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

2. I. Introduction 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

3. 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 ‡ Luminous Matter Bullock & Johnston 2005 Dark Matter Diemand+ 2006 † Klypin+ 1999, Moore+ 1999, Benson+ 2002 ‡ Somerville 2002, Benson+ 2002; Stoehr+ 2002; Kravtsov+ 2004 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

4. 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? Cetus dSph Whiting+ 1999 Subaru g,r,i And IX 27 July 2009Daniel 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 27 July 2009Daniel 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 27 July 2009Daniel 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 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

8. A Field of Streams...and Dots NGC 5466 NGC 5272 Canes Venatici I 27 July 2009Daniel 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 (< 10 8 M  )  most sensitive to reionisation and feedback processes Walsh+ 2008 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

10. INT Data And Now, for Something Different: Leo T, The Smallest Star-Forming Galaxy Leo T not dead yet: stars formed within few x 10 8 yr HIPASS, GMRT, WSRT: ~3 x 10 5 M  H I, RV  ~ 35 km s -1 M H I /M  ~ 2, (Virialized) M dyn ~ 7x10 6 M  -- how did it keep/accrete gas? Are there many more like Leo T? INT g,r Irwin+ 2007, Ryan-Weber+ 2008 GMRT WSRT 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

11. M V vs. r h for the New Dwarfs Few objects with r h 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   Gap?  No DMDM 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

12. A Common (Minimum) Mass for Galaxies? Strigari+ 2008 Matteo+ 1993 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

13. 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan 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?

14. II. Stellar Populations of Low- Luminosity Dwarfs* 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan “The stellar populations of ultra-faint dwarfs are still a mystery” – O. Gnedin (2008)

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 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

16. Hercules M V = -6.6 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

17. CVn I M V = -8.6 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

18. Leo T M V = -8.0 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

19. 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

20. 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) HerCVn ILeo T 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

21. Current Star Formation in Extreme Dwarfs 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? INT g+r, GALEX NUV, FUV Leo T Zucker+, in prep. Gemini Hα, Swift UVW1, UVW2 Leo T 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

22. Star Formation History vs. D G D G (kpc)MVMV >10 Gyr (z>2) 5-8 Gyr (0.5<z<1 ) <~1 Gyr (z<0.1) And XI≥100-7.3Old And XIII≥100-6.9Old+ Her140-6.6Old CVn II150-4.9Old+ Leo IV160-5.0Old+ CVn I220-8.6Old+Inter+ Leo T420-8.0Old+Inter+Young! 27 July 2009Daniel 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? 27 July 2009Daniel 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 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan

25. 27 July 2009Daniel Zucker Extreme Star Formation in Dwarf Galaxies Ann Arbor, Michigan