Post on 17-Jan-2016
description
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.