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Population Synthesis at the Crossroads Claus Leitherer (STScI) and Sylvia Ekström (Geneva Obs.)

Population Synthesis at the Crossroads Claus Leitherer (STScI) and Sylvia Ekström (Geneva Obs.). Cosmology. Galaxy Evolution. Stellar Populations. Stellar Astrophysics. Nuclear and Atomic Physics. Errors • Information Flow. Popularity • Mobility • Front Page News.

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Population Synthesis at the Crossroads Claus Leitherer (STScI) and Sylvia Ekström (Geneva Obs.)

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  1. Population Synthesis at the CrossroadsClaus Leitherer (STScI)andSylvia Ekström (Geneva Obs.) Claus Leitherer: Population Synthesis

  2. Cosmology Galaxy Evolution Stellar Populations Stellar Astrophysics Nuclear and Atomic Physics Errors • Information Flow Popularity • Mobility • Front Page News Claus Leitherer: Population Synthesis

  3. Evolutionary Population Synthesis: Basics • Goal: model SEDs of nearby and distant galaxies • Star formation law (IMF, SF history, clustering…) • Stellar evolution models (conversion of mass into luminosity and introduction of the time-scale) • Spectral libraries (empirical and theoretical) • Other (dust, geometry…) Claus Leitherer: Population Synthesis

  4. Most widely used, public model packages: • GALEV (Schulz et al. 2002; Kotulla et al. 2009) • GALEXEV(Bruzual & Charlot 1993; 2003) • PEGASE (Fioc & Rocca-Volmerange 1997; Le Borgne et al. 2004) • STARBURST99 (Leitherer et al. 1999; Leitherer & Chen 2009)  generally good agreement Claus Leitherer: Population Synthesis

  5. Broadened scope beyond stellar populations: • Panchromatic SEDs including stellar and gaseous contributions (Dopita et al. 2005; 2006a; 2006b; 2008) • SEDs with self-consistent inclusion of dust (Dwek & Scalo 1980; Dwek 2009) • Chemical evolution due to stars and galactic outflows and infall (Matteucci 2009; 2010) Claus Leitherer: Population Synthesis

  6. New Developments: the New Normal • Stochasticity • Rapid eruptive evolutionary phases • Stellar multiplicity • Convective overshooting • Stellar rotation Claus Leitherer: Population Synthesis

  7. Stochasticity • Bruzual (2002): • Cluster simulations of sparsely populated evolutionary phases due to stochastic fluctuations of the IMF • Cerviño et al. (2002): • Relevant for clusters with M < 105 M⊙, depending on wavelength Claus Leitherer: Population Synthesis

  8. Da Silva et al. (2011): SLUG – Stochastically Light Up Galaxies • SLUG synthesizes stellar populations using a Monte Carlo technique with stochastic sampling, including the effects of clustering, the stellar IMF, star formation history, stellar evolution, and cluster disruption Claus Leitherer: Population Synthesis

  9. Eldridge (2011): BPASS – Binary population and spectral synthesis • Application: comparison with sample of 300 star-forming galaxies within 11 Mpc (Lee et al. 2009) • SFR(H)/SFR(UV) declines for small SFR(UV) • Photon leakage? • Eldridge (2011): can also be accounted for by stochastic effects Claus Leitherer: Population Synthesis

  10. Crowther et al. (2010): the most massive stars in R136 (LMC) • Four stars have masses between 150 and 300 M⊙ • These stars account for ~50% of the ionizing photon output of R136 • Correspondingly, they account for ~10% of the ionizing luminosity of the entire LMC Claus Leitherer: Population Synthesis

  11. Rapid Eruptive Evolutionary Phases • Maraston (2011) • Contribution to LBol of SSP by TP-AGB stars • Evolution/atmospheres still poorly understood Claus Leitherer: Population Synthesis

  12. Conroy & Gunn (2010): FSPS – flexible stellar population synthesis • SPS code combines stellar evolution calculations with stellar spectral libraries to produce simple stellar populations • Manually adjust stellar phases and estimate errors • Allows estimate of uncertainties of, e.g., the AGB phase Claus Leitherer: Population Synthesis

  13. Smith et al. (2011): transient, eruptive phases in the upper HRD • Continuum in M(peak) from novae to SNe • AGB in the red, vs. LBV in the blue • AGB important for mass loss and luminosity • LBV only important for mass loss • LBVs are critical predecessors of WR stars Claus Leitherer: Population Synthesis

  14. Stellar Multiplicity • Sana & Evans 2011: >50% of all massive stars in binaries; significant fraction in close binaries Claus Leitherer: Population Synthesis

  15. de Mink (2010): evolution of a massive close binary On the main-sequence: spin-up and higher rotation; mixing and evolution to higher L and Teff. Post-main-sequence: Roche-lobe overflow and mass transfer to secondary: “rejuvenation”. Claus Leitherer: Population Synthesis

  16. Eldridge & Stanway (2009): rejuvenation effect in SSP • Evolution of Hβ equivalent width with time in SF galaxy • Solid: single stars with different Z; dashed: binaries with different Z • Hot, ionizing population appears after ~10 Myr • Relevance: age spread of star formation? LINERs? Claus Leitherer: Population Synthesis

  17. Convective Overshooting Energy production of massive stars in convective core Claus Leitherer: Population Synthesis

  18. Stellar Rotation • Hunter et al. (2009): N/H and v sin i in LMC OB stars • Significant N enrichment on the main sequence Claus Leitherer: Population Synthesis

  19. How to reproduce the N enhancement • Stellar winds to remove outer, stellar layers • Mass loss and mass transfer in close binaries • Enhanced convective overshooting • Correlates with rotation rotationally induced mixing Claus Leitherer: Population Synthesis

  20. Brott et al. (2011): evolutionary models with rotation M < 2 M⊙: rotation negligible because of magnetic braking 2 M⊙ < M < 15 M⊙: Teff decrease caused by centrifugal forces M > 15 M⊙: larger convective core with higher Teff and L Claus Leitherer: Population Synthesis

  21. Ekström et al. (2012): full set of evolution models with rotation • 0.8 M⊙ < M < 120 M⊙ • Z = Z⊙ (other Z in preparation) • vrot = 0.4 vbreakup on ZAMS • Calibrated extensively via local stars and star clusters • Implemented in Starburst99 Claus Leitherer: Population Synthesis

  22. MBol vs. time (SSP, 106 M⊙, Z⊙, Kroupa IMF) • Models with rotation are more luminous by ~0.4 magbecause of the higher L/M and Teff of individual stars Claus Leitherer: Population Synthesis

  23. Lyman photons vs. time (SSP, 106 M⊙, Z⊙, Kroupa IMF) • The number ionizing photons increases by a factor of ~4 when hot, massive stars are present Claus Leitherer: Population Synthesis

  24. H equivalent width vs. time (continuous SF, Z⊙, Kroupa IMF) • W(H) increases by ~0.2 dex. If used as an IMF indicator in late-type galaxies, the new models change the IMF exponent from, e.g, 2.3 to 2.6 Claus Leitherer: Population Synthesis

  25. Br luminosity vs. time (SFR = 100 M⊙ yr-1, Z⊙, Kroupa IMF) • Applied to IR-luminous galaxies: models with rotation lead to, e.g., SFR = 100 M⊙yr-1, whereas models without give SFR = 175 M⊙yr-1. Claus Leitherer: Population Synthesis

  26. Take-Away Points • Replacemement of traditional population synthesis models by a new generation of models • Ready for quantitative testing, beyond mere concept studies • Stochastic effects, stellar multiplicity, rotation • Significant decrease of M/L, and therefore revision of IMF and SF rates • The new models need to be tested by comparison with galaxy properties Claus Leitherer: Population Synthesis

  27. Cosmology Galaxy Evolution Stellar Populations Stellar Astrophysics Nuclear and Atomic Physics Information Flow Claus Leitherer: Population Synthesis

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