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Local Group Galaxies

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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Local Group Galaxies

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  1. Advanced Lectures on Galaxies (2008 INAOE): Chapter 1 and 3a Local Group Galaxies Divakara Mayya INAOE http://www.inaoep.mx/~ydm

  2. Galaxies in the Universe: An Introduction Linda S. Sparke and John S. Gallagher Reference Ultra Deep Field

  3. Star Formation in the Local Group Binggeli Adopted from Eva K. Grebel Astronomical Institute University of Basel Astro-ph/0508147

  4. 3 Grebel 1999

  5. LMC: Multi-wavelength view

  6. LMC and SMC: Milky Way’s satellites

  7. 4 Grebel 1999 dSphs dEs dSph/dIrrs dIrrs The Local Group

  8. 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. 9 Buonanno et al. 1998

  10. 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.

  11. 14 105 Star CMDs from WFPC2: LMC Star Formation Histories Disk Bar Smecker-Hane, Gallagher, Cole, Stetson, 2002, ApJ, 566, 239

  12. CMDs: Galactic bulge vs LMC disk

  13. Fornax dwarf spheroidal galaxy

  14. Carina dwarf spheroidal galaxy: M/L=74

  15. 10 Luminosity function of Ursa Minor: Indistinguishable from Galactic globulars Feltzing et al. 1998; Wyse et al. 2002

  16. 5 Morphological Segregation Gas-poor, low-mass dwarfs Grebel 2000 Gas-rich, higher-mass dwarfs

  17. 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

  18. 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

  19. 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.)‏

  20. 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

  21. 12 Star formation activity in low-mass galaxies (~107 M)‏ Grebel & Gallagher 2004 Cosmology: flat, m = 0.27, H0 = 71 km/s/Mpc

  22. 16 Correlation between SFH and distance

  23. 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.

  24. 18

  25. 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…

  26. 20 Harbeck, Gallagher, & Grebel 2004 More luminous dwarf No obvious distance correlation for M31 dSphs

  27. 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.

  28. 22 Hierarchical structure formation: Numerous mergers leave imprint on halo (and disk)‏ Thus expected: • Overdensities • Lots of streams • Identification photometrically / kinematically 2MASS + Johnston streams

  29. 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”.

  30. 25 Ibata et al. 2001, Ferguson et al. 2002 + ongoing HST follow-up Zucker et al. 2004

  31. 26 Extremely deep HST imaging of M31’s halo Brown et al. 2003 Old populations present, but intermediate-age, metal-rich populations dominate.

  32. 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)‏

  33. 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

  34. Ellipticals, lenticulars, spirals and irregulars fit into the classical Tunic-fork diagram What about the rest?

  35. Irregular II or Amorphous galaxies

  36. Ring galaxies (Romano et al 2008)

  37. 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

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