The blue plume population in dwarf spheroidal galaxies: genuine blue stragglers or young stellar population?

TL;DR

This study investigates whether blue plume stars in dwarf spheroidal galaxies are genuine blue stragglers or young stars, finding that most are likely primordial binaries with a significant anti-correlation between BSS frequency and galaxy luminosity.

Contribution

It provides the first systematic analysis of BSS frequency in non star-forming dwarf galaxies, distinguishing genuine BSS from young stars based on their anti-correlation with galaxy luminosity.

Findings

BSS frequency is higher in dwarf galaxies than in globular clusters of similar luminosity.

The BSS frequency in low-luminosity dwarfs matches that of the Milky Way halo.

An anti-correlation exists between BSS frequency and galaxy luminosity, indicating primordial origin.

Abstract

Abridged... Blue stragglers (BSS) 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. Assuming that their blue plume populations are made of BSS, we estimate the BSS frequency for 8 Local Group non star-forming dwarf galaxies, using a compilation of ground and space based photometry. Our results can be summarized as follows: (i) The BSS frequency in dwarf galaxies, at any given Mv, 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 most interestingly (iii) derive a statistically significant anti-correlation between the BSS frequency and the galaxy Mv. 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 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.