Abstract
We present updated chemical evolution models of two dwarf spheroidal galaxies (Sculptor and Carina) and the first detailed chemical evolution models of two ultra-faint dwarfs (Hercules and Boötes I). Our results suggest that the dwarf spheroidals evolve with a low efficiency of star formation, confirming previous results, and the ultra-faint dwarfs with an even lower one. Under these assumptions, we can reproduce the stellar metallicity distribution function, the [α/Fe] versus [Fe/H] abundance patterns and the total stellar and gas masses observed at the present time in these objects. In particular, for the ultra-faint dwarfs we assume a strong initial burst of star formation, with the mass of the system being already in place at early times. On the other hand, for the classical dwarf spheroidals the agreement with the data is found by assuming the star formation histories suggested by the colour–magnitude diagrams and a longer time-scale of formation via gas infall. We find that all these galaxies should experience galactic winds, starting in all cases before 1 Gyr from the beginning of their evolution. From comparison with Galaxy data, we conclude that it is unlikely that the ultra-faint dwarfs have been the building blocks of the whole Galactic halo, although more data are necessary before drawing firm conclusions.
Highlights
Orbiting around the Milky Way, there are a large number of satellite galaxies, most of which have so low average surface brightnesses and small effective radii that their detection was very difficult in the past: from 1937 up to 1994, only nine of them were discovered and that number remained unchanged until 2005
Colour–magnitude diagram (CMD) fitting analysis revealed star formation rate (SFR) in dwarf spheroidal galaxies (dSphs) to have been either continuous for a long time or occurring in bursts
We changed the values of ν and ω, in order to find a model able to reproduce the observed properties of the galaxy, such as the abundance ratios, the metallicity distribution function (MDF) and the present time gas and stellar masses
Summary
Orbiting around the Milky Way, there are a large number of satellite galaxies, most of which have so low average surface brightnesses and small effective radii that their detection was very difficult in the past: from 1937 up to 1994, only nine of them were discovered and that number remained unchanged until 2005. Very few dSphs have been observed to host younger stars, which populate the so-called ‘blue plume’ in the CMD, sign of a relatively recent star formation activity, which occurred up to ∼2–3 Gyr ago (Monelli et al 2003) All such features led cosmologists to hypothesize dSphs to be the evolved small progenitor systems which merged in the past to form the actual large structures in the Universe as the stellar halo of the Milky Way, in the framework of the cold dark matter standard cosmological model (Helmi & White 1999; Bullock, Kravtsov & Weinberg 2001; Harding et al 2001; Bullock & Johnston 2005).
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