1. The ages and metallicities of Hickson Compact Group galaxies. Rob Proctor Swinburne University of Technology May 2005 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. 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) 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(N X /N Y ) * -log(N X /N Y )  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(N X /N Y ) * -log(N X /N Y ) 

5. Determining star formation histories: The challenges The age-metallicity degeneracy: Young, metal-rich populations strongly resemble old, metal-poor populations. The age-metallicity degeneracy: Young, metal-rich populations strongly resemble old, metal-poor populations. Age=6 Gyr, [Fe/H]=0.2 Age=12Gyr, [Fe/H]=0.0 15 Gyr 1.0 Gyr 1.5 Gyr [Fe/H]=-0.4 [Fe/H]=-2.25 2.0 Gyr 7 Gyr Models: Bruzual & Charlot (2003) Models: Sanchez-Blazquez (Ph.D. thesis); Vazdekis et al. 2005 (in prep)

6. Different sensitivities of Lick indices result in a breaking of the age/metallicity degeneracy. Breaking the degeneracy with Lick indices. Age =1 Gyr Z=0.5 Age=15 Gyr Z= - 2.25

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 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 ). 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. 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. Squares: S0s Circles: Ellipticals Solids: Spirals Star: Star-burst galaxy Large symbols: HCGS Proctor et al. 2004

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 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 ). 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) 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) Squares: S0s Circles: Ellipticals Solids: Spirals Star: Star-burst galaxy Large symbols: HCGS NGC 821

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? 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? Solids: HCGs Proctor et al. 2004

12. Metallicity in spiral bulges ‘Mass’-metallicity relation. But… Again, no discernable difference between cluster/field and HCGS (however, we note small numbers). 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 extremely rapidly) OR.. HCGs possess a common dark matter halo which promotes stability. (e.g. Verdes-Montenegro et al. 2005) 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 extremely rapidly) OR.. HCGs possess a common dark matter halo which promotes stability. (e.g. Verdes-Montenegro et al. 2005)