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

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

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  • Observational Properties of Extragalactic Globular Cluster SystemsDuncan A. ForbesCentre for Astrophysics & Supercomputing, Swinburne University

  • Globular Clusters and Galaxy FormationDuncan A. ForbesCentre for Astrophysics & Supercomputing, Swinburne University

  • Collaborators - SAGES ProjectMike 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-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 ? 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) Photometry CMD Morphology [0.1 ~ 0.4 pc] Optical colours Infrared colours Sizes Spectra (V ~ 16) Metallicities Abundances Relative Ages System Kinematics3-6m telescopesBrodie & Huchra 1990, 1991

  • 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 SystemSub-populations:Metal-rich ~50Bulge (RGC < 5 kpc)Thick disk (RGC > 5 kpc)Metal-poor ~100Old Halo (prograde)Young Halo (retrograde)Young halo + 4 Sgr dwarf GCs = Sandage noise

  • Milky Way Bulge ClustersThe inner metal-rich GCs are: spherically distributed similar metallicity to bulge stars similar velocity dispersion to bulge follow the bulge rotation=> associated with the bulgeMinniti 1995

  • Bulge Clusters in M31 and M81The 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

  • MetallicitiesNumber of metal-rich GCs scale with the bulgeForbes, Brodie & Larsen 2001

  • Globular Clusters in M104M104 M31 MWSaSbSbc667100530.800. typeMetal-rich GCsBulge-to-totalDisk SNBulge SNThe metal-rich GCs in M104 must be associated with the bulge not disk component.

  • GCs in SpiralsInner 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 GroupM31+MW+M33+LMC+SMC+...gE with MV = 22.0and N = 700 +/- 125Universal luminosity function

  • Local Group EllipticalMetallicity peaks at[Fe/H] = 1.55, 0.64Ratio 2.5:1Forbes, Masters, Minniti & Barmby 2000SN = 1.1 -> 2.5

    S + S -> gE with low SN

  • 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.

  • LuminositiesA Universal Globular Cluster Luminosity Function MV Ellipticals 7.33 +/- 0.04 1.36 +/- 0.03 Spirals 7.46 +/- 0.081.21 +/- 0.05 Ho = 74 +/- 7 km/s/Mpc GCLFHo = 72 +/- 8 km/s/Mpc HST Key ProjectHarris 2000

  • SizesLarsen et al. 2001For Sp S0 E cD the GCs reveal a sizecolour trend. The blue GCs are larger by ~20%.This trend exists for a range of galaxy types and galactocentric radii.

  • MetallicitiesAll ( 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 Galaxy MassForbes, Larsen & Brodie 2001Blue GCs
  • Metallicity vs Galaxy MassForbes, Larsen & Brodie 2001Red GC relation has similar slope to galaxy colour relation.Red GCs and galaxy stars formed in the same star formation event.

  • Colour - ColourForbes & Forte 2001Galaxy 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 Blue GCs = halocommon to all galaxies[Fe/H] ~ 1.5constant colour Pregalactic ?Red GCs = bulge[Fe/H] ~ 0.5colour varies with galaxy massformed in same event as bulge stars

  • GC AgesblueredLRIS spectra, 2hrs~3.5hrs on Keck 10mH error = +/- 0.2 to 0.3 A

    NGC 1399-2.2 < [Fe/H] < 0.3ages ~ 12 Gyrs(some young GCs)NGC 1399NGC 13991.6 Gyr2.3 Gyr12 GyrCaveat: 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 KinematicsM49M49red low ~ galaxyblue high , rotate

    M87red ~ blue, rotate

    No general trendsZepf 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 ModelAshman & Zepf 1992

  • Merger Model

  • Merger ModelMeanwhile, 10 billion years later...

  • Merger Model

  • Multi-phase collapse modelForbes, Brodie & Grillmair 1997Pre-galactic Phaseclumpy collapse of gas cloudformation of metal-poor globulars and a few halo starsT = 0 Dormant Phasestar formation stopped by SNIIgas reheated by SNIagas coolsGalactic Phaseadditional gas collapseformation of metal-rich globulars and bulge starsT = 2

  • Multi-phase collapse model

  • Mike BeasleyDuncan ForbesRay 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 LuminosityThe 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 appearbursty.

    Old ages and particularly bimodal metallicity distributions lead to bimodal colour distributions.


  • 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.

  • Evolution in SNFor 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 ellipticalsGalaxy Merger AgeSNNGC31561 Gyr0.3NGC17002 Gyr1.4 -> 2NGC67022 Gyr2.3 -> 4NGC13163 Gyr1.5 -> 2NGC36103 Gyr1.1

    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 PredictionsThe 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.AgeNAgelog

  • GC AgesblueredLRIS spectra, 2hrs~3.5hrs on Keck 10mH error = +/- 0.25 A

    NGC 1399-2.2 < [Fe/H] < 0.3ages ~ 12 Gyrs(some young GCs)NGC 1399NGC 13991.6 Gyr2.3 Gyr12 Gyr

  • 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 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 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 TimelineBlue GCs form in metal-poor gaseous fragments with little or no knowledge of potential well. Halo formation.8-11 Gyrs12 GyrsTimeLate epoch mergers of Sp + Sp (low SN) EClumpy collapse/merger of gaseous fragments form metal-rich red GCs and `bulge stars.http://astronomy.swin.edu.au/dforbes

  • Number per unit StarlightMcLaughlin (1999) proposed a universal GC formation efficiency = MGC / Mgas + Mstars= 0.26 % Mgas = current Xray gas massNtot ~ L2 = 0.2%

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