Binggeli

1. Star Formation in the Local Group Binggeli • Eva K. Grebel • Astronomical Institute • University of Basel

2. 2 Why the Local Group? • Proximity • Resolution(individual stars) • Depth(faintest absolute luminosities) • Measurements of: • Lowest stellar masses • Oldest stellar ages • Metallicities, element abundances • Detailed stellar and gas kinematics • Highest level of detail and accuracy • Variety(of galaxy types) • Range of masses, ages, metallicities • Range of morphological types • Range of environments • Tests of galaxy evolution theories • Understanding distant, unresolved galaxies Ultra Deep Field

3. 3 Grebel 1999

4. 4 Grebel 1999 dSphs dEs dSph/dIrrs dIrrs The Local Group

5. 5 Morphological Segregation Gas-poor, low-mass dwarfs Grebel 2000 Gas-rich, higher-mass dwarfs

6. 6 1. The Earliest Epoch of Star Formation • Cold dark matter models predict: • Low-mass systems: • first sites of star formation (z ~ 30) • Larger systems form through hierarchical merging of smaller systems • Re-ionization may Kravtsov & Klypin (CfCP & NCSA) • squelch star formation in low-mass substructures • Galaxies less massive than 109 M lose star-forming material during re-ionization

7. 7 1. The Earliest Epoch of Star Formation • Consequences: • Low-mass galaxies must form stars prior to re-ionization; must contain ancient populations • Sharp drop / cessation of star formation activity after re-ionization, may resume only much later • High-mass galaxies’ oldest populations must be as old as low-mass galaxies’ populations or younger Testable predictions! • Redshifts of 20 - 30 not (yet?) accessible • Dwarf galaxies at those redshifts would be extremely difficult targets anyway • Exploit fossil record in nearby Universe instead • Local Group ideal target since oldest populations resolved and accessible with HST

8. 8 1. The Earliest Epoch of Star Formation Age-dating old populations: • Most accurate ages for resolved stellar populations • Most accurate ages from main-sequence turn-offs • Absolute ages model-dependent (isochrones) • Relative ages with internal accuracies of fractions of a Gyr; typically compared against ancient Galactic globular clusters of the same metallicity and [/Fe](differential age dating) • Requires at least 2 mag below MSTO • High resolution (crowding) • Depth (e.g., at M31 distance MSTO > 29 mag) • HST for anything but closest Milky Way satellites

9. 9 Buonanno et al. 1998

10. 10 Luminosity function of Ursa Minor: Indistinguishable from Galactic globulars Feltzing et al. 1998; Wyse et al. 2002

11. 10 1. The Earliest Epoch of Star Formation Results (largely based on HST): • Old populations ubiquitous but fractions vary • Evidence for a common epoch of star formation • Globular clusters with main-sequence photometry (Galactic halo & bulge,Sgr, LMC,For) • Field populations with main-sequence photometry (Sgr,LMC,Dra,UMi,Scl,Car,For,LeoII) • Inferred from globular clusters (e.g., BHBs, spectra): M31, WLM, NGC 6822) • Inferred from BHBs in field populations: Leo I, Phe, And I, II, III, V, VI, VII, Cet, Tuc • Possible evidence for delayed formation? • Inferred from GC MS: SMC’s NGC 121 (2-3 Gyr). (However, lack of ancient globulars does not imply absence of ancient field population.)

12. 11 1. The Earliest Epoch of Star Formation • Limitations: • Deep data for direct (MSTO) age measurements lack in dwarfs beyond ~ 300 kpc. • True fraction of old stars still poorly known (incomplete area coverage & unknown tidal loss) • No data on Population III stars and their ages • Confirmed: • Ancient Population II in Milky Way, LMC, and dwarf spheroidal galaxies ~ coeval (± 1 Gyr) • Consistent with building block scenario • All galaxies studied in sufficient detail so far contain ancient populations • In contrast to CDM predictions: • No cessation of star formation activity in low-mass galaxies during re-ionization • Considerable enrichment: Episodes of several Gyr Grebel & Gallagher 2004

13. 12 Star formation activity in low-mass galaxies (~107 M) Grebel & Gallagher 2004 Cosmology: flat, m = 0.27, H0 = 71 km/s/Mpc

14. 13 2. Global star formation histories Age structure in a synthetic color-magnitude diagram Gallart et al. 1999 Shown: Constant star formation rate from 15 Gyr to the present, no photom. errors.

15. 14 105 Star CMDs from WFPC2: LMC Star Formation Histories Disk Bar Smecker-Hane, Gallagher, Cole, Stetson, 2002, ApJ, 566, 239

16. 15 Population box Star formation rate Metallicity [Fe/H] Lookback time Age 2. Global star formation histories Requires: Ages and SFR(t)  photometry + metallicity (t)  spectroscopy + abundance ratios (t)  spectroscopy

17. 16 Correlation between SFH and distance

18. 17 Star formation history - distance correlation • Faint (MV > -14) Milky Way companions: • Increasingly higher intermediate-age population fractions with increasing distance from the MW • Environmental influence of Milky Way? Star-forming material might have been removed earlier on from closer companions via • ram-pressure stripping • SN-driven winds from Milky Way • high UV flux from proto-Milky Way • tidal stripping(van den Bergh 1994) If environment is primarily responsible for gas-poor dSphs, then existence of isolated Cetus & Tucana is difficult to understand. Caveat: Argument considers only present-day distances; orbits still poorly known / unknown.

19. 18

20. 19 If the apparent trend of low-mass galaxy properties with distance from the primary generally holds, we should also find it for M31’s low-mass companions…

21. 20 Harbeck, Gallagher, & Grebel 2004 More luminous dwarf No obvious distance correlation for M31 dSphs

22. 21 3. Harrassment and Accretion • Can we find evidence for this • in the surroundings of massive galaxies in the Local Group? • in the massive galaxies of the Local Group themselves? • Structural properties of nearby galaxies • Stellar content and population properties of nearby galaxies (including abundance patterns) • Streams around and within massive galaxies Dwarf galaxies might be considered the few survivors of a once more numerous dark matter “building block” galaxy population.

23. 22 • Hierarchical structure formation: • Numerous mergers leave imprint on halo (and disk) • Thus expected: • Overdensities • Lots of streams • Identification photometrically / kinematically 2MASS + Johnston streams

24. 23 3. Dwarf galaxy accretion: Sagittarius’ tidal stream within the Milky Way Majewski et al. 2003 2MASS: Detection of Sgr’s tidal stream across the entire sky (area coverage advantage of shallow of all-sky survey). Recent detection of second dSph in state of advanced accretion: Monoceros (SDSS, Newberg et al. 2002); “CMa dSph”.

25. 24 3. Dwarf galaxy accretion: Sagittarius’ tidal stream around & within the Milky Way Majewski et al.

26. 25 Ibata et al. 2001, Ferguson et al. 2002 + ongoing HST follow-up Zucker et al. 2004

27. 26 Extremely deep HST imaging of M31’s halo Brown et al. 2003 Old populations present, but intermediate-age, metal-rich populations dominate.

28. 27 4. Are dwarf galaxy abundance patterns consistent with the building block scenario? • Old populations in dSphs and dIrrs are metal-poor. • The mean metallicity decreases with decreasing parent galaxy luminosity(see Figure). • Star formation history details vary widely, and all dwarfs show considerable abundance spreads. • In terms of mean overall metallicity, • consistency for Milky Way halo • inconsistency for M31 halo (too metal-rich and dominated by 6 - 8 Gyr populations; Brown et al. 2003)

29. 28 The metallicity - luminosity relation for different types of dwarf galaxies

30. 29 4. Are dwarf galaxy abundance patterns consistent with the building block scenario? Type II SNe: M ≥ 10 M Mainly -elements (O, Ne, Mg, Si, S, Ca) Timescale < ~ 3·107 yr Type Ia SNe: white dwarfs accreting mass Mainly Fe peak elements Timescale ~ 3·107 yr to 15 Gyr [/Fe] vs. [Fe/H]: mainly determined by time delay between SNe II and Ia and SFH (IMF, nucleosynth.) If dSphs were dominant contributors to the build-up of the Galactic halo, then their abundance patterns should match those observed in the halo. Matteucci 2002

31. 30 0.6 0.4 0.2 0.0 After Matteucci 2002 [/Fe]  0.0 -1.0 -2.0 -3.0 -4.0 [Fe/H] High star formation rates (bulges, ellipticals): Larger plateau for [/Fe], more SN II () contribution. Low star formation rates (bursty or continuous; disks, Irrs): Short plateau in [/Fe] at low metallicities. Slow star formation rates (dwarfs): SNe Ia contribute large amounts of Fe sooner and already at very low metallicities, so that solar [/Fe] reached sooner.

32. 31 Tolstoy & Venn 2004 Blue, black: Galactic disk and halo stars Other colors: Galactic dSphs • Lower [/Fe] @ [Fe/H] in dSphs than in Galactic halo: • Low SFRs (little contribution from massive SNe II ()), or • Loss of metals and SN ejecta, or • Larger contribution from SNe Ia (Fe enhanced over ) • Present-day dSphs cannot have been dominant contributors to the build-up of the Galactic halo Shetrone, Côté, & Sargent 2001

33. 32 • Essential science:The Local Group • as a test case for galaxy evolution theories • What we know now: • All nearby galaxies contain ancient populations; fractions vary; ~ coeval Population II. • No two galaxies alike in star formation histories, population fractions, mean metallicities and abundance spreads. • But: global correlations (e.g., mass-metallicity) • Environmental impact and CDM building blocks: • Morphology-density • Distance - HI content • Accretion events • Coeval ancient SF • But: • Tucana, Cetus • Uncertain distance - SFH • Number and [ / Fe] • Extended SF in low-mass galaxies (vs. reionization)

34. 33 • Essential science:The Local Group • as a test case for galaxy evolution • Essential and unique HST science for the future: • Not possible from the ground • Not possible with JWST or • Prepares the ground for JWST and others • HST uniqueness:resolution (crowding) • sensitivity (sky background) • wavelength coverage • (Optical: highest age and metallicity resolution) • SFHs and oldest ages of M31 subsystem + beyond • (I)MFs in cluster & field populations in nearby galaxies; variations, dissolution times, dependencies • Constraints on orbits via parallaxes prior to SIM • Exploit synergy with ground-based spectroscopic capabilities for abundances and kinematics!