Monopole Fluctuations in Galaxy Surveys

TL;DR

This paper calculates monopole fluctuations in galaxy surveys, revealing their impact on clustering measurements and emphasizing the need for modeling these fluctuations to improve cosmological inferences.

Contribution

It introduces the first computation of monopole fluctuations in galaxy surveys and analyzes their effect on galaxy clustering measurements.

Findings

Monopole fluctuations can be as large as 7% at z=0.5 in idealized surveys.

Fluctuations decrease to below 1% at redshifts above 2.

Modeling monopole fluctuations is essential for accurate galaxy clustering analysis.

Abstract

Galaxy clustering provides a powerful way to probe cosmology. This requires understanding of the background mean density of galaxy samples, which is estimated from the survey itself by averaging the observed galaxy number density over the angular position. The angle average includes not only the background mean density, but also the monopole fluctuation at each redshift. Here for the first time we compute the monopole fluctuations in galaxy surveys and investigate their impact on galaxy clustering. The monopole fluctuations vary as a function of redshift, and it is correlated with other fluctuations, affecting the two-point correlation function measurements. In an idealized all-sky survey, the rms fluctuation at $z=0.5$ can be as large as 7% of the two-point correlation function in amplitude at the BAO scale, and it becomes smaller than 1% at $z>2$. The monopole fluctuations are unavoidable, but they can be modeled. We discuss its relation to the integral constraint and the implications for the galaxy clustering analysis.