Chemical Evolution of Dwarf Spheroidal Galaxies

1. Probes of internal and external ‘Feedback’ Chemical Evolution of Dwarf Spheroidal Galaxies Rosemary Wyse Ann Arbor, August 2007

2. Necessary ingredients include: • Star formation rate • Smooth? Stochastic? • Gas flows • Out/In flows a priori or self-consistent? • Prescription for energy and momentum injection from supernovae etc • Phase transitions? Radiative transfer.. • Stellar IMF • Fixed at Kroupa, Tout & Gilmore function -- no evidence for significant variations, but NOT Salpeter • Model of Type Ia supernovae -- rates • Yields

3. Zeroth order model: Shallow potential well • Cannot sustain a burst of star formation + Supernovae: • B.E ~ 3GM2/5R ~ 5x1051erg (M/107 M)2 (1kpc/R) equivalent to only several SNe • In extreme case of rapid, extensive mass loss, systems expands to a new, diffuse, equilibrium leading to dSph (Saito 1979; Dekel and Silk 1986) • Binding energy per unit mass increases with mass, leading to decreasing efficiency of winds and mass-metallicity relationship (Larson 1974; Vader 1986; Arimoto & Yoshii 1987; Kauffmann & Charlot 1998…etc etc) • ~10km/s velocity dispersion also corresponds to 104K, sensitive to (re)ionization (Efstathiou 1990; Bullock et al 01)

4. Simplest models derived early SNe-driven winds • dSph should have simple stellar population! • Also if reionization suppresses further star formation • Now deep CMDs reveal extended star formation histories

5. Hernandez, Gilmore & Valls-Gabaud 2000 Carina dSph Leo I dSph Intermediate-age population dominates in typical dSph satellite galaxies, and star formation rate is low < 0.0001M/yr cf. Smecker-Hane et al 1994

6. Outflows, Inflows • Extended star formation history obviously means either managed to retain significant gas after onset of star formation, or the gas went out, but came back in. • Gas-free now – why? Ram pressure stripping? • Low mean stellar metallicity, typically less than a tenth solar, combined with invariant IMF, means gas removal from star formation, not by star formation • Why are many (most?) stars a few Gyr old? Special epoch of inflow/interactions? • Tie to formation of the Local Group (Silk, Wyse & Shields 1987) ?

7. Self-Regulation? • Recent SPH simulations with self-regulating SNe heating – cooling – star formation cycle have faster variations in SFR than observed, and overall declining SFR (Stinson et al 07) • Regulation of gas phase is promising (Robertson talk)

8. Extended, low-rate star formation and slow enrichment with gas retention, leads to expectation of ~solar (or below) ratios of [/Fe], such as in LMC stars Hiatus then burst Smith et al 2003 Local disk Gilmore & Wyse 1991

9. Observational constraints from dSph: • Want to develop self-consistent models of dSph evolution, with any SNe-driven outflows constrained by star formation rate and potential well, and with inflows motivated cosmologically, rather than totally ad hoc • Multi-object medium-resolution spectroscopy provides large samples with radial velocities accurate to few km/s and metallicity to ~0.2 dex • We can now obtain even high-resolution spectra of stars in dSph, and can thus derive detailed elemental abundances • HST CMDs for star formation histories

10. Carina dSph • Part of Large Program on VLT (PI Gilmore, co-Is Wyse, Grebel, Wilkinson, Kleyna, Koch, Evans). • Determine radial velocities and metallicities for large sample of candidate members, across the face of several dSph, plus elemental abundances for a smaller sample of known members: FLAMES/GIRAFFE and FLAMES/UVES • Constrain potential well from internal kinematics (Gilmore talk; Walker talk)

11. Carina dSph: Broad Metallicity Distribution 437 RGB stars, Ca T spectra Mean -1.7dex; σ=0.9 Koch et al 2006 Weak dependence of metallicity on galactocentric distance

12. Closed-box, IRA, outflow proportional to SFR (Hartwick 1976): K-giant problem: Pre-enrichment?

13. Wind-dominated models: Lanfranchi & Matteucci (2004); parameter is star formation efficiency

14. Tuned wind-dominated model: Lanfranchi et al. 07

15. Stochastic Enrichment model Searle 1977 • Independent star-forming regions, each with identical ‘enrichment events’ occuring at a fixed mean rate • Metallicity of each fragment proportional to the number of enrichment events in it • Enrichment is then a Poisson process • Each fragment assumed to be well-mixed • Model parameter is the mean number of enrichment events per region – changes shape. Adopted value of 3 here.

16. Inefficient stochastic enrichment:

17. Elemental abundances with bursts of star formation Gilmore & Wyse 1991 Carina data: bursts + inhomogeneous star formation Massive star IMF invariant Koch et al 2007

18. Main sequence luminosity functions of UMi dSph and of globular clusters are indistinguishable.  normal stellar M/L HST star counts Wyse et al 2002 Massive-star IMF constrained by elemental abundances – also normal M92  M15  0.3M

19. Stars in satellite galaxies have different elemental ratios than do field halo stars… Milky Way field stars satellites Geisler et al 07

20. Bulk of stellar halo is OLD, as is bulge: Did not form from typical satellites disrupted later than a redshift of 2 Unavane, Wyse & Gilmore 1996 Scatter plot of [Fe/H] vs B-V for local high-velocity halo stars (Carney): few stars bluer (younger) than old turnoffs (5Gyr, 10Gyr, 15Gyr Yale)

21. Satellites/star forming regions that formed the field halo must have been accreted/disrupted prior to self-enrichment by Type Ia supernovae • And also formed stars only a long time ago, so if similar to surviving satellites and would have extended SFH, need to have been accreted a long time (~10Gyr) ago • (Λ)CDM predicted much late merging, incompatible with the Unavane et al limits • Can newer models be fine-tuned?

22. State-of-the art ΛCDM prediction, constrained to fit observations such as total field halo luminosity: Peak of stellar halo  Robertson et al 2006 Curve: Model predictions from one progenitor, with 68% of stars to lie on bold solid curve Crosses: Venn et al 04 compilation of field halo

23. Concluding remarks: • Data ahead of models… • Need to look closely at inhomogeneous chemical evolution models for dSph • Stars form in clusters/associations, natural? • Bulk of the stellar halo of the Milky Way does not look like the predictions from ΛCDM model so far..

24. Type II Supernova yields Ejecta Salpeter IMF gives [/Fe] ~ 0.4 Gibson 1998 Progenitor mass

25. Schematic [O/Fe] vs [Fe/H] Wyse & Gilmore 1993 IMF biased to most massive stars Type II only Plus Type Ia Fast Slow enrichment SFR, winds.. Self-enriched star forming region. Assume good mixing so IMF-average yields

27. M15 Cannot use color on RGB to estimate metallicity – there is presumably an age-metallicity relationship that produces a narrow RGB (cf. Smecker-Hane et al 1999) despite large metallicity spread. 47 Tuc Blue/red symbol for [Fe/H] > < –1.7

28. Targets selected from ESO Imaging Survey 5 fields, each 25 diameter, ~110 stars per pointing