Inclusion of horizontal branch stars in the derivation of star formation histories of dwarf galaxies: The Carina dSph

Abstract

We present a detailed analysis of the Horizontal Branch of the Carina Dwarf Spheroidal Galaxy by means of synthetic modelling techniques, taking consistently into account the star formation history and metallicity evolution as determined from main sequence and red giant branch spectroscopic observations. We found that a range of integrated red giant branch mass loss values of 0.1-0.14 M, increasing with metallicity, is able to reproduce the colour extension of the old Horizontal Branch. However, leaving the mass loss as the only free parameter is not enough to match the detailed morphology of Carina Horizontal Branch. We explored the role played by the star formation history on the discrepancies between synthetic and observed Horizontal Branches. We derived a toy bursty star formation history that reproduces the horizontal branch star counts, and also matches qualitatively the red giant and the turn off regions. This star formation history is made of a subset of age and [M/H] components of the star formation history based on turn off and red giants only, and entails four separate bursts of star formation of different strenghts, centred at 2, 5, 8.6 and 11.5 Gyr, with mean [M/H] decreasing from \sim -1.7 to \sim -2.2 for increasing ages, and a Gaussian spread of 0.1 dex. The comparison between the metallicity distribution function of our star formation history and the one measured from the infrared CaT feature using a CaT-[Fe/H] calibration shows a qualitative agreement, once taken into account the range of [Ca/Fe] abundances measured in a sample of Carina stars, that biases the derived [Fe/H] distribution toward too low values. In conclusion, we have shown how the information contained within the horizontal branch of Carina (and dwarf galaxies in general) can be extracted and interpreted to refine the star formation history derived from red giants and turn off stars only. Abridged