Cosmic Multipoles in Galaxy Surveys Part I: How Inferences Depend on Source Counts and Masks

TL;DR

This paper introduces a Bayesian method for accurately estimating cosmic multipoles, including dipoles, in galaxy surveys, even with masked skies or small survey footprints, aiding in understanding cosmic dipoles.

Contribution

A novel Bayesian approach for constructing and fitting multipoles in galaxy surveys that is robust to masks and effective with small survey areas.

Findings

Method effectively recovers dipoles in small survey footprints.

Technique remains robust despite sky masks.

Facilitates analysis of cosmic dipole anomalies.

Abstract

We present a new approach to constructing and fitting dipoles and higher-order multipoles in synthetic galaxy samples over the sky. Within our Bayesian paradigm, we illustrate that this technique is robust to masked skies, allowing us to make credible inferences about the relative contributions of each multipole. We also show that dipoles can be recovered in surveys with small footprints, determining the requisite source counts required for concrete estimation of the dipole parameters. This work is motivated by recent probes of the cosmic dipole in galaxy catalogues. Namely, the kinematic dipole of the Cosmic Microwave Background, as arising from the motion of our heliocentric frame at $\approx 370\ \text{km}\,\text{s}^{-1}$, implies that an analogous dipole should be observed in the number counts of galaxies in flux-density-limited samples. Recent studies have reported a dipole aligning with the kinematic dipole but with an anomalously large amplitude. Accordingly, our new technique will be important as forthcoming galaxy surveys are made available and for revisiting previous data.