Evolutionary Population Synthesis: Insights and Developments

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