Origin of the inferred ∼1700 km/s motion: Solar-system vs. Milky Way bulk motion

Determine whether the ∼1700 km s−1 peculiar velocity inferred from the dipole anisotropy in the redshift distribution of 1.3 million quasars in the Quaia Gaia–unWISE catalog arises from Solar-system motion within the Milky Way toward the Galactic Centre or from bulk motion of the Milky Way galaxy (possibly as part of a larger structure) in a direction that coincides with the Galactic Centre as seen from Earth.

Background

The paper analyzes dipole anisotropy in the redshift distribution of 1.3 million quasars from the Quaia (Gaia–unWISE) catalog and infers a Solar peculiar velocity with magnitude roughly 4–5 times the CMB-based value and a direction close to the Galactic Centre. This result contrasts with the CMB dipole direction and with several number-count dipoles that generally align with the CMB direction.

Given this discrepancy, the authors raise the unresolved issue of whether the large inferred velocity reflects motion of the Solar system within the Milky Way toward its centre, or instead bulk motion of the Milky Way (possibly as part of a larger structure) whose apparent direction coincides with the Galactic Centre from Earth. The current data and method cannot distinguish between these possibilities.