The Progenitors of the Milky Way Stellar Halo: Big Bricks Favoured over Little Bricks

TL;DR

This study compares the ratios of blue straggler to blue horizontal branch stars in dwarf galaxies, globular clusters, and the Milky Way halo, providing insights into the galaxy assembly process and the origins of the stellar halo.

Contribution

It offers the first comprehensive comparison of BS-to-BHB ratios across different stellar systems, supporting the idea that massive dwarfs primarily built the Milky Way's stellar halo.

Findings

Dwarf galaxies show increasing BS-to-BHB ratio with luminosity.

Globular clusters exhibit decreasing BS-to-BHB ratio with luminosity.

The Milky Way halo has a high BS-to-BHB ratio, favoring massive dwarf progenitors.

Abstract

We present a census of blue horizontal branch (BHB) and blue straggler (BS) stars belonging to dwarf galaxies and globular clusters, and compare these counts to that of the Milky Way stellar halo. We find, in agreement with earlier studies, that the ratio of BS-to-BHB stars in these satellite populations is dependent on stellar mass. Dwarf galaxies show an increasing BS-to-BHB ratio with luminosity. In contrast, globular clusters display the reverse trend, with N_BS/N_BHB (< 1) decreasing with luminosity. The faintest (L < 10^5 L_Sun) dwarfs have similar numbers of BS and BHB stars (N_BS/N_BHB ~ 1), whereas more massive dwarfs tend to be dominated by BS stars (N_BS/N_BHB ~ 2-40). We find that the BS-to-BHB ratio in the stellar halo is relatively high (N_BS/N_BHB ~ 5-6), and thus inconsistent with the low ratios found in both ultra-faint dwarfs and globular clusters. Our results favour more massive dwarfs as the dominant "building blocks" of the stellar halo, in good agreement with current predictions from LambdaCDM models.