Published in The Astronomy and Astrophysics Review, Volume 18, Issue 1-2, pp. 1-65, 2010.
astro-ph/0909.4234

For a PDF version of the article, click here.

STAR FORMATION HISTORIES OF DWARF GALAXIES FROM THE COLOUR-MAGNITUDE DIAGRAMS OF THEIR RESOLVED STELLAR POPULATIONS

Michele Cignoni 1,2 and Monica Tosi 2


1 Astronomy Department, Bologna University, Via Ranzani 1, I-40127 Bologna, Italy
2 INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna, Italy


Abstract. In this tutorial paper we summarize how the star formation (SF) history of a galactic region can be derived from the colour-magnitude diagram (CMD) of its resolved stars. The procedures to build synthetic CMDs and to exploit them to derive the SF histories (SFHs) are described, as well as the corresponding uncertainties. The SFHs of resolved dwarf galaxies of all morphological types, obtained from the application of the synthetic CMD method, are reviewed and discussed. In short: 1) Only early-type galaxies show evidence of long interruptions in the SF activity; late-type dwarfs present rather continuous, or gasping, SF regimes; 2) A few early-type dwarfs have experienced only one episode of SF activity concentrated at the earliest epochs, whilst many others show extended or recurrent SF activity; 3) No galaxy experiencing now its first SF episode has been found yet; 4) No frequent evidence of strong SF bursts is found; 5) There is no significant difference in the SFH of dwarf irregulars and blue compact dwarfs, except for the current SF rates. Implications of these results on the galaxy formation scenarios are briefly discussed.


Keywords: Galaxies: dwarfs - Galaxies: evolution - Galaxies: stellar content


Table of Contents

INTRODUCTION

STAR FORMATION HISTORIES FROM COLOUR-MAGNITUDE DIAGRAMS: THE METHOD
Building a synthetic population
Stellar ages from a CMD
Deep is better

DEDUCING THE SFH: GUIDELINES
A changing landscape
To grid or not to grid
Maximum Likelihood
Wondering in the parameter space
A practical example
Uncertainties affecting the synthetic CMD procedures
Completeness
IMF
Binaries
Metallicity and metallicity spread
Helium content
Reddening and distance
Additional issues

STAR FORMATION HISTORIES OF RESOLVED DWARFS FROM SYNTHETIC CMD ANALYSES: RESULTS

CONCLUDING REMARKS

REFERENCES

Next