Claudia Maraston Institute of Cosmology and Gravitation University of Portsmouth, UK

1. Spectra of stellar populations: where to look at Claudia Maraston Institute of Cosmology and Gravitation University of Portsmouth, UK Carmen 2007

2. Outline • Synthetica Spectra of Stellar Population Models • What parameters we measure at different wavelenghts • Breaking the age/metallicity degeneracy • Summary Claudia MarastonCarmen2007

3. Stellar Population Models Integrating the energetics of the various stellar phases stellar tracks or empirical recipes + distributing the energetics over stellar spectra empirical libraries or model atmospheres Claudia MarastonCarmen2007

4. Energetics of Stellar Populations:Three Ages Three Phases Main Sequence plus He-burning stars dominate the light of young stellar populations AGB stars dominate the light of intermediate-age stellar populations RGB stars dominate the light of old stellar populations Claudia MarastonCarmen2007

5. Phases and wavelenght UV: massive Main Sequence stars and old, hot stars on post-RGB phases Visual: Main Sequence, RGB and HB Near-IR: red supergiants (short lifetime ~10^6) AGB stars ---> Carbon stars RGB stars Claudia MarastonCarmen2007

6. MUV FUV UV: spectra of starbursts/age dating Nieves, Maraston, Bendersubmitted Claudia MarastonCarmen2007

7. Visual: element abundance ratios Abundance ratios modify lines hence matter on the derivation of age and total metallicity Thomas, Maraston, Bender 03 Thomas, Maraston, Korn 04 Korn, Maraston, Thomas 05 Claudia MarastonCarmen2007

8. Visual: element abundance ratios 2. What lines should be used? [/Fe] ratio: classical (Lick) indices like Mg2 or Mgb and Fe @ 5000 Å because are well calibrated and the measurements can be related to objects in the local universe Ziegler et al. 05 Age: H is a better age indicator at higher redshifts (> 0.4) where hot, old stars (post-RGBs, hot HBs, etc.) are less likely to be present Plus: it is rather insensitive to the [/Fe] The higher-order Balmer lines H, H are strongly sensitive to [/Fe] Thomas, Maraston, Korn 05 At higher redshift one can use Fe4383 Thomas et al. 2003 N.B.: with the new empirical stellar libraries (e.g. MILES) one can define any index, but we need robust models Claudia MarastonCarmen2007

9. The near-IR: finding TP-AGB stars Spectral bands due to TP-AGB stars appear only in populations of ~ 1 Gyr The C2 @ 1.77 in particular is only enhanced by Carbon stars These effects are modeled Maraston 2005 Claudia MarastonCarmen2007

10. Dating star formation in AGN Detection of CN molecules in the near-IR Carbon stars 0.2 -- 2 Gyr Riffel et al. 07 ApJL Claudia MarastonCarmen2007

11. Age/metallicity degeneracy revisited Worthey94: 2/3 ‘rule’ The use of the near-IR breaks the degeneracy Claudia MarastonCarmen2007

12. Korn, Maraston, Thomas 2005 Caveats Defect in model atmospheres and empirical library Missing opacity of molecular lines, problems in G and later-type stars Kurucz 1979; Castelli & Kurucz 2006 Missing lines -- purely theoretical SEDs are risky Korn et al. 05; Coehlo et al. 07 Empirical libraries limited to some chemical pattern Stelib, Elodie, Miles Uncertainties in stellar evolutionary tracks Temperature of the RGB at high metallicities Maraston 2005 Overshooting/ RGB onset Ferraro et al. 2004; Maraston 2005 Rotation in massive stars G. Meynet in prep. Claudia MarastonCarmen2007

13. Summary * For robust age and metallicity determination abundance ratios cannot be neglected, in particular the [/Fe] ratio * Classical Lick indices such as Mg2, Mgb + Fe (or some others, check the Thomas, Maraston & Bender speiselist ) are at present the best choice because the models are well calibrated and we know what the indices trace *The age/metallicity degeneracy of the optical can be broken by using also the rest-frame near-IR In particular, the C2 molecular absorption is an unambiguous signature of Carbon stars, e.g. 1 Gyr SP (as early suggested by Tinsley & Gunn 1976!) * New interesting UV absorption lines have been modeled and calibrated with star clusters and can be used to date high-redshift starbursts Claudia MarastonCarmen2007