The Star Formation Histories of Red Sequence Galaxies Mike Hudson U. Waterloo / IAP

1. The Star Formation Histories of Red Sequence Galaxies Mike Hudson U. Waterloo / IAP Steve Allanson (Waterloo)Allanson, MH et al 09, ApJ 702, 1275 Russell Smith (Durham) Coma/Hectospec team, esp. Ron Marzke

2. 70% of the stellar mass density is in present red-sequence galaxies Red-sequence galaxies dominated the star formation rate at z > 1

3. Outline “Semi-empirical” galaxy formation models: Fit star formation models to low-z spectra Compare with high-z observations What quenches star formation: the role of environment

4. Cluster Samples Red and emission-free but not morphologically-selected. Large dynamic range in galaxy mass Shapley 340 galaxies in 3 rich clusters from deep 8hr AAOmega. Smith et al. 07 MNRAS 381 1035; Smith et al. 09 Coma Cluster Survey Deep Hectospec of 90 dwarfs. Smith et al. 08; Smith et al. 09a

5. Ages, Metallicities & -Enhancements from Spectra Measure Lick indices (typically H, H, H, several Fe lines, Mgb) Compare with a-enhanced models e.g. Thomas/Maraston et al., Schiavon. Get (“luminosity-weighted”) ages, metallicity, -enhancement

6. The “Driving Parameter” of Stellar Populations Velocity dispersion, ,is the driving scaling parameter, with little or no residual dependence on stellar mass. Smith, Lucey, MH ‘09 Graves & Faber ‘09

7. Shapley (Smith et al 07, Allanson et al 09) 70 km/s ~ L*+1.5 The End of Downsizing?

8. Ages from Balmer Lines … are luminosity-weighted ages. SSP EXP Quenched FR Very hard to distinguish these toy models with spectra

9. Ages from Balmer Lines only old stars Linestrengths cannot easily distinguish SF history.

10. The End of Downsizing?

11. Stellar M/L vs Dynamical M/L Old SSP, Exponential SFR: M*/L > Mdyn/L Quenched: OK Young SSP: OK Allanson, MH, Smith & Lucey 2009 Stellar M/L (Kroupa) Dynamical M/L

12. Outline “Semi-empirical” galaxy formation models: Fit star formation models to low-z spectra Compare with high-z observations What quenches star formation: the role of environment

13. Buildup of the red sequence If age is a strong function of mass/velocity dispersion then the red-sequence itself is built “top-down”

14. Dwarf-to-Giant Ratio as a function of redshift DGR evolution rules out frosting and exponential models.

15. The “typical” star formation history Pure SSP is certainly unrealistic, but high -enhancement suggests most of the stellar mass formed in short (<1 Gyr) burst. Any residual star formation must have been quenched quite abruptly. It does not continue to the present-day.

16. Outline “Semi-empirical” galaxy formation models: Fit star formation models to low-z spectra Compare with high-z observations What quenches star formation: the role of environment

17. Environmental Dependence For giant (~L*) galaxies, dependence of stellar populations on environment is very weak. Smith et al. ‘06 found a 15% variation in age from the cluster core to the virial radius. For dwarfs the story is very different …

18. Recently-quenched Coma red dwarfs Smith et al. 2008, 2009 NGC4839 Smith et al ‘10 in prep SSP

19. Recently-quenched Coma dwarfs Coma red dwarfs consistent with having been “quenched” within the last 1-2 Gyr. Effect seems stronger in NGC 4839 group Morphologically, most of the recently-quenched galaxies appear to be early-type with Sersic n ~ 2 (but ~40% do have disks).

20. Cluster Red Sequence Galaxies • Strong age (“downsizing”), metallicity and a-enhancements along red velocity dispersion sequence. • Downsizing in stellar age stops at ~ 70 km/s. • Comparisons with both z ~ 0 and high-z RS data favour a star formation history in which the younger ages are due to star formation quenching at intermediate epochs and not due to a small amount of very recent star formation. • Red sequence dwarf ages show strong environmental dependence.