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The Stellar Populations, Mass-to-Light Ratios and Dark Matter in Spiral Galaxies Roelof S. de Jong Steward Observatory Eric Bell Rob Kennicutt Rob Swaters Rob Olling Don McCarthy Cedric Lacey Overview • Introduction • Ages and metallicities of stellar populations 26 February 2001 Rijks Universiteit Groningen – 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 Galaxy Formation and Evolution • Huge progress, both observational and theoretical: 26 February 2001 Rijks Universiteit Groningen – observational: e.g. the star formation history of the Universe and of local group galaxies – theoretical: hierarchical galaxy formation models in CDM-like universes • Something is missing: We do not not where, when and especially why stars are forming in particular galaxies Galaxy Evolution and Structural Parameters 26 February 2001 Rijks Universiteit Groningen • What drives parameters: the Star Formation History and the Structural luminosity, scale size, Chemical Evolution within galaxies? surface brightness, mass,disk velocity distribution – current star formation in disks semi-regular, related to morphology andstudies: structural parameters Statistical scaling relations – 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)? Stellar populations Color-Color diagrams 26 February 2001 Rijks Universiteit Groningen Gyr Bruzual & Charlot models Gyr Data & Samples • Face-on disk galaxies with – data in at least 3 passbands (of which one IR) 26 February 2001 Rijks Universiteit Groningen – good colors over at least 2 disk scale lengths • 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 Total sample of 121 galaxies 26 February 2001 R-K Rijks Universiteit Groningen Radial Color-Color Observations BR BR Maximum Likelihood Fitting • Make model grid of e-t/τ Star Formation History and metallicity Rijks Universiteit Groningen – 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 26 February 2001 • Use Monte Carlo simulations to determine uncertainties 26 February 2001 Rijks Universiteit Groningen Local Age & Local Metallicity versus Local Surface Brightness 26 February 2001 Rijks Universiteit Groningen Age vs Surface Brightness & Luminosity 26 February 2001 Rijks Universiteit Groningen Metals vs Surface Brightness & Luminosity 26 February 2001 Rijks Universiteit Groningen What determines SFH and Metals? Surface Brightness or Luminosity? Remember luminosity and surface brightness are correlated! The Galaxy Space Density 26 February 2001 Rijks Universiteit Groningen Surface Brightness & Magnitude Space density of spiral galaxies corrected for selection effects (de Jong & Lacey 2000) 26 February 2001 Rijks Universiteit Groningen Are Ages mainly determined by Surface Brightness or Luminosity? 26 February 2001 Rijks Universiteit Groningen Is metallicity mainly determined by Surface Brightness or Luminosity? Summary observations 26 February 2001 Rijks Universiteit Groningen • 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 So what are the caveats? – Changes in the IMF – Other Stellar Population Synthesis models – The effect of dust reddening 26 February 2001 Rijks Universiteit Groningen IMF uncertainty Salpeter IMF Scalo IMF Spectral synthesis model uncertainty Bruzual & Charlot 26 February 2001 Rijks Universiteit Groningen Kodama & Arimoto The effect of Dust Extinction • Extinction will mainly effect metallicity determinations 26 February 2001 Rijks Universiteit Groningen 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 The effect of Dust Extinction • Extinction will mainly effect metallicity determinations 26 February 2001 Rijks Universiteit Groningen 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 – 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 26 February 2001 Rijks Universiteit Groningen Dust modeling with scattering • Scattering preferably to face on direction • Reddening follows absorption curve, not extinction curve • For low optical depth reddening insignificant Conclusion Age & Metallicity Caveats • Only very unusual IMFs can mimic our results 26 February 2001 Rijks Universiteit Groningen • Other Spectral Synthesis Models will only change the absolute age and metallicity values • Dust will at most effect metallicities a bit The relative rankings of Ages & Metallicities are Robust Simple Galaxy Evolution Models • Simple closed box models: – Start with exponential gas disk 26 February 2001 Rijks Universiteit Groningen – Form stars according to Schmidt law: (surface density)n – Instantaneous recycling of metals – Maximum likelihood fitting on resulting integrated colors • 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 26 February 2001 Rijks Universiteit Groningen Galaxy evolution models Mass dependent formation epoch model with star burst Closed box model 26 February 2001 Rijks Universiteit Groningen 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 • What about masses instead of luminosities? Why stellar M/Ls? 26 February 2001 Rijks Universiteit Groningen • 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 • 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 26 February 2001 Rijks Universiteit Groningen Galaxy evolution models Closed box 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 Color-ML for hierarchical galaxy model 26 February 2001 Rijks Universiteit Groningen B I Even a hierarchical galaxy formation model shows strong correlation between color and M/L K Cole et al. (2000) models 26 February 2001 Rijks Universiteit Groningen Different population synthesis models • The slope of the color-M/L relation is independent of stellar population synthesis models used 26 February 2001 Rijks Universiteit Groningen Different Initial Mass Functions • The slope of the color-M/L relation is independent of models and IMFs used • The normalization of the relation depends on the IMF used, i.e. the amount of low mass stars 26 February 2001 Rijks Universiteit Groningen Rotation curve M/L constraint Maximum disk constraints • A Salpeter IMF is too massive • Distribution suggests IMF similar in most galaxies and close to maximum disk for a fraction of the galaxies bad data point due to beam smearing data Verheijen (1997) 26 February 2001 Rijks Universiteit Groningen • The color-M/L relation must be normalized below all maximum disk values 26 February 2001 Rijks Universiteit Groningen Stellar Mass Tully-Fisher relation • Raw Tully Tully-Fisher Stellar etmasses al. (1998) derived relation has from different different extinction slopes corrections passbands and offsets using in makes different the thecolor-M/L slopes passbands steeper, relation but do not agree bring to them within 10%agreement into rms • The Tully-Fisher relations derived from different passbands are identical to within the errors • The slope is very steep Vrot ~ M*4.5 Baryonic Tully-Fisher relation 26 February 2001 Rijks Universiteit Groningen • 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 26 February 2001 Rijks Universiteit Groningen Future work: Stellar Velocity Dispersions An isothermal disk yields: 26 February 2001 Rijks Universiteit Groningen Future work: Rotation Curves Future work: stellar populations 26 February 2001 Rijks Universiteit Groningen Ages and metallicities of resolved stellar populations in nearby disk galaxies Ages of young star clusters in merging galaxies Conclusions • Local star formation history in disks mainly set by local surface density, resulting in inside-out disk formation 26 February 2001 Rijks Universiteit Groningen • 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