A-type stars in the Canada-France Imaging Survey I. The stellar halo of the Milky Way traced to large radius by blue horizontal branch stars

TL;DR

This paper maps the Milky Way's outer stellar halo using blue horizontal branch stars identified through deep imaging, revealing a broken power-law density profile with a steeper inner slope and a shallower outer slope, challenging previous RR Lyrae-based measurements.

Contribution

It introduces a new photometric method to select BHB stars with low contamination, enabling detailed modeling of the stellar halo's density profile at large radii.

Findings

Outer halo well modeled by a broken power law

Inner slope of the halo is approximately 4.24

Outer slope of the halo is approximately 3.21

Abstract

We present the stellar density profile of the outer halo of the Galaxy traced over a range of Galactocentric radii from $15< R_{GC} < 220$ kpc by blue horizontal branch (BHB) stars. These stars are identified photometrically using deep $u-$band imaging from the new Canada-France-Imaging-Survey (CFIS) that reaches 24.5 mag. This is combined with $griz$ bands from Pan-STARRS 1 and covers a total of $\sim4000$ deg$^2$ of the northern sky. We present a new method to select BHB stars that has low contamination from blue stragglers and high completeness. We use this sample to measure and parameterize the three dimensional density profile of the outer stellar halo. We fit the profile using (i) a simple power-law with a constant flattening (ii) a flattening that varies as a function of Galactocentric radius (iii) a broken power law profile. We find that outer stellar halo traced by the BHB is well modelled by a broken power law with a constant flattening of $q=0.86 \pm 0.02$, with an inner slope of $\gamma=4.24 \pm 0.08$. This is much steeper than the preferred outer profile that has a slope of $\beta=3.21\pm 0.07$ after a break radius of $r_b=41.4^{+2.5}_{-2.4}$ kpc. The outer profile of the stellar halo trace by BHB stars is shallower than that recently measured using RR Lyrae, a surprising result given the broad similarity of the ages of these stellar populations.