1. The LCID Project (Local Cosmology from Isolated Dwarfs) A Brief Overview by Evan Skillman Prepared for the Michigan Workshop: “Extreme Star Formation in Dwarf Galaxies” July 28, 2009

2. Methodology: To obtain and analyze deep CMDs for a representative sample of isolated dwarf galaxies of the Local Group Deep  ACS/HST data with S/N~10 at the level of the oldest main sequence turn-off (absolute I~+3, apparent I~27) Representative  2 dIrr, 2 dIrr/dSph (“transition-types”), 2 dSph Isolated  far from the Milky Way and M31 and free from environmental effects that could affect their evolution Purpose: study the star formation histories (SFHs) and age- metallicity relationships (AMRs) of six isolated nearby dwarf galaxies to address the following questions: Was there any delay in the first star formation event? Did reionization play a role? Do these galaxies manage to retain gas after the first star formation event? Are there systematic traces of quiescence in the star formation? How does the spatial structure evolve? Do they have halos? El Cid (1048-1099) Statue by Quesada in Burgos The LCID project L ocal C osmology from I solated D warfs

3. 1) P.I. C. Gallart ACS - Cycle 14: 95 orbits Co-I: A. Aparicio (ULL-IAC), E. Bernard (IAC), I. Drozdovsky (IAC), M. Monelli (IAC), S. Hidalgo (IAC), G. Bertelli (U. Padova), S. Cassisi (INAF-OA-Teramo), P. Demarque, (U. Yale), H.C. Ferguson (STScI), L. Mayer (U. Zurich), M. Mateo (U. Michigan), J. Navarro (U. Victoria), A. Cole, E. Skillman (U. Minnesota), P.B. Stetson (DAO), E.Tolstoy (Kapteyn), A. Dolphin (U. Arizona), D. Weisz (U. Minnesota) 2) P.I.: A. Cole (U. Minnesota)ACS - Cycle 14: 16 orbits Co-Is: A. Aparicio (ULL-IAC), C. Gallart (IAC) A. Dolphin (U. Arizona), J. Gallagher (U. Wisconsin), E.Skillman (U.Minnesota), P. Stetson (DAO), E. Tolstoy (Kapteyn), D. Weisz (U. Minnesota) 3) P.I.: A. Aparicio WFPC2 - Cycle 9 + archive Cycle 6: 24 orbits LCID galaxy sample Project team and data sources LeoA PHO TUC CET IC1613 LGS3 32 25 22 wfpc2 12 16 24 # Orbit 24.77 24.39 23.30 23.98 24.50 24.41 (m-M) 0 dSph dIrr/dSph dIrr Type 899 / 1340 755 / 680 457 / 610 625 / 270 794 / 1180 769 / 520 D(kpc) MW / M31 Tucana Cetus Phoenix LGS3 Leo A IC1613 Galaxy M31 MW M33

4. Our view of dwarf galaxy evolution is biased by the MW companions.

5. Our new HST targets are the nearest isolated dwarf galaxies.

6. IC 1613 Outer Field Deep HST/WFPC2 observations (24 orbits EDS et al. 2003) At the time, this was the deepest color magnitude diagram of an isolated dwarf irregular galaxy.

7. IC 1613 - 24 Orbits ACS - note depth

8. LGS-3 and Leo A Comparison

9. Results on LCID data I V - I

10. Results on LCID data Solved CMD for IC1613 solved data

11. Results on LCID data

12. Results for 2 age binnings, with 1  random errors on SFR Leo A --- From Cole et al. (2007)

13. SFHs for the LCID sample (courtesy Sebastian Hidalgo)

15. Results on LCID data Cumulative mass fraction

16. Blue Stragglers? As emphasized by Momany et al. (2007) and Mapelli et al. (2007) the interpretation of the blue plume population in dSphs is critical. Typically, SFH codes interpret these as young stars, but they could represent coalescing binaries from an older population. If these are younger stars, then the star formation rates are so low that it seems unlikely that the IMF would ever be fully populated.

17. Results on LCID data I V - I Blue stragglers

18. Results on LCID data Chemical enrichment law

19. Results on LCID data Variable stars Bernard et al (2008)

20. Results on LCID data Variable stars Bernard et al (2008)

21. Baade (1944) Nature of the extended structures remains unsolved Pop I : Bluish Pop II : Reddish Gradients on stellar populations

22. What is the nature of these structures? No kinematic data. Are stellar halos like in Spiral galaxies? Are composed only by old low metallicity stars like the MW? Gradients on stellar populations

23. PHOENIX: Spatial distribution of 1 2 3 4 5 6 Hidalgo et al (2008) Gradients on stellar populations

24. PHOENIX: Age at 5% and 95% of the age distribution Age of 5% oldest Age of 5% youngest Hidalgo et al (2008) Gradients on stellar populations

25. PHOENIX: Scale length as a function of time Hidalgo et al (2008) Gradients on stellar populations

26. VI. CONCLUSIONS We have developed new, flexible software to derive quantitative SFHs of resolved stellar systems. Solutions by the LCID project have been tested against the photometry method, errors in the estimation of distance modulus and reddening, number of binaries, stellar evolution models, and different IMFs. Leo A clearly shows a delay in start of the main event of the SF, and we find evidence in IC1613 for a strong peak which extends for of 4 ~ 5 Gyr. All the galaxies in our sample except Leo A show significant star formation older than 10 Gyr. The early episodes of star formation start at different times and are extended in time. Future work: - Address the open questions on the extended structures of these galaxies. -Study the variable star distributions.