4 The Local Group Grebel 1999 dSphs dEs dSph/dIrrs dIrrs
2 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
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 squelch star formation in low-mass substructures Galaxies less massive than 10 9 M lose star- forming material during re-ionization Kravtsov & Klypin (CfCP & NCSA)
7 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 1. The Earliest Epoch of Star Formation
10 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):
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
12 Grebel & Gallagher 2004 Star formation activity in low-mass galaxies (~10 7 M ) Cosmology: flat, m = 0.27, H 0 = 71 km/s/Mpc
17 Star formation history - distance correlation Faint (M V > -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 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…
20 No obvious distance correlation for M31 dSphs More luminous dwarf Harbeck, Gallagher, & Grebel 2004
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.
3. Dwarf galaxy accretion: Sagittarius’ tidal stream within the Milky Way 23 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
25 Ibata et al. 2001, Ferguson et al. 2002 Zucker et al. 2004 + ongoing HST follow-up
26 Brown et al. 2003 Extremely deep HST imaging of M31’s halo Old populations present, but intermediate -age, metal-rich populations dominate.
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)
Galaxies (Class III): Types Giant galaxies Dwarf galaxies Galaxies with a prominent nucleus E Sp IrrI, IrrII Peculiar LSB dE dSph dIrr HII, BCD, Haro … Starburst, post-starburst AGN
Ellipticals, lenticulars, spirals and irregulars fit into the classical Tunic-fork diagram What about the rest?
Ellipticals: 4 Flavors Giant cDs, centers of clusters/groups, masses 10^13-10^14 Msolar Normal Es: Masses from 10^8 (not many, M32 holds down the low mass range of most correlations…) to 10^13 Msolar Spheroidals: dSphs in the local group, lower surface brightness dwarfs (10^7-10^9) in clusters Dwarf Ellipticals