Correcting for small-displacement interlopers in BAO analyses

TL;DR

This paper develops a model to correct for small-displacement interloper galaxies in BAO analyses, crucial for future space surveys like Roman and Euclid, ensuring accurate cosmological measurements despite line mis-identification.

Contribution

It introduces a method to account for small-displacement interlopers in BAO analyses by modeling their cross-correlation with target galaxies, improving measurement accuracy.

Findings

The model accurately measures BAO dilation parameters with known cross-correlation.

Correcting for interlopers prevents bias in BAO peak measurements.

Method is effective if the interloper-target cross-correlation is well-estimated.

Abstract

Due to the low resolution of slitless spectroscopy, future surveys including those made possible by the Roman and Euclid space telescopes will be prone to line mis-identification, leading to interloper galaxies at the wrong redshifts in the large-scale structure catalogues. The most pernicious of these have a small displacement between true and false redshift such that the interloper positions are correlated with the target galaxies. We consider how to correct for such contaminants, focusing on $\rm H\beta$ interlopers in [OIII] catalogues as will be observed by Roman, which are misplaced by $\Delta d = 97 \,h^{-1}\,{\rm Mpc}$ at redshift $z = 1$. Because this displacement is close to the BAO scale, the peak in the interloper-target galaxy cross-correlation function at the displacement scale can change the shape of the BAO peak in the auto-correlation of the contaminated catalogue, and lead to incorrect cosmological measurements if not accounted for properly. We consider how to build a model for the monopole and quadrupole moments of the contaminated correlation function, including an additional free parameter for the fraction of interlopers. The key input to this model is the cross-correlation between the population of galaxies forming the interlopers and the main target sample. It will be important to either estimate this using calibration data or to use the contaminated small-scale auto-correlation function to model it, which may be possible if a number of requirements about the galaxy populations are met. We find that this method is successful in measuring the BAO dilation parameters without significant degradation in accuracy, provided the cross-correlation function is accurately known.