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Advanced Lectures on Galaxies (2008 INAOE): Chapter 1 and 3a. Local Group Galaxies. Divakara Mayya INAOE http://www.inaoep.mx/~ydm. Galaxies in the Universe: An Introduction Linda S. Sparke and John S. Gallagher. Reference. Ultra Deep Field. Star Formation in the Local Group. Binggeli.
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Advanced Lectures on Galaxies (2008 INAOE): Chapter 1 and 3a Local Group Galaxies Divakara Mayya INAOE http://www.inaoep.mx/~ydm
Galaxies in the Universe: An Introduction Linda S. Sparke and John S. Gallagher Reference Ultra Deep Field
Star Formation in the Local Group Binggeli Adopted from Eva K. Grebel Astronomical Institute University of Basel Astro-ph/0508147
3 Grebel 1999
4 Grebel 1999 dSphs dEs dSph/dIrrs dIrrs The Local Group
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
9 Buonanno et al. 1998
13 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.
14 105 Star CMDs from WFPC2: LMC Star Formation Histories Disk Bar Smecker-Hane, Gallagher, Cole, Stetson, 2002, ApJ, 566, 239
10 Luminosity function of Ursa Minor: Indistinguishable from Galactic globulars Feltzing et al. 1998; Wyse et al. 2002
5 Morphological Segregation Gas-poor, low-mass dwarfs Grebel 2000 Gas-rich, higher-mass dwarfs
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 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
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.)
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 Star formation activity in low-mass galaxies (~107 M) Grebel & Gallagher 2004 Cosmology: flat, m = 0.27, H0 = 71 km/s/Mpc
16 Correlation between SFH and distance
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 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 Harbeck, Gallagher, & Grebel 2004 More luminous dwarf No obvious distance correlation for M31 dSphs
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.
22 Hierarchical structure formation: Numerous mergers leave imprint on halo (and disk) Thus expected: • Overdensities • Lots of streams • Identification photometrically / kinematically 2MASS + Johnston streams
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 Ibata et al. 2001, Ferguson et al. 2002 + ongoing HST follow-up Zucker et al. 2004
26 Extremely deep HST imaging of M31’s halo Brown et al. 2003 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 E Sp IrrI, IrrII Peculiar LSB Giant galaxies Dwarf galaxies Galaxies with a prominent nucleus 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?
Ring galaxies (Romano et al 2008)
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