Investigating Galaxy Evolution with Empirical Population Synthesis
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Transcript of Investigating Galaxy Evolution with Empirical Population Synthesis
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
• ...