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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:

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The ages and metallicities of Hickson Compact Group galaxies.

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  1. 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)

  2. 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.

  3. 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)

  4. 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)

  5. 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)

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. ‘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

  13. 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

  14. 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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