What does NB515 imaging reveal about the Andromeda stellar halo out to 120 kpc?

Subaru/HSC narrowband photometry separates M31 halo giants from foreground stars and maps stellar debris.

The structure of the stellar halo of the Andromeda galaxy explored with the NB515 for Subaru/HSC. I.: New Insights on the stellar halo up to 120 kpc

The structure of the stellar halo of the Andromeda galaxy explored with the NB515 for Subaru/HSC. I.: New Insights on the stellar halo up to 120 kpc. Ogami, Itsuki, et al., Monthly Notices of the Royal Astronomical Society, 536, 530 (2025).

arXiv:2401.00668

Approach

Gravity-sensitive NB515 imaging is combined with CFHT broad-band data to select halo stars, estimate photometric distances and metallicities, and measure surface-brightness profiles.

Main result

The selection reaches about 90 per cent foreground separation accuracy. Three new substructures are identified, and the halo shows spatially uneven metallicities despite a relatively smooth overall light profile.

Scientific implications

Distance gradients and metallicity patterns provide constraints on the accretion events that built the M31 halo.

Scope and limitations

Photometric metallicities and distance estimates depend on stellar-population assumptions. Associations inferred from these quantities need kinematic follow-up.