The Pristine Survey. XXX. Mapping substructures in the inner and outer halo with blue horizontal branch stars
Abstract
Blue horizontal branch (BHB) stars serve as good standard candles and kinematic tracers of the Milky Way halo. The halo displays various overdensities and kinematical substructures of different shapes and sizes. Our objective is to investigate the substructure in three dimensions and the bulk motion of the halo up to distances well exceeding the limit of the Gaia parallaxes. We selected BHB stars in the (u−CaHK) 0-(g−r) 0 colour-colour diagram with a hierarchical Bayesian mixture model of three populations: BHB stars, blue straggler stars, and outliers. Using CaHK photometry derived from Gaia XP spectra and the Pristine survey, we provide both a shallow and a deep catalogue of BHB stars of 7,057 and 8,824 stars, respectively, where the deep catalogue is restricted to the deeper Pristine footprint compared to the shallower with Gaia XP. Using the shallow catalogue, we trace to larger distances 6D substructures identified earlier within a 2.5 kpc from the Sun, including accepted populations; namely, Gaia-Enceladus, Thamnos, Helmi streams, Sequoia, and other smaller halo substructures such as ED-3, ED-4, and ED-6. The deep catalogue provides reliable distance measurements for a more detailed 3D mapping of the Sagittarius stream and of the Pisces Overdensity. We add new candidate BHB stars with line-of-sight velocities to existing catalogues and confirm previously reported differences between the northern and southern Galactic hemispheres, likely due to the infall of the Large Magellanic Cloud. This research demonstrates that probabilistic photometric classification can effectively identify distant candidate BHB stars, and can provide catalogues of these stars that enhance our understanding of the 3D and 6D structures in the halo of the Milky Way.
Related
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- The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) XII. Constraints on large bodies embedded in discs
- The Pristine survey XXIX. Data Release 2: Photometric metallicities for 15 millions stars down to G∼ 20.5
- The Pristine survey XXXI. A stream offset from the Stream: very metal-poor stellar debris trailing 100∘ from the LMC-SMC system
- Multi-branch classification of diffuse cluster radio emission from the LOFAR two-metre sky survey
BibTeX
@misc{bayer2026pristine,
title = {{The Pristine Survey. XXX. Mapping substructures in the inner and outer halo with blue horizontal branch stars}},
author = {Manuel Bayer and Else Starkenburg and Annemarijn Zwerver and Fraser McKibben and Maya Bohak and Nicolas F. Martin and Emma Dodd and Hanneke C. Woudenberg and Guillaume F. Thomas and Amina Helmi and Julio F. Navarro},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.41731},
eprint = {2610.41731},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.41731},
}
RIS
TY - UNPB
TI - The Pristine Survey. XXX. Mapping substructures in the inner and outer halo with blue horizontal branch stars
AU - Manuel Bayer
AU - Else Starkenburg
AU - Annemarijn Zwerver
AU - Fraser McKibben
AU - Maya Bohak
AU - Nicolas F. Martin
AU - Emma Dodd
AU - Hanneke C. Woudenberg
AU - Guillaume F. Thomas
AU - Amina Helmi
AU - Julio F. Navarro
PY - 2026
DA - 2026/10/30
PB - arXiv
DO - 10.48550/arxiv.2610.41731
UR - https://arxiv.org/abs/2610.41731
AB - Blue horizontal branch (BHB) stars serve as good standard candles and kinematic tracers of the Milky Way halo. The halo displays various overdensities and kinematical substructures of different shapes and sizes. Our objective is to investigate the substructure in three dimensions and the bulk motion of the halo up to distances well exceeding the limit of the Gaia parallaxes. We selected BHB stars in the (u−CaHK) _0-(g−r) _0 colour-colour diagram with a hierarchical Bayesian mixture model of three populations: BHB stars, blue straggler stars, and outliers. Using CaHK photometry derived from Gaia XP spectra and the Pristine survey, we provide both a shallow and a deep catalogue of BHB stars of 7,057 and 8,824 stars, respectively, where the deep catalogue is restricted to the deeper Pristine footprint compared to the shallower with Gaia XP. Using the shallow catalogue, we trace to larger distances 6D substructures identified earlier within a 2.5 kpc from the Sun, including accepted populations; namely, Gaia-Enceladus, Thamnos, Helmi streams, Sequoia, and other smaller halo substructures such as ED-3, ED-4, and ED-6. The deep catalogue provides reliable distance measurements for a more detailed 3D mapping of the Sagittarius stream and of the Pisces Overdensity. We add new candidate BHB stars with line-of-sight velocities to existing catalogues and confirm previously reported differences between the northern and southern Galactic hemispheres, likely due to the infall of the Large Magellanic Cloud. This research demonstrates that probabilistic photometric classification can effectively identify distant candidate BHB stars, and can provide catalogues of these stars that enhance our understanding of the 3D and 6D structures in the halo of the Milky Way.
ER -