Deriving galaxy ages and metallicities using 6dF 6dFGS Workshop April 2005 Rob Proctor (Swinburne University of Technology) Collaborators: Philip Lah (ANU) Duncan Forbes (Swinburne University of Technology) Warrick Couch (UNSW) Matthew Colless (AAO)
Aim and Outline • Aim: • To test theories of galaxy formation using galactic-archeology. • Outline: • The challenges. • Our approach to them using 6dFGS data. • Some preliminary results. • The future. • Some conclusions
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)
The challenges • Abundance-ratio variations (e.g. [Mg/Fe] †) †[X/Y]=log(NX/NY)*-log(NX/NY) • A new opportunity? • ‘’-element abundance ratios in stellar populations are indicators of the time-scale of star formation.
Lick indices (Worthey 1994) 25 spectral features with a variety of sensitivities to age, overall metallicity ([Z/H]) and ‘’-element abundance ratio ([Mg/Fe]). Models of Thomas, Maraston & Korn (2004) used here. Model simple stellar populations (SSPs) with ages up to 15 Gyr and [Z/H] from -2.25 to +0.4 dex. ‘’-element abundance ratios of from -0.3 to +0.5 dex modelled using the spectral synthesis of Tripicco & Bell (1995)
Breaking the degeneracy with Lick indices. • Differences in sensitivities leads to the breaking of the age/metallicity degeneracy. N=1200 Age =1 Gyr Z=-2.25 • Data require extrapolation of models in metallicity • A population apparently older than 15 Gyr. • Observational error. • Modelling uncertainties. • Horizontal-branch morphology? Z=0.5 Age=15 Gyr
Our approach. • Use Lick indices to estimate luminosity-weighted age, [Fe/H], [/Fe] and [Z/H] for ~5000 6dFGS DR1 galaxies(Already ~50x larger than any previous study of its kind). • Employ as many indices as possible (up to 25) in the derivation of galaxy properties using a 2-fitting procedure (Proctor & Sansom 2002; Proctor et al. 2004a,b). • This: • Minimises effects of most reduction and calibration errors (sky-subtraction, flux calibration, stray cosmic rays, poor calibration to Lick system etc). • Minimises effects of modelling errors. • Utilises the fact that ALL indices contain SOME information about age, [Fe/H], [/Fe] and [Z/H] (Proctor et al. 2005). • Provides some of the most reliable age and metallicity estimates from integrated spectra to-date (I.e. work to low S/N).
Results from 6dFGS spectra: Emission • Use emission to isolate a sample dominated by early-type galaxies. • From ~35,000 DR1 galaxies with index measurements we find: • 9000 with S/N>15 • 5000 emission free • 2000 with HII region emission • 2000 with AGN emission • 3000 with S/N>22 • 1800 emission free • 600 with HII region emission • 600 with AGN emission Ref……….. HII regions AGN H, OII and NII emission strengths supplied by Philip Lah.
6dFGS: Age with velocity dispersion • Both AGN and HII region galaxies lower velocity dispersion (mass) than the emission free. • Emission line galaxies dominate at low velocity dispersion. • Consistent with the notion that we are excluding late-type galaxies. N=7500
6dFGS: Age with velocity dispersion N=3000 • Suggests a mass-age correlation in opposite sense to hierarchical collapse models of Kauffmann (1996). • i.e. Highest mass galaxies tend to be old. • Range of ages inconsistent with models of primordial collapse. • BUT………..
6dFGS: Age with velocity dispersion N=3000 NGC 821 • “Frosting” • A busrt of SF of only a few % of galaxy mass can easily provide the majority of the sampled luminosity. e.g. NGC 821: Proctor et al. 2005
6dFGS: Age with velocity dispersion MB=-19 MB=-21 • Lines of constant luminosity estimated using FJ-relation and [M/L] models of BC03. • Sampling effects probably cause apparent age-mass relation. • Recall sample is essentially luminosity limited.. • Can infer Forbes & Ponman (1999) finding that young galaxies tend to have high luminosity for their velocity dispersion N=2500
The Faber-Jackson Relation Red: Young • Confirms Forbes & Ponman (1999) finding that residuals to the FJ-relation correlate with galaxy age. • Suggests age/metallicity degeneracy has been broken. N=1500
The Colour-Magnitude Relation (CMR)(The ‘red-sequence’) • Normally assumed to indicate a mass/metallicity relation and to imply a small range of ages. • Data suggest true picture not so clear-cut • However, the sample is limited to high luminosity galaxies. • (photometric bimodality becomes significant R>-17) • Nevertheless, argues against common belief that low scatter in CMR implies old ages. (At least in high luminosity galaxies)
6dFGS: Results for [Z/H] Red: Low mass Red: Young An age-metallicity relation A mass-metallicity relation [Z/H]=0.7log()-0.6log(age)-1.0 (a mass-metallicity relation that evolves with time)
6dFGS: -element abundance ratios. Red: Young Red: Low mass Pure Fe N=3500 Pure Fe Suggests less continuous SF than solar neighbourhood An [/Fe]-age relation
The future. • Refine age/metallicity measurements (This is a work in progress). • Probe ages and metallicities in emission line galaxies (Consider ages<1.0 Gyr). • Investigate emission line characteristics (HII/AGN, Balmer decrements, gas metallicities). • Quantify trends in galaxy parameters (FJ-relation, CMR and age/mass/metallicity planes). • Test idea of ‘frosting’ (Compare spectroscopic results for central regions to global photometry). • Investigate variations with environment. • DR2 and DR3.
Conclusions. • We have used Lick indices to break the age-metallicity degeneracy in by far the largest study of its kind to-date. • Results show trends in ALL metallicity parameters with both mass AND age. • These provide challenges to both primordial and monolithic collapse models of galaxy formation. • The 6dFGS will prove to be an invaluable testing ground for galaxy formation models. • The addition of reliable age and metallicity estimates for a large number of galaxies will significantly enhance the value of the 6dFGS.
Lick indices (Worthey 1994) • Integration of stellar properties (weighted by IMF) along isochrones of given age and metallicity yields model properties for an SSP. • Spectral synthesis of Tripicco & Bell (1995) models ‘’-elements(Models used here : Thomas, Maraston & Korn 2004) Properties of single stellar populations (SSPs) are estimated using: Stellar spectral libraries (Teff, log g and [Fe/H]). Isochrones (age and [Fe/H]). A Stellar Initial Mass Function(IMF: No. with mass).
Age-metallicity degeneracy1. Photometry [Fe/H]=-0.4 15 Gyr 7 Gyr 1.0 Gyr [Fe/H]=-2.25 2.0 Gyr 1.5 Gyr - Tight locus of all combinations of age and metallicity in the range 2.0 -15 Gyr, -2.25≤[Fe/H]≤-0.4(Models: Bruzual & Charlot 2003)
Our approach. • Estimate Age, [Fe/H], [/Fe] and [Z/H] • Use as many indices as possible (up to 25) • Thus: • Minimise effects of most errors (reduction and calibration) • Utilise the fact that ALL indices contain SOME information about age, [Fe/H] and [E/Fe].