On the Use of the Number Count of Blue Horizontal--Branch Stars to Infer the Dominant Building Blocks of the Milky Way Halo

TL;DR

This paper challenges previous claims by showing that the ratio of blue horizontal-branch to blue straggler stars is biased by globular cluster contributions, questioning its use as an indicator of the Milky Way halo's building blocks.

Contribution

The study demonstrates that the BS-to-BHB ratio is not a reliable indicator for the origin of the Milky Way halo due to biases from globular cluster star populations.

Findings

High BS-to-BHB ratios in outer halo are likely from globular clusters.

Dynamical models show preferential removal of first-generation stars from GCs.

Globular clusters can significantly contribute to the outer halo stars.

Abstract

The formation of the Milky Way stellar halo is thought to be the result of merging and accretion of building blocks such as dwarf galaxies and massive globular clusters. Recently, Deason et al. (2015) suggested that the Milky Way outer halo formed mostly from big building blocks, such as dwarf spheroidal galaxies, based on the similar number ratio of blue straggler (BS) stars to blue horizontal-branch (BHB) stars. Here we demonstrate, however, that this result is seriously biased by not taking into detailed consideration on the formation mechanism of BHB stars from helium enhanced second-generation population. In particular, the high BS-to-BHB ratio observed in the outer halo fields is most likely due to a small number of BHB stars provided by GCs rather than to a large number of BS stars. This is supported by our dynamical evolution model of GCs which shows preferential removal of first generation stars in GCs. Moreover, there are a sufficient number of outer halo GCs which show very high BS-to-BHB ratio. Therefore, the BS-to-BHB number ratio is not a good indicator to use in arguing that more massive dwarf galaxies are the main building blocks of the Milky Way outer halo. Several lines of evidence still suggest that GCs can contribute a significant fraction of the outer halo stars.