The blue plume population in dwarf spheroidal galaxies^*

Genuine blue stragglers or young stellar population?

¹ INAF: Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: [yazan.almomany;enrico.held;marco.gullieuszik;simone.zaggia]@oapd.inaf.it
² European Southern Observatory, A. de Cordova 3107, Santiago, Chile e-mail: isaviane@eso.org
³ Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA e-mail: rizzi@ifa.hawaii.edu


Received: 22 December 2006
Accepted: 3 April 2007

Abstract

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 ∼ Gyr old star forming episode.

Methods. Assuming that their blue plume populations are made of BSS, we estimate the BSS frequency (; 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 , 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 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 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.