The blue plume population in dwarf spheroidal galaxies

Aims.Blue stragglers (BSS) in the Milky Way field and globular/open clusters are thought to be the product of either primordial or collisional binary systems. In the context of dwarf spheroidal galaxies it is hard to firmly disentangle a genuine BSS population from young main sequence (MS) stars tracing a ∼$ 1{-}2$ Gyr old star forming episode. Methods. Assuming that their blue plume populations are made of BSS, we estimate the BSS frequency ($F^{\rm BSS}_{\rm HB}$; as normalized to the horizontal branch star counts) for 8 Local Group non star-forming dwarf galaxies, using a compilation of ground and space based photometry. Results. (i) The BSS frequency in dwarf galaxies, at any given ${ M_{V}}$, is always higher than that in globular clusters of similar luminosities; (ii) the BSS frequency for the lowest luminosity dwarf galaxies is in excellent agreement with that observed in the Milky Way halo and open clusters; and most interestingly (iii) derive a statistically significant $F^{\rm BSS}_{\rm HB}-{ M_{V}}$ anti-correlation for dwarf galaxies, similar to that observed in globular clusters. Conclusions.The low density, almost collision-less, environments of our dwarf galaxy sample allow us to infer (i) their very low dynamical evolution; (ii) a negligible production of collisional BSS; and consequently (iii) that their blue plumes are mainly made of primordial binaries. The dwarf galaxies $F^{\rm BSS}_{\rm HB}-{ M_{V}}$ anti-correlation can be used as a discriminator: galaxies obeying the anti-correlation are more likely to possess genuine primordial BSS rather than young main sequence stars