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 projectLocal Cosmology from Isolated Dwarfs

1) P.I. C. Gallart ACS - Cycle 14: 95 orbitsCo-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 orbitsCo-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

TUCCET

IC1613LGS3

32

25

22wfpc2

12

16

24

# Orbit

24.77

24.39

23.30

23.98

24.50

24.41

(m-M)0

dSph

dSph

dIrr/dSph

dIrr/dSph

dIrr

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

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

Our new HST targets are the nearest isolated dwarf galaxies.

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.

IC 1613 - 24 Orbits ACS - note depth

LGS-3 and Leo A Comparison

Results on LCID data

I

V - I

Results on LCID data

• Solved CMD for IC1613

solveddata

Results on LCID data

Results for 2 age binnings, with 1 random errors on SFR

Leo A --- From Cole et al. (2007)

SFHs for the LCID sample (courtesy Sebastian Hidalgo)

SFHs for the LCID sample (courtesy Sebastian Hidalgo)

Results on LCID data

• Cumulative mass fraction

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.

Results on LCID data

IV - I

• Blue stragglers

Results on LCID data

• Chemical enrichment law

Results on LCID data

• Variable stars

Bernard et al (2008)

Results on LCID data

• Variable stars

Bernard et al (2008)

PHOENIX: Spatial distribution of ( , )t z

1

2

3

4

5

6

Hidalgo et al (2008)

Gradients on stellar populations

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

PHOENIX: Scale length as a function of time

Hidalgo et al (2008)

Gradients on stellar populations

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.