Secular Extragalactic Parallax: Measurement Methods and Predictions for Gaia

Abstract

Secular extragalactic parallax caused by the solar system's velocity relative to the cosmic microwave background rest frame may be observable as a dipole proper motion field with amplitude 78 μas yr⁻¹ Mpc. Nearby galaxies also exhibit proper motions caused by their transverse peculiar velocities that prevent detection of secular parallax for any single galaxy, although a statistical detection may be made instead. Such a detection could constrain the local Hubble parameter. We present methods to measure secular parallax using correlated extragalactic proper motions and find a first limit on the secular parallax amplitude using proper motions of 232 nearby galaxies from Gaia Data Release 2. The recovered dipole has an insignificant upper limit of 3500 μas yr⁻¹ Mpc. This measurement will be improved by a larger sample size and reduced proper motion uncertainties in future data releases. Using the local peculiar velocity field derived from Cosmicflows-3, we simulate galaxy proper motions and predict that a significant detection (5–10σ) of the secular parallax amplitude will be possible by Gaia's end of mission. The detection does not correspond to a constraint on the Hubble parameter because it depends on nearby (<5 Mpc), bright (G < 15 mag) galaxies and the underlying peculiar motion dipole. We further investigate the implications of our simulations for the study of transverse peculiar velocities. The peculiar velocity field additionally results in low multipole-correlated proper motions on the order of 0.3 μas yr⁻¹ that may be confounded with other cosmological proper motion measurements, such as limits on the gravitational-wave background and the anisotropy of the Hubble expansion.