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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: - PowerPoint PPT Presentation
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Deriving galaxy ages and metallicities using 6dF
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)
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 OutlineAim 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
• 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 challengesThe 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]=-2.252.0 Gyr 7 Gyr
Models: Bruzual & Charlot (2003) Models: Sanchez-Blazquez (Ph.D. thesis); Vazdekis et al. 2005 (in prep)
The challengesThe 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)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)
• 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)
• Differences in sensitivities leads to the breaking of the age/metallicity degeneracy.
Breaking the degeneracy with Lick
indices.
Breaking the degeneracy with Lick
indices.N=1200
• Data require extrapolation of models in metallicity
• A population apparently older than 15 Gyr.
• Observational error.
• Modelling uncertainties.• Horizontal-branch morphology?
Age =1 GyrZ
=0.5
Age=15 Gyr
Z=-2.25
Our approach.Our approach.
• 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).
• 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).
• 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).
Results from 6dFGS spectra: Emission
Results from 6dFGS spectra: Emission
Ref………..
H, OII and NII emission strengths supplied by Philip Lah.
• 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
HII regions AGN
6dFGS: Age with velocity dispersion
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
6dFGS: Age with velocity dispersion
• 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………..
N=3000
6dFGS: Age with velocity dispersion
6dFGS: Age with velocity dispersion
N=3000
“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
NGC 821
N=2500
6dFGS: Age with velocity dispersion
6dFGS: Age with velocity dispersion
• 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
MB=-21MB=-19
• Lines of constant luminosity estimated using FJ-relation and [M/L] models of BC03.
The Faber-Jackson Relation
The Faber-Jackson Relation
• Confirms Forbes & Ponman (1999) finding that residuals to the FJ-relation correlate with galaxy age.
• Suggests age/metallicity degeneracy has been broken.
• Confirms Forbes & Ponman (1999) finding that residuals to the FJ-relation correlate with galaxy age.
• Suggests age/metallicity degeneracy has been broken.N=1500
Red: Young
The Colour-Magnitude Relation (CMR)
(The ‘red-sequence’)
The Colour-Magnitude Relation (CMR)
(The ‘red-sequence’)
• 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)
• 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)
• Normally assumed to indicate a mass/metallicity relation and to imply a small range of ages.• Data suggest true picture not so clear-cut
6dFGS: Results for [Z/H]
6dFGS: Results for [Z/H]
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)
Red: Low mass Red: Young
6dFGS: -element abundance ratios.
6dFGS: -element abundance ratios.Red: Young Red: Low mass
N=3500
An [/Fe]-age relation Suggests less continuous SF than solar neighbourhood
Pure Fe
Pure Fe
The future.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.
• 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.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.
• 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.
Abrat issues 1 - ?Abrat issues 1 - ?
Lick indices (Worthey 1994)Lick indices (Worthey 1994)
• 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).
• 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).• 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)
15 Gyr
1.0 Gyr
1.5 Gyr
[Fe/H]=-0.4
[Fe/H]=-2.25
Age-metallicity degeneracy
1. Photometry
Age-metallicity degeneracy
1. Photometry
- 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)
2.0 Gyr
7 Gyr
6dFGS: [Fe/H] results6dFGS: [Fe/H] results
Our approach.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].
• 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].