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

• ...