Connecting the local stellar halo and its dark matter density to dwarf galaxies via blue stragglers

Abstract

The Gaia HR diagram shows the presence of apparently young stars at high tangential velocities. Using a simple analytical model I show that these stars are likely to be blue stragglers. Once normalized to red giant stars, the fraction of nearby halo blue stragglers is of order 20 percent, and remarkably close to that measured in dwarf galaxies. Motivated by this similarity, I apply to field blue stragglers scaling relations inferred from blue stragglers in dwarf galaxies. Doing this for the Milky Way halo returns an average stellar density of 3.4 x 10^-5 Msun/pc^3 and a dark matter density of ~0.006 Msun/pc^3 ~ 0.22 GeV/cm^3 within 2 kpc from the Sun. These values compare favourably to other determinations available in the literature, but are based on an independent set of assumptions. A few considerations of this methodology are discussed, most notably that the correlation between the dark matter halo core-density and stellar mass seen in dwarf galaxies seems to hold also for the nearby Milky Way halo.

ApJ accepted. Video of Figure 1 at https://www.dropbox.com/s/3z183bjti0p7vi4/video.mp4?dl=0

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