Measurement of the dipole in the cross-correlation function of galaxies

TL;DR

This paper presents the first measurement of the galaxy cross-correlation dipole, validating the method and paving the way for detecting relativistic effects in future large-scale structure surveys.

Contribution

It introduces a method to measure the galaxy cross-correlation dipole and demonstrates its effectiveness using BOSS survey data, confirming theoretical predictions.

Findings

Relativistic distortions, evolution, and wide-angle effects are too small to detect in current data.

The large-angle effect can be measured with high significance using specific angle combinations.

The measured large-angle dipole agrees with theoretical models, validating the measurement approach.

Abstract

It is usually assumed that in the linear regime the two-point correlation function of galaxies contains only a monopole, quadrupole and hexadecapole. Looking at cross-correlations between different populations of galaxies, this turns out not to be the case. In particular, the cross-correlations between a bright and a faint population of galaxies contain also a dipole. In this paper we present the first attempt to measure this dipole. We discuss the four types of effects that contribute to the dipole: relativistic distortions, evolution effect, wide-angle effect and large-angle effect. We show that the first three contributions are intrinsic anti-symmetric contributions that do not depend on the choice of angle used to measure the dipole. On the other hand the large-angle effect appears only if the angle chosen to extract the dipole breaks the symmetry of the problem. We show that the relativistic distortions, the evolution effect and the wide-angle effect are too small to be detected in the LOWz and CMASS sample of the BOSS survey. On the other hand with a specific combination of angles we are able to measure the large-angle effect with high significance. We emphasise that this large-angle dipole does not contain new physical information, since it is just a geometrical combination of the monopole and the quadrupole. However this measurement, which is in excellent agreement with theoretical predictions, validates our method for extracting the dipole from the two-point correlation function and it opens the way to the detection of relativistic effects in future surveys like e.g. DESI.