Deriving galaxy ages and metallicities using 6dF

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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)

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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

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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
Age6 Gyr , Fe/H0.2 Age12Gyr, Fe/H0.0
2.0 Gyr
Fe/H-2.25
Models Bruzual Charlot (2003)
Models Sanchez-Blazquez (Ph.D. thesis)
Vazdekis et al. 2005 (in prep)
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The challenges

• Abundance-ratio variations (e.g. Mg/Fe )
• X/Ylog(NX/NY)-log(NX/NY)?

• A new opportunity?
• ?-element abundance ratios in stellar
  populations are indicators of the time-scale of
  star formation.

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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)

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Breaking the degeneracy with Lick indices.

• Differences in sensitivities leads to the
  breaking of the age/metallicity degeneracy.

N1200
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?

Z0.5
Age15 Gyr
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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).

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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/N15
• 5000 emission free
• 2000 with HII region emission
• 2000 with AGN emission
• 3000 with S/N22
• 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.
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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.

N7500
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6dFGS Age with velocity dispersion
N3000

• 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..

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6dFGS Age with velocity dispersion
N3000
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

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6dFGS Age with velocity dispersion
MB-21
MB-19

• 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

N2500
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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.

N1500
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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)

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6dFGS Results for Z/H
Red Low mass
Red Young
An age-metallicity relation
A mass-metallicity relation
Z/H0.7log(?)-0.6log(age)-1.0 (a
mass-metallicity relation that evolves with time)
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6dFGS ?-element abundance ratios.
Red Young
Red Low mass
Pure Fe
N3500
Pure Fe
Suggests less continuous SF than solar
neighbourhood
An ?/Fe-age relation
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The future.

• Refine age/metallicity measurements (This is a
  work in progress).
• Probe ages and metallicities in emission line
  galaxies (Consider ages
• 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.

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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.

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Abrat issues 1 - ?
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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).

• 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)

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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,
-2.25Fe/H-0.4 (Models Bruzual Charlot 2003)
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6dFGS Fe/H results
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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.

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