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The Least Luminous Galaxies: Faint But Not Dull

The Least Luminous Galaxies: Faint But Not Dull. Daniel Zucker Macquarie University/ Anglo-Australian Observatory. I. Introduction. Luminous Matter. Bullock & Johnston 2005. Dark Matter. Diemand+ 2006. The “Missing Satellite” Problem, circa 2005.

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The Least Luminous Galaxies: Faint But Not Dull

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  1. The Least Luminous Galaxies: Faint But Not Dull Daniel Zucker Macquarie University/ Anglo-Australian Observatory

  2. I. Introduction Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  3. Luminous Matter Bullock & Johnston 2005 Dark Matter Diemand+ 2006 The “Missing Satellite” Problem, circa 2005 • CDM models predict far more low-mass dark subhalos and substructure than dwarfs and streams observed -- “missing satellites”† • Some theoretical solutions: inhibited star formation; observed satellites much more massive; observed satellites originally more massive but tidally stripped‡ † Klypin+ 1999, Moore+ 1999, Benson+ 2002 ‡ Somerville 2002, Benson+ 2002; Stoehr+ 2002; Kravtsov+ 2004 Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  4. And IX Cetus dSph Subaru g,r,i Whiting+ 1999 The Observational Perspective, circa 2005 • 2003: only 9 Milky Way (MW) dwarf spheroidal galaxies (dSphs), still fewer around M31 • 2004: Andromeda IX (Zucker+) • 2005: Ursa Major(Willman+), Andromeda X (Zucker+); runts or tip of an iceberg? Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  5. The Sloan Digital Sky Survey • SDSS-I: Large-area imaging and spectroscopic survey, covered ~25% of the sky • SDSS-II: just finished, SDSS-III now underway Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  6. Stellar Density in SDSS Data • SDSS data for the North Galactic Cap cover over 8000 sq. deg. • ~56 million stellar objects identified by SDSS pipeline in DR5 Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  7. The SDSS “Field of Streams” Density composite from magnitude slices:probing distances in upper main sequence and turn-off stars Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  8. Canes Venatici I NGC 5466 NGC 5272 A Field of Streams...and Dots Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  9. The Ultra-Low-Luminosity Explosion • Wide-area surveys (SDSS,CFHT)  ~20 new low-luminosity LG dwarfs since 2004, almost all dSphs • Many of the new dwarfs have very low measured masses (< 108 M)  most sensitive to reionisation and feedback processes Walsh+ 2008 Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  10. INT Data GMRT WSRT INT g,r And Now, for Something Different: Leo T, The Smallest Star-Forming Galaxy • Leo T not dead yet: stars formed within few x 108 yr • HIPASS, GMRT, WSRT: ~3 x 105M H I, RV ~ 35 km s-1 • MH I/M ~ 2, (Virialized) Mdyn~ 7x106 M -- how did it keep/accrete gas? Are there many more like Leo T? Irwin+ 2007, Ryan-Weber+ 2008 Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  11.  Gap?  No DM DM MV vs. rh for the New Dwarfs • Few objects with rh between ~40 pc and ~100 pc  characteristic size scale for objects with dark matter? Implications+? • What is the nature of the objects in the “gap” --star clusters or dwarfs? + e.g. Gilmore+ 2008, Strigari+2008 Luminosity  Size  Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  12. Strigari+ 2008 Matteo+ 1993 A Common (Minimum) Mass for Galaxies? Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  13. Some Big Questions about Little Galaxies • How many low-luminosity dwarfs are there? What are their properties, and how are they distributed? • Is there a minimum galaxy mass? (Is there a common galaxy mass?) If so, what does this tell us about dark matter/ star formation/ both? • Is there a minimum galaxy size? What are the objects in the “gap”? • What are their stellar populations, and what can they tell us about the conditions of star formation in such extreme systems over a cosmic time? Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  14. II. Stellar Populations of Low-Luminosity Dwarfs* “The stellar populations of ultra-faint dwarfs are still a mystery” – O. Gnedin (2008) Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  15. Stellar Populations with HST • 101-orbit HST program to observe 4 new Milky Way satellites, 3 M31 satellites and Leo T, using F606W (broad ~V) and F814W (~ I) filters • WFPC2 (RIP): 4-chip batwing, 160” on a side And XIII F606W Zucker+, in prep; Martin+, in prep Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  16. Hercules MV = -6.6 Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  17. CVn I MV = -8.6 Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  18. Leo T MV = -8.0 Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  19. Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  20. Her CVn I Leo T Star Formation/Chemical Enrichment Histories • Hercules: purely old (~14 Gyr), metal poor • CVn I: extended star formation (14 - 8+ Gyr), no clear sign of younger population (cf. Martin et al. 2008), increasing metallicity • Leo T: extended, multi-episodic star formation (14 - 6 Gyr, 4 Gyr - 1 Gyr, 250 Myr) Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  21. Leo T INT g+r,GALEX NUV,FUV Current Star Formation in Extreme Dwarfs Leo T • Leo T: a unique (?) laboratory • H (Gemini) and UV (pointed GALEX + Swift/UVOT) observations to study recent star formation: no detected H II regions  no ongoing SF… or no O stars? Gemini Hα,Swift UVW1,UVW2 Zucker+, in prep. Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  22. Star Formation History vs. DG Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  23. Summary • The “ultra-faint” dwarfs appear to be part of the same continuum of properties with the previously-known dwarfs – e.g., derived star formation histories are consistent with the apparent proximity effect: ultrafaintluminosity-challenged • None of the dwarfs studied (Hercules? And XI?) look like purely ancient, single stellar populations  what impact did reionization have on such objects? • Leo T has had multiple epochs of star formation, as recently as 200 Myr ago – and still has neutral gas – but no H II regions (or no O stars). Is Leo T a “Rosetta Stone” for understanding this extreme regime of star formation? Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  24. Epilogue: Some Things Coming Up Down Under… • Skymapper: southern all-sky 6-filter photometric survey • HERMES: 400 fiber high-res (R~30000) spectrograph with 2° FOV for AAT • WALLABY: southern all-sky H I survey proposed for ASKAP Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

  25. Daniel Zucker  • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

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