The ages and metallicities of hickson compact group galaxies
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The ages and metallicities of Hickson Compact Group galaxies. Rob Proctor Swinburne University of Technology May 2005. Collaborators: Duncan Forbes (Swinburne University of Technology) George Hau (Durham) Mike Beasley (Santa Cruz). Aim and Outline. Aim:

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The ages and metallicities of Hickson Compact Group galaxies.

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The ages and metallicities of hickson compact group galaxies

The ages and metallicities of Hickson Compact Group galaxies.

Rob Proctor

Swinburne University of Technology

May 2005

Collaborators:

Duncan Forbes (Swinburne University of Technology)

George Hau (Durham)

Mike Beasley (Santa Cruz)


Aim and outline

Aim and Outline

  • Aim:

    • To determine galaxy star formation histories using galactic-archeology.

    • Test galaxy formation theories using Hickson Compacts Groups (HCGs) as an extreme of environment.

  • Outline

    • The challenges.

    • Our approach to cracking them using Lick indices.

    • Some results and conclusions.


Why hcgs

Why HCGS?

  • Space densities and early-type galaxy fractions are abnormal outside the centres of large clusters (Hickson 1988).

  • But velocity dispersions are low.

  • Conditions therefore conducive to merging.

  • However, interaction rates and AGN activity are lower than expected.

    (Zepf & Whitmore 1991; Coziol et al. 1998; Verdes-Montenegro et al. 1998)

  • And systems are virialised, suggesting longevity.

    (Ponman et al. 1996)


Determining star formation histories the challenges

Determining star formation histories: The challenges

  • Integrated light only:

    • Requires models.

  • The age-metallicity degeneracy:

    • Young, metal-rich populations strongly resemble old, metal-poor populations.

  • Abundance-ratio variations (e.g. [Mg/Fe] †):

    • A new opportunity.

      † [X/Y]=log(NX/NY)*-log(NX/NY)


  • Determining star formation histories the challenges1

    Determining star formation histories: The challenges

    • The age-metallicity degeneracy:

      • Young, metal-rich populations strongly resemble old, metal-poor populations.

    [Fe/H]=-0.4

    1.5 Gyr

    1.0 Gyr

    15 Gyr

    7 Gyr

    Age=6 Gyr , [Fe/H]=0.2

    Age=12Gyr, [Fe/H]=0.0

    2.0 Gyr

    [Fe/H]=-2.25

    Models: Bruzual & Charlot (2003) Models: Sanchez-Blazquez (Ph.D. thesis); Vazdekis et al. 2005 (in prep)


    Breaking the degeneracy with lick indices

    Breaking the degeneracy with Lick indices.

    Age =1 Gyr

    Z=-2.25

    • Different sensitivities of Lick indices result in a breaking of the age/metallicity degeneracy.

    Z=0.5

    Age=15 Gyr


    Abundance ratios fe

    Abundance ratios ([‘’/Fe])

    • Thought to measure ‘duration’ of star formation.

      • This assumes that:

        • C, Mg (and other -elements) made mostly in SN.

        • Fe peak elements made predominantly in SNa.

    • Use [E/Fe] where E is sum of C,N,O,Mg,Na,Si


    Results central values

    Results (central values).

    Proctor et al. 2004

    • Field/cluster results from:

      • Trager et al. (2000)

      • Proctor & Sansom (2002)

      • Proctor et al. (2004) (small symbols)

    • HCG results from:

      • Proctor et al. (2004) (large symbols)

    • Correlation?

      • Note luminosity limited studies.

    Large symbols: HCGS

    Squares: S0s

    Circles: Ellipticals

    Solids: Spirals

    Star: Star-burst galaxy


    Age profile of ngc821 an important caveat

    Age profile of NGC821: An important caveat.

    • Young central age.

      But…

    • Strong age gradient.

      So….

    • Recent burst must be <10 % by Mass!

      (in actuality probably ≤1%)

    • I.e .amounts to a ‘frosting’ of younger stars

    Proctor et al. 2005


    Results central values1

    Results (central values).

    NGC 821

    • Old ages of most massive galaxies are ANTI-hierarchical (Kauffmann 1996).

      AND…

    • Age range is inconsistent with the simple primordial collapse picture.

      BUT….

    • Frosting effects must be considered (e.g. NGC 821)

    Large symbols: HCGS

    Squares: S0s

    Circles: Ellipticals

    Solids: Spirals

    Star: Star-burst galaxy


    Metallicity in early type galaxies

    Metallicity in early-type galaxies.

    • HCGs consistent with trends in cluster/field galaxies

    • Suggests relation of form:

      log()=log(age) +[Fe/H]+ (see Proctor et al. 2004)

    • Several interpretations

      (My favourite is an evolving mass/metallicity relation.)

    • ‘Frosting’ effects?

    • Inconsistent with pure primordial collapse.

    • Hierarchical merging?

    Proctor et al. 2004

    Solids: HCGs


    Metallicity in spiral bulges

    • ‘Mass’-metallicity relation.

    • But…

    • Again, no discernable difference between cluster/field and HCGS (however, we note small numbers).

    Metallicity in spiral bulges

    Proctor et al. 2004


    Early type galaxies age metallicity distributions in hcgs clusters and the field

    Early-type galaxies: Age/metallicity distributions in HCGS, clusters and the field

    • Early-type field galaxies ~2 Gyr younger than those in clusters (confirmed in many other studies).

    • HCGs possess age and [Fe/H] distributions more similar to those of cluster galaxies than field galaxies (confirmed in Mendes de Oliveira et al. 2005).

    Proctor et al. 2004


    Conclusions

    Conclusions.

    • Results are inconsistent with simple models of both primordial collapse and hierarchical merging.

      However…

    • Early-type galaxies in HCGs more similar to cluster galaxies than those in the field.

    • According to Mendes de Oliveira et al. this implies either:

      • HCGs are highly transient (I.e. collapse to form a merger remnant extremelyrapidly)

        OR..

      • HCGs possess a common dark matter halo which promotes stability.(e.g. Verdes-Montenegro et al. 2005)


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