 %

Mgas = current Xray gas mass

Ntot ~ L22

= 0.2%= 0.2%

NGC 5128NGC 5128

Harris etal 1999

Reveals a mostly metal-rich halo, which has similar metallicity to the metal-rich globular clusters.