Mapping the anisotropic Galactic stellar halo with blue horizontal branch stars

¹ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), Martí i Franquès, 1, 08028 Barcelona, Spain
² Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Received: 12 April 2024
Accepted: 25 June 2024

Abstract

We used Legacy Survey photometric data to probe the stellar halo in multiple directions of the sky using a probabilistic methodology to identify blue horizontal branch (BHB) stars. The measured average radial density profile follows a double power law in the range 5 < r_gc/kpc < 120, with a density break at r_gc ≈ 20 kpc. This description, however, falls short, depending on the chosen line of sight, with some regions showing no signature of a break in the profile and a wide range of density slopes, such as an outer slope −5.5 ≲ α_out ≲ −4, pointing towards a highly anisotropic stellar halo. This explains, in part, the wide range of density profiles reported in the literature owing to different tracers and sky coverage. Using our detailed 3D stellar halo density map, we quantified the shape of the Pisces overdensity associated with the transient wake response of the Galaxy’s (dark) halo to the Large Magellanic Cloud (LMC). Measured in the LMC’s coordinate system, Pisces stands above the background, is 60° long and 25° wide, and is aligned with the LMC’s orbit. This would correspond to a wake width of ∼32 kpc at ∼70 kpc. We do not find a statistically significant signature of the collective response in density as previously reported in the literature measured with K giant stars, despite our larger numbers. We release the catalogue constructed in this study with 95 446 possible BHB stars and their BHB probability.