Kinematically induced dipole anisotropy in line-emitting galaxy number counts and line intensity maps

TL;DR

This paper demonstrates that spectral-line intensity maps exhibit a dipole anisotropy caused by our solar system's motion, which can be used to measure cosmic velocities and test cosmological models.

Contribution

It introduces the concept of dipole anisotropy in line-emitting galaxy counts and intensity maps, proposing methods to measure and utilize it for cosmological insights.

Findings

Dipole anisotropy exists in spectral-line intensity maps.

Multi-frequency measurements can accurately determine solar velocity.

Upcoming surveys like SPHEREx are promising for detecting this anisotropy.

Abstract

The motion of the solar system against an isotropic radiation background, such as the cosmic microwave background, induces a dipole anisotropy in the background due to the Doppler effect. Flux-limited observation of the continuum radiation from galaxies also has been studied extensively to show a dipole anisotropy due to the Doppler effect and the aberration effect. We show that a similar dipole anisotropy exists in spectral-line intensity maps, represented as either galaxy number counts or the diffuse intensity maps. The amplitude of these dipole anisotropies is determined by not only the solar velocity against the large-scale structures but also the temporal evolution of the monopole (sky-average) component. Measuring the dipole at multiple frequencies, which have mutually independent origins due to their occurrence from multiple redshifts, can provide a very accurate measure of the solar velocity thanks to the redundant information. We find that such a measurement can even constrain astrophysical parameters in the nearby universe. We explore the potential for dipole measurement of existing and upcoming surveys, and conclude that the spectral number count of galaxies through SPHEREx will be optimal for the first measurement of the dipole anisotropy in the spectral-line galaxy distribution. LIM surveys with reasonable accuracy are also found to be promising. We also discuss whether these experiments might reveal a peculiar nature of our local universe, that seems to call for a non-standard cosmology other than the simple LambdaCDM model as suggested by recent measures of the baryon acoustic oscillation signatures and the Alcock-Paczynski tests.