Old stellar populations in galaxy clusters
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Old Stellar Populations in Galaxy Clusters. Scott Trager (Kapteyn Institute, Groningen) with Sandy Faber (UCO/Lick) Alan Dressler (Carnegie Observatories). Introduction: Why Cluster Galaxies?. Why early-type galaxies?

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Old Stellar Populations in Galaxy Clusters

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Old stellar populations in galaxy clusters

Old Stellar Populations in Galaxy Clusters

Scott Trager

(Kapteyn Institute, Groningen)


Sandy Faber (UCO/Lick)

Alan Dressler (Carnegie Observatories)

Stellar Populations, MPA

Introduction why cluster galaxies

Introduction: Why Cluster Galaxies?

  • Why early-type galaxies?

    • Spheroidal galaxies (E’s, S0’s, and bulges) contain 50% (Schechter & Dressler 1987) to 75% (Fukugita et al. 1998) of stellar mass in Universe

  • Why clusters?

    • Easy place to find them—80% of ellipticals live in clusters, at least 50% of S0s

  • Aren’t cluster early-types boring, forming all their stars at z>3?

    • Maybe…maybe not…

Stellar Populations, MPA

Observed clusters

Observed clusters

  • Local Universe: rich and nearby…

    • Nearby:

      • Virgo (Gonzàlez 1993; Caldwell et al. 2003; Yamada et al.)

      • Fornax (Kuntschner 2000)

      • Hydra (Trager, in prep.)

    • Rich:

      • Coma (cf. Jørgensen 1999; Mehlert et al. 2000; Terlevich et al. 2001; Poggianti et al. 2001; Kuntschner et al. 2001; Moore et al. 2002; Sánchez-Blázquez et al.; Trager et al.)

  • Distant Universe: rich clusters (so far…)

    • Abell 851 (=CL0939+4713), z=0.41

    • GHO1322+3114 (z=0.69), GHO1322+3027 (z=0.75)

Stellar Populations, MPA

Coma our study

Coma: our study

  • Centered around NGC 4874, one of two cDs in center of Coma

    • 30s “white light” picture from LRIS

  • Early-types with Mb<-16

Stellar Populations, MPA

Really good spectra





Really good spectra!

  • Coma early-type galaxy spectra taken with Keck/LRIS

    • 2 hours through clouds, 0”8 seeing

    • Typical S/N>150/Å

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Ok the boring stuff

OK, the boring stuff

  • Calibration to Lick/IDS system

    • Relative flux correction

    • Smooth to Lick/IDS resolution

    • Measure line strengths and errors (à la Gonzàlez 1993) through correct bandpasses

      • Fixed by cross-correlation of each index with HR 6018, the template star for the Lick/IDS system

    • Calibrate stellar indices onto Lick/IDS stellar system

      • Small additive corrections needed for some indices

    • Velocity dispersion corrections to get galaxies onto stellar system (à la Trager et al. 1998)

  • Et voilà! We’re on the Lick/IDS system…

Stellar Populations, MPA

Stellar populations of coma early types

Stellar Populations of Coma Early-types


Red: field & Virgo Es from Gonzàlez (1993);

Green: Fornax early-types from Kuntschner (2000)

Models from Thomas,

Maraston & Bender (2003)


  • Early-types in Coma are nearly coeval

    • but not exactly: Factor of two spread in age, even in the massive galaxies!

      • NGC4874 is 12 Gyr old, NGC4867 is 5 Gyr old

        • note that error bars are mostly smaller than points

  • But much more than even early-types in the field (red) or even Fornax (green)!

Stellar Populations, MPA

The z plane

The Z-plane

  • Trager et al. (2000b) found that local early-type galaxies project onto a plane in the four-space of log t, [Z/H], [E/Fe], and logσ

    • Controlling parameters can be identified with log t and log σ

  • Coma early-types obey this plane as well

    • Except D158 and GMP3565, which seem to be too metal-poor for their masses

Models from Worthey (1994) + Trager et al (2000a)

Stellar Populations, MPA

Correlations with

Correlations with σ

  • [E/Fe]-σ relation, so strong in field galaxies (Trager et al. 2000b), clearly evident in Coma galaxies

    • But note slight offset and GMP3565

      • Are there two [E/Fe]-σ relations?

  • No age-σ relation, but obvious factor of two spread in age at all masses

    • Late episodes of star formation are stochastic, independent of galaxy mass

  • Mass-metallicity relation exists in Coma (if not the field)

    • Note again that D158 and GMP3565 too metal-poor for their masses

Stellar Populations, MPA

Stellar population parameters from h and h effects of e fe

Stellar population parameters from Hδ and Hγ: effects of [E/Fe]

  • No corrections for effect of [E/Fe] on Hδ and Hγ currently published

  • Excellent Coma spectra allow us to make empirical corrections

    • Predict line strengths from models of Hβ, Mgb, <Fe>

  • Strong effect!

    • cf. Korn, Maraston et al.

Stellar Populations, MPA

The effect of e fe and c fe on c 2 4668 and fe4383

The effect of [E/Fe] and [C/Fe] on C24668 and Fe4383

  • Again, use Hβ, Mgb, <Fe> to predict C24668 and Fe4383

    • Highly useful for intermediate-z galaxies

  • Doesn’t quite work

  • Instead, allow [C/E] to vary independently of other “E” elements

    • Works very well, only requires typical [C/E]=0.04±0.02(!)

    • cf. Sánchez-Blázquez et al. (2003)

    • Note: also solves offset in [E/Fe]-σ relation

Stellar Populations, MPA

Putting it all together

Putting it all together…

  • Stellar parameters from Hδ or Hγ, C24668, Fe4383 vs. Hβ, Mgb, <Fe>

    • Make empirical corrections to higher-order Balmers

  • Pretty good fit for t

  • [Z/H] slightly too high

  • [E/Fe] is off but linearly related to original values

Stellar Populations, MPA

Distant galaxies the new frontier

Distant galaxies: The New Frontier

Trager, Dressler & Faber, in prep.

  • Stellar populations of early-type galaxies in A851 (CL0939+4713) at z=0.4

  • Can now determine SSP ages and abundances of galaxies out to z=0.8 (at least), using corrections determined from Coma galaxies

Stellar Populations, MPA

What s next

What’s next?





  • PCA analysis of Coma spectra

  • Can begin to study interesting elements besides Mg, Fe, CN, Ca, Na

    • in the blue: Mn, Sr (see Rose 1985), Cr, (Zn?)

    • in the red: Ba (others?)

    • but still not O!

Trager, in prep

Stellar Populations, MPA



  • Coma early-types are not coeval, even in cluster center

    • Not just faint galaxies—even massive ellipticals show age spread of 2x

    • But much more coeval than local field or even Fornax S0s

  • Galaxies follow Z-plane, but low-mass galaxies are off

    • Separate relation for dEs?

    • No mass-age relation  star formation events are stochastic

Stellar Populations, MPA

Conclusions cont

Conclusions (cont.)

  • [C/E]>0 ?

    • Possible, but not highly enhanced

      • <[C/E]>=0.04±0.02 for massive Coma early-types

    • Maybe C stars?

  • Can determine stellar populations from higher-order Balmers (Hγ, Hδ) and C24668, Fe4383 with confidence

    • Allows us to study higher-redshift galaxies!

  • End of days for line strengths?

    • Much better spectra and much better libraries  new tools for stellar population analysis (cf. Vazdekis 1999)

    • But still need to correct for [X/Fe]...

Stellar Populations, MPA

Old stellar populations in galaxy clusters

  • Special thanks to Guy Worthey, Dan Kelson, Daniel Thomas, Claudia Maraston, Harald Kuntschner, and Bianca Poggianti

Stellar Populations, MPA

Continuing annoyances

Continuing Annoyances

  • O abundances

    • need accurate O abundances for precise age dating of old stellar populations (Worthey), but no good way of measuring in optical

      • what about IR OH lines?

  • Stellar libraries

    • still dependent on Lick/IDS stellar library, but errors are large compared with current galaxy and GC data

    • do not (yet?) include metal-rich, α-enhanced stars

  • BHB stars in metal-rich populations

    • very likely to be small contribution in gE’s (Rose 1985; Maraston & Thomas 2000)

    • BHBs have been seen in metal-rich bulge GCs (Rich et al. 1997)

  • Blue stragglers

    • unlikely to be significant contributors to Balmer-line strengths (Rose 1985)

  • What is the temperature of the RGB tip?

Stellar Populations, MPA

Metallicity distributions

Metallicity distributions

  • Multiple ages cause problems...

  • ...but metallicity distributions aren’t so bad

    • for M32, assuming 8.5 Gyr population and Grillmair et al. (1996) MDF, find

      • Hβ=2.0Å, Mgb=2.9Å, <Fe>=2.3Å (model)

      • Hβ=1.9Å, Mgb=3.0Å, <Fe>=2.4Å (observed)

    • gives [Z/H]SSP=-0.32

      • compare to light-weighted peak of [Z/H]=-0.25 from CMD

    • but only works in absence of metal-poor tail







Stellar Populations, MPA

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