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Eva K. GrebelAstronomical Institute University of Basel

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Why the Local Group?

Ultra Deep Field

• 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

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Grebel 1999

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The Local Group

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dSphsdEsdSph/dIrrsdIrrs

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Morphological Segregation

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Gas-poor, low-mass dwarfs

Gas-rich, higher-mass dwarfs

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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

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squelch star formation in low-mass substructures

• Galaxies less massive than 109 M lose star-forming material during re-ionization

Kravtsov & Klypin (CfCP & NCSA)

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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 youngerTestable 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

1. The Earliest Epoch of Star Formation

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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

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Luminosity function of Ursa Minor: Indistinguishable from Galactic globulars

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1. The Earliest Epoch of Star Formation

• 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.)

Results (largely based on HST):

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1. The Earliest Epoch of Star FormationLimitations:• 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 agesConfirmed:• 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

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Grebel & Gallagher 2004

Star formation activity in low-mass galaxies (~107 M)

Cosmology: flat, m = 0.27, H0 = 71 km/s/Mpc

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Age structure

in a synthetic color-magnitude diagram

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Shown:Constant star formation rate from 15 Gyr to the present, no photom. errors.

2. Global star formation histories

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Smecker-Hane, Gallagher, Cole, Stetson, 2002, ApJ, 566, 239

105 Star CMDs from WFPC2: LMC Star Formation Histories

Disk Bar

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2. Global star formation histories

Star formation

rate Metallicity[Fe/H]

Lookback time

Age

Population box

Requires: Ages and SFR(t) photometry + metallicity (t) spectroscopy + abundance ratios (t) spectroscopy

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Correlation between SFH and distance

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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.

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19If 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…

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No obvious distance correlation for M31 dSphs

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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 galaxiesDwarf galaxies might be considered the few survivors of a once more numerous dark matter “building block” galaxy population.

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Hierarchical structure formation:Numerous mergers leave imprint on halo (and disk)Thus expected: Overdensities Lots of streams

Identification photometrically / kinematically

2MASS + Johnston streams

3. Dwarf galaxy accretion: Sagittarius’ tidal stream within the Milky Way

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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”.

Majewski et al. 2003

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3. Dwarf galaxy accretion: Sagittarius’ tidal stream around & within the Milky Way

Majewski et al.

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+ ongoingHST follow-up

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Extremely deep HST imaging of M31’s halo

Old populations present, but intermediate-age, metal-rich populations dominate.

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4. Are dwarf galaxy abundance patternsconsistent 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)

28The metallicity - luminosity relation

for different types of dwarf galaxies

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4. Are dwarf galaxy abundance patternsconsistent 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 massMainly Fe peak elementsTimescale ~ 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

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[Fe/H]

[/

Fe]

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0.0 -1.0 -2.0 -3.0 -4.0

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.

After Matteucci 2002

31 Tolstoy & Venn 2004

Blue, black: Galactic disk and halo starsOther 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

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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)

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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!