investigating galaxy evolution with empirical population synthesis
DESCRIPTION
Investigating Galaxy Evolution with Empirical Population Synthesis. Laerte Sodré Jr. Departamento de Astronomia Instituto de Astronomia, Geofísica e Ciências Atmosféricas Universidade de São Paulo Challenges of New Physics in Space Campos do Jordão, 25 – 30 April 2009. log [OIII] / H b. - PowerPoint PPT PresentationTRANSCRIPT
Investigating Galaxy Evolution Investigating Galaxy Evolution with Empirical Population with Empirical Population
SynthesisSynthesis
Laerte Sodré Jr.Departamento de Astronomia
Instituto de Astronomia, Geofísica e Ciências Atmosféricas Universidade de São Paulo
Challenges of New Physics in SpaceCampos do Jordão, 25 – 30 April 2009
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SEAGal Collaboration(Semi-Empirical Analysis of Galaxies)
• Roberto Cid Fernandes (Florianópolis)• Grazyna Stasinska (Meudon)• LSJ (SP)• Abílio Mateus (SP, Florianópolis)
+ several PhD students:• Natalia Asari (Florianópolis, Meudon)• Juan Torres-Papaqui (INAOE, Florianópolis)• William Schoenell (Florianópolis)• Jean M. Gomes (Florianópolis)• Luis Vega Neme (Córdoba)• Tiago F. Triumpho (SP)• Marcus V. Costa Duarte (SP)• ...
• Cid Fernandes et al., 2005; Sodré et al. 2006; Mateus et al., 2006; Stasinska et al., 2006; Mateus et al., 2007; Cid Fernandes et al., 2007; Asari et al. 2008; Stasinska et al. 2008
log [NII] / H
log
[OII
I] /
H
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some questions about galaxy evolution:
• how star formation evolved?
• how metallicity evolved?
• what is the role played by galaxy mass?
• ...
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some questions about galaxy evolution:
• how star formation evolved?
• how metallicity evolved?
• what is the role played by galaxy mass?
• ...
• these are examples of problems that can be addressed by spectral synthesis
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Empirical Population Synthesis
• Fitting of a set of observables of a given galaxy by means of a linear combination of simpler systems of known characteristics, like individual stars or Simple Stellar Populations (SSP) to recover galaxy properties
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Why spectral synthesis?
• SS allows to retrieve the stellar history of galaxies from galaxy spectra
• galaxy spectrum: encodes information on the age and metallicity distributions of the constituent stars
• it is an expression of the galaxy star-formation and chemical history
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what is a galaxy spectrum?• energy flux per
wavelength interval
• continuum + absorption lines: stars
• emission lines: ionized gasproduced by star-forming regions or AGNs
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Why spectral synthesis?
• SS provides information on:
- Stellar population mix – galaxy history: star-formation, metallicity
- Gas properties – ionizing source: stars x AGN
- Kinematics & Dust – σ* , AV
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Model spectrum
Mλ0: synthetic flux at the normalization wavelength λ0 = 4020A
b j,λ: spectrum of the j-th SSP normalized at λ0
(N* SSP)
x j: fractional contribution of the j-th SSP to the model flux at λ0
reddening term (foreground dust): rλ = dex[-0.4(Aλ-Aλ0)](Cardelli, Clayton & Mathis 1989)
Gaussian with dispersion σ*
our approach: code Starlight – chi2 fitting
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Spectral base (B&C03):• N* = 45-150 SSP
• 3-6 metallicities
0.2, 1, 2.5 Zsun
(+ 0.005, 0.02, 0.4)
• 15 - 25 ages 0.001 to 13 Gyr (now: up to 18 Gyr)
STELIB library + Padova (1994) tracks + Chabrier (2003) IMF
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The SDSS sample
• SDSS: enormous amount of good quality, homogeneously obtained spectra
• Data from DR2 to DR7
samples from 20,000 to ~1,000,000 galaxies
• Median S/N ~14 (range 5 – 30)
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Examples:
Observed spectrum, model spectrum, error spectrum, masked pixels
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Emission line measurements
• Emission lines are measured from the “pure emission”, residual spectra
• Intensities are computed for many lines
• Galaxies with emission lines are classified according to their position in the BPT diagram ([OIII]/Hβ x [NII]/Hα):
- normal star-forming galaxies
- AGNs
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Empirical relations(useful to constrain models and for sanity checks)
relation between the meanstellar metallicity and thenebular metallicity
[O/H]: “empirical methods”(= Tremonti et al. 2004)
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Empirical relations
relation between velocitydispersion and stellar mass
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Empirical relations
AV (Balmer) ~ 2 AV (Stellar)
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Bimodality of the galaxy population
sequence x bimodality
“Early and late, in spite of their temporal connotations, appear to be the most convenient adjectives available for describing relative positions in the sequence”
(Hubble 1926)
• SDSS: Strateva et al. (2001), Kauffmann et al. (2003), ...
• Here: Mateus et al. (2006) * Volume limited sample (M(r) < -20.5)* ~50,000 galaxies (DR2)
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Bimodality of the galaxy population• Many galaxy properties present a bimodal
distribution: early-type / late-type • AGN hosts: preference for passive populations, but
everywhere
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Bimodality of the galaxy population
• The mean light-weighted stellar age provides a better separation between classes than stellar mass
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Galaxy downsizing
• Massive galaxies stoped to form stars more than 10 Gyr ago
• Galaxies forming stars today tend to have low masses
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A nature via nurture scenario for galaxy evolution
• Light (SF) is more sensitive to environment than stellar mass
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A nature via nurture scenario for galaxy evolution
• Galaxies in dense environments are older and more massive
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A nature via nurture scenario for galaxy evolution
• Galaxies in dense environments have more metals
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A nature via nurture scenario for galaxy evolution
• PCA: <log t>L log M* log Σ10 log Lr M* /Lr
• Most of the variance in galaxy properties are due to 1) environment and 2) age
• Galaxy evolution is accelerated in denser environments
• Galaxy evolution is accelerated for higher masses
• “Nature” necessarily acts via “nurture” effects (c.f. Abilio)
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Chemical enrichment and mass-assembly histories of SF galaxies
Bins inZneb
Cid Fernandes et al. (2007), Asari et al. (2007)
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Chemical enrichment and mass-assembly histories of SF galaxies
mass
Z
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Chemical enrichment and mass-assembly histories of SF galaxies
• Cid Fernandes et al. (2007), Asari et al. (2007):
- Low Zneb galaxies are slow in forming stars and reached Z* ~1/3 Zsun in the last ~100 Myr
- High Zneb galaxies formed most of their stars long ago, reaching Z* ~1 Zsun several Gyr ago
- Actually, more evidence of downsizing
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-bands not fitted in massive ellipticals ... new models will fix this!H–missfit with STELIB ... MILES fixes this!
Ellipticals SF-galaxies
technical challenges:
Residuals ~ within errors, but systematic!
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What changes with the new spectral bases???
Refits using CB07 models(MILES + Martins libraries)
2003
2007
2003 2007
SF-galaxies
Ellipticals
Residuals are smallerie., spectral fits are better!!
SFHs are smoother
Mean ages decrease a bit
<Z> increase a bit
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new surveys
• SDSS/DR7
• new photometric callibration!
• ~1,000,000 galaxies
• ... more to come!
EUCLID, WFMOS, ...
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some scientific challenges:
• uncertain stages of stellar evolution
• downsizing
• initial mass function
• chemical evolution
• dust evolution
• ...