Bruzual

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Bruzual Charlot 1993

• Spectral Evolution of Stellar Populations Using
  Isochrone Synthesis

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The Storm of the Century

• Georgia!!
• (32 N Latitude)

• Severe weather from Cuba to Quebec
• 270 human deaths
• Several billion dollars damage
• Record snowfall and temperature lows in many
  Southeastern states

March 12 13, 1993
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Outline

• Background
• History of Spectral Synthesis
• Charlot Bruzual 1991 (CB91)
• The Paper (BC93)
• Generation of spectra
• Testing the spectra
• Suggested applications
• Actual applications
• Spectral Synthesis since 1993

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Spectral Synthesis

• to predict the spectral evolution of stellar
  populations.
• Trial and Error Approach
• Spinrad Taylor 1971
• Faber 1972
• Pickles 1985
• Assemble library of
• local stellar observations
• Combine linearly to
• try to match populations

young Sandra Faber at the 120-in. Shane
Telescope, Lick Observatory
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Spectral SynthesisTrial and Error

• Most of these methods do incorporate
  astrophysical constraints
• 38 colors in this case (narrow features and
  continuum sampling)
• These methods were abandoned in 1980s too many
  free parameters

from Faber 1972
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Spectral Synthesis

• Evolutionary Population Synthesis
• Gunn, Stryker, Tinsley 1981 ? pioneer model
• Want to model stellar populations of giant
  ellipticals
• Use broad populations of stars (e.g. empirical
  lower MS, theoretical isochrones through base of
  giant branch)
• Minimizes free parameters
• Allows direct analysis of star formation history

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Stellar Population ModelingOlder Methods

• Want to generate CMD isochrones for a range of
  ages
• Start w/ Salpeter IMF
• Use evolutionary tracks
• (e.g. Maeder Meynet 1989)
• Integrate over CMD to
• get photometric evolution
• of galaxies/clusters

APOD 23 Feb. 2001
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Stellar Population ModelingOlder Methods

• Conventional
• population synthesis
• Tinsley 1980
• Bruzual 1983
• Guiderdoni Rocca-Volmerange 1987
• Mass binning makes burst population photometry
  oscillate
• Requires a posteriori smoothing

CB91 Fig. 7b
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Stellar Population ModelingOlder Methods

• Fuel Consumption Theorem
• Renzini 1981
• Renzini Buzzoni 1986
• Buzzoni 1990
• Can handle starbursts (unlike conventional
  synthesis)
• Assigns evolutionary track and IMF weight of
  turn-off mass to ALL post-MS stars
• As a result, underestimates age of burst
  populations (although otherwise compares nicely
  with data)

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CB91 to the rescue

• Brand new idea Isochrone synthesis
• Interpolate between stellar tracks in CMD for
  stars of intermediate masses
• Produces smoothly varying photometry
• Avoids problematic assumptions of fuel
  consumption method

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CB91 vs. The Data

• Comparison with data encouraging, with some
  caveats (solar metallicity only, so predictions
  too red for metal-poor clusters)

CB91 Fig. 10
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Two years pass, and then

• BC93 does spectral evolution using CB91
• We combine the photometric model of isochrone
  synthesis recently published by Charlot Bruzual
  with an updated library of stellar spectra to
  predict the spectral evolution of stellar
  populations with solar metallicity.
• Assign a spectrum to each point
• in theoretical CMD
• Correct evol. tracks for 1.3 2.5
• M_solar stars (MS lifetimes over-
• estimated by as much as 100)
• improves CB91s results

Gustavo Bruzual
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Spectral Assignment

• Visible spectra from Gunn Stryker 1983
• Near-IR from Persson 1987
• Four far-IR fluxes from IRAS
• UV spectra from IUE
• ?Bottom line continuous spectral coverage from 5
  Å to 2.56 µm (broadest ever)
• BUT for Teff 50,000 K, just get black body!
• For Teff 40,000 K, use Kurucz (1979) models
• For cool stars, just assign spectrum w/ nearest
  Teff
• For all others, minimize residuals b/w spectra
  and colors matched to evol. tracks

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Star Formation Histories

• Synthesizing isochrones does not limit you
• can convolve with arbitrary star formation
  history!
• with f? an instantaneous-burst SED, F? the
  spectrum of a population with arbitrary SFR ?(t)
• NOTE that this assumes time-invariant IMF!
• Commonly used is exponentially decaying SFR

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SF Histories The Extremes
BC93 Fig. 4a
BC93 Fig. 4d
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Models vs. Data Ellipticals

• Best-fitting age model and composite elliptical
  spectrum
• Fairly good fit over entire spectral range
• Note UV-rising branch, highlighting importance of
  accurate AGB modeling
• Authors admit that these are large-aperture
  spectra, so metallicity will be roughly solar

BC93 Fig. 5
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Models vs. Data Irregulars

• Note that emission lines will never be fit (these
  models only include stars!)
• Overprediction of flux at blue end, probably
  because of internal extinction due to dust
• ?So this plot highlights limitations of
  stellar-only models

BC93 Fig. 6
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Models vs. Data Spirals

• Increasing SF time scales produce good fits for
  increasingly late-type Spirals
• Note good fit of 4000 Å break and main absorption
  features in all cases

BC93 Fig. 7
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Suggested Applications

• Color-redshift relations (see right), compared to
  data and previous models
• Modeling evolution of spectral features (4000 Å,
  912 Å breaks) over time
• Both of these useful to photometric redshifts!

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Wrapping it up (the paper)

• SF timescales poorly constrained
• They look at relative contributions of different
  stellar types to various frequency bands at
  different ages
• Stellar conclusions (e.g. PNNs) read the paper!
• Conclusion good fits, good spectral coverage,
  good modeling of tricky AGB
• ? its a hit!

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Why is this paper cited so often?? 1227 fans
cant all be wrong!

• PHOTOMETRIC REDSHIFTS
• templates for fitting spectra to multiband
  photometry
• would generally prefer to use empirical, but
  spectrophotometric calibration is sometimes
  inadequate if you want to work at high-z, models
  are best
• Integrated stellar mass/light ratios
• Try to guess at star formation history of an
  interesting-looking galaxy

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Blanton et. al. 2003

• A code for k-correcting SDSS galaxy photometry
• Can also be used as photometric redshift
  estimator
• Forms orthonormal basis of BC93 spectra to fit
  to photometry
• Available online at
• http//physics.nyu.edu/mb144/kcorrect

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Spectral Synthesis since 1993

• Fioc Rocca-Volmerange 1997
• Includes nebular emission (but theyd been doing
  that since 1987)
• Better treatment of AGB phase
• Bruzual Charlot 2003
• Z0.0001, 0.0004, 0.004, 0.008, 0.02 (solar) and
  0.05
• Future?
• AGB evolution, convective overshooting still
  uncertain
• more

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Accessing the Spectra

• Fioc Rocca-Volmeranges PEGASE code ftp.iap.fr
  ? /pub/from_users/pegase/
• Bruzual Charlot Atlas (1995)
• Bruzual Charlots 2003 GALAXEV library
• http//www2.iap.fr/users/charlot/bc2003/

http//www.stsci.edu/hst/observatory/cdbs/cdbs_bc9
5.html
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Summary

• Isochrone synthesis method solved previous
  problems in population synthesis
• Addition of spectra allowed probing of many more
  galactic observables for populations with known
  initial conditions
• Enabled work on inverse problem
• Still more improvements to be made!

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Happy Holidays!