The Stellar Populations, Mass-to-Light Ratios and Dark Matter in Spiral Galaxies

1. The Stellar Populations,Mass-to-Light Ratios andDark Matterin Spiral Galaxies Roelof S. de JongSteward Observatory Eric Bell Rob Kennicutt Rob Swaters Rob Olling Don McCarthy Cedric Lacey

2. Overview • Introduction • Ages and metallicities of stellar populations • description of method • scaling laws with structural parameters • Galaxy evolution modeling • Mass-to-light ratios of stellar populations • correlation with population colors • constraints from rotation curves • application to Tully-Fisher relation • Future work Rijks Universiteit Groningen

3. Galaxy Formation and Evolution • Huge progress, both observational and theoretical: • observational: e.g. the star formation history of the Universe and of local group galaxies • theoretical: hierarchical galaxy formation models in CDM-like universes Rijks Universiteit Groningen • Something is missing: We do not not where, when and especially why stars are forming in particular galaxies

4. Galaxy Evolution and Structural Parameters • What drives the Star Formation History and the Chemical Evolution within disk galaxies? • current star formation in disks semi-regular, related to morphology and structural parameters • are spirals determined by initial conditions or are infall and outflow important? • how is galaxy evolution related to the luminous and dark matter distribution and galaxy dynamics? • What is the distribution of dark and luminous matter? • can we explain the Tully-Fisher relation? • does dark matter really follow NFW profile distributions? • do we need alternative gravity (e.g. MOND)? Structural parameters: luminosity, scale size, surface brightness, mass, velocity distribution Statistical studies: scaling relations Rijks Universiteit Groningen

5. Stellar populations Color-Color diagrams Gyr Gyr Rijks Universiteit Groningen Bruzual & Charlot models

6. Samples: • de Jong & van der Kruit 1994 • BVRIHK of 64 face-on field spirals • Verheijen et al. 1998 • BVRK of 34 Ursa Major Cluster spirals • Bell et al. 1999 • BVRIK of 23 Low Surface Brightness galaxies Data & Samples • Face-on disk galaxies with • data in at least 3 passbands (of which one IR) • good colors over at least 2 disk scale lengths Rijks Universiteit Groningen Total sample of 121 galaxies

7. Radial Color-Color Observations Rijks Universiteit Groningen R-K B-R B-R

8. Make model grid of e-t/τ Star Formation History and metallicity parameterize SFH by average age <A> Determine minimum Χ2 between models and data use all available passbands take calibration, flatfield and sky errors into account Repeat for all radii Use Monte Carlo simulations to determine uncertainties Maximum Likelihood Fitting Rijks Universiteit Groningen

9. Local Age & Local Metallicity versusLocal Surface Brightness Rijks Universiteit Groningen

10. Age vs Surface Brightness & Luminosity Rijks Universiteit Groningen

11. Metals vs Surface Brightness & Luminosity Rijks Universiteit Groningen

12. What determines SFH and Metals?Surface Brightness or Luminosity? Remember luminosity and surface brightness are correlated! Rijks Universiteit Groningen

13. The Galaxy Space DensitySurface Brightness & Magnitude Space density ofspiral galaxies corrected for selection effects(de Jong & Lacey 2000) Rijks Universiteit Groningen

14. Are Ages mainly determined by Surface Brightness or Luminosity? Rijks Universiteit Groningen

15. Is metallicity mainly determined by Surface Brightness or Luminosity? Rijks Universiteit Groningen

16. Summary observations • Ages are mainly determined by surface brightness, suggesting inside-out disk formation • Metallicity is determined by surface brightness and total luminosity • The observed scatter is larger than observational errors Rijks Universiteit Groningen So what are the caveats? • Changes in the IMF • Other Stellar Population Synthesis models • The effect of dust reddening

17. IMF uncertainty Salpeter IMF Scalo IMF Rijks Universiteit Groningen

18. Spectral synthesis model uncertainty Bruzual & Charlot Kodama & Arimoto Rijks Universiteit Groningen

19. The effect of Dust Extinction • Extinction will mainly effect metallicity determinations i.e. reddening vector runs parallel to metallicity color gradients • Reddening not the main cause of the observed trends because: • we are using face-on galaxies • of the limits set by overlapping and edge-on galaxy Rijks Universiteit Groningen

20. The effect of Dust Extinction • Extinction will mainly effect metallicity determinations i.e. reddening vector runs parallel to metallicity color gradients • Reddening not the main cause of the observed trends because: • we are using face-on galaxies • of the limits set by overlapping and edge-on galaxy Rijks Universiteit Groningen • we see no dependence on galaxy inclination • colors are mainly determined by least obscured stars • patchy dust structure reduces reddening effect • reddening is caused by absorption only, not by scattering

21. Dust modeling with scattering • Scattering preferably to face on direction • Reddening follows absorption curve, not extinction curve • For low optical depth reddening insignificant Rijks Universiteit Groningen

22. Conclusion Age & Metallicity Caveats • Only very unusual IMFs can mimic our results • Other Spectral Synthesis Models will only change the absolute age and metallicity values • Dust will at most effect metallicities a bit Rijks Universiteit Groningen The relative rankings of Ages & Metallicities are Robust

23. Simple Galaxy Evolution Models • Simple closed box models: • Start with exponential gas disk • Form stars according to Schmidt law: (surface density)n • Instantaneous recycling of metals • Maximum likelihood fitting on resulting integrated colors Rijks Universiteit Groningen • Additional bells and whistles: • Mass dependent metal free gas infall • Mass dependent enriched gas blowout • Mass dependent epoch of formation • Fluctuations due to small starbursts

24. Galaxy evolution models Rijks Universiteit Groningen Closed box model Mass dependent formation epoch model with star burst Mass dependent formation epoch model

25. Modeling conclusions • Simple closed box models with a star formation rate dependent on local gas density explains the basic observed trends between stellar ages & metallicities and galaxy surface brightness parameters • Enriched gas blowout or mass dependent formation epoch models are needed to explain the metallicity dependence on total luminosity of the galaxy • Small burst of star formation explains the scatter on the observed relations Rijks Universiteit Groningen • What about masses instead of luminosities?

26. Why stellar M/Ls? • Stellar M/Ls needed to do dynamics in situations where we have more matter than just stars, e.g. • (baryonic) Tully-Fisher and other scaling relations • rotation curve decomposition • Dynamics is needed to model star formation and galaxy evolution Rijks Universiteit Groningen • How? Many approaches possible: • Milky Way kinematics • galaxy kinematics • streaming motions induced by bars or spiral arms • vertical velocity dispersion in stellar disks • stellar population synthesis

27. Galaxy evolution models Rijks Universiteit Groningen Closed box model Mass dependent formation epoch model Mass dependent formation epoch model with star bursts The optical color of a stellar population is a good M/L indicator Even in K mass-to-light ratio varies by factor of 2

28. B I K Color-ML for hierarchical galaxy model Even a hierarchical galaxy formation model shows strong correlation between color and M/L Rijks Universiteit Groningen Cole et al. (2000) models

29. The slope of the color-M/L relation is independent of stellar population synthesis models used Different population synthesis models Rijks Universiteit Groningen

30. The slope of the color-M/L relation is independent of models and IMFs used Different Initial Mass Functions Rijks Universiteit Groningen • The normalization of the relation depends on the IMF used, i.e. the amount of low mass stars

31. Rotation curve M/L constraint Rijks Universiteit Groningen

32. The color-M/L relation must be normalized below all maximum disk values Salpeter IMF Maximum disk constraints • A Salpeter IMF is too massive Rijks Universiteit Groningen • Distribution suggests IMF similar in most galaxies and close to maximum disk for a fraction of the galaxies data Verheijen (1997) bad data point due to beam smearing

33. Stellar masses derived from different passbands using the color-M/L relation agree to within 10% rms The Tully-Fisher relations derived from different passbands are identical to within the errors The slope is very steep Vrot ~ M*4.5 Stellar Mass Tully-Fisher relation • Raw Tully-Fisher relation has different slopes and offsets in different passbands • Tully et al. (1998) extinction corrections makes the slopes steeper, but do not bring them into agreement Rijks Universiteit Groningen

34. Add in the HI gas mass to calculate the baryonic Tully-Fisher relation The slope is less steep than stars only and less than Vrot ~ Mbar3.5 Slope problematic for MOND, but consistent wit hierarchical CDM galaxy formation models with some fine-tuning Baryonic Tully-Fisher relation Rijks Universiteit Groningen

35. An isothermal disk yields: Future work: Stellar Velocity Dispersions Rijks Universiteit Groningen

36. Future work: Rotation Curves Rijks Universiteit Groningen

37. Ages of young star clusters in merging galaxies Future work: stellar populations Ages and metallicities of resolved stellar populations in nearby disk galaxies Rijks Universiteit Groningen

38. Conclusions • Local star formation history in disks mainly set by local surface density, resulting in inside-out disk formation • Metallicity regulated by both surface density and mass • Realistic galaxy evolution models predict a strong correlation between population color and M/L • Maximum disk constraints support this observationally and suggest that a Salpeter IMF is too massive • The stellar mass Tully-Fisher relation is independent of originating passband • The baryonic Tully-Fisher relation has a maximal slope of about 3.5 +/- 0.2 Rijks Universiteit Groningen