Correcting correlation functions for redshift-dependent interloper contamination

Daniel J Farrow,Ariel G Sánchez,Karl Gebhardt,Shun Saito,Jan Snigula,Robin Ciardullo,Henry S Grasshorn Gebhardt,Donghui Jeong,Eiichiro Komatsu,Eric Gawiser,Dustin Davis,Maximilian Fabricius,Isak G B Wold,Martin Landriau,Erin Mentuch Cooper,Gary J Hill,Chenxu Liu

doi:10.1093/mnras/stab1986

Abstract

ABSTRACT The construction of catalogues of a particular type of galaxy can be complicated by interlopers contaminating the sample. In spectroscopic galaxy surveys this can be due to the misclassification of an emission line; for example in the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) low-redshift [O ii] emitters may make up a few per cent of the observed Ly α emitter (LAE) sample. The presence of contaminants affects the measured correlation functions and power spectra. Previous attempts to deal with this using the cross-correlation function have assumed sources at a fixed redshift, or not modelled evolution within the adopted redshift bins. However, in spectroscopic surveys like HETDEX, where the contamination fraction is likely to be redshift dependent, the observed clustering of misclassified sources will appear to evolve strongly due to projection effects, even if their true clustering does not. We present a practical method for accounting for the presence of contaminants with redshift-dependent contamination fractions and projected clustering. We show using mock catalogues that our method, unlike existing approaches, yields unbiased clustering measurements from the upcoming HETDEX survey in scenarios with redshift-dependent contamination fractions within the redshift bins used. We show our method returns autocorrelation functions with systematic biases much smaller than the statistical noise for samples with at least as high as 7 per cent contamination. We also present and test a method for fitting for the redshift-dependent interloper fraction using the LAE–[O ii] galaxy cross-correlation function, which gives less biased results than assuming a single interloper fraction for the whole sample.

Highlights

The measurement of a redshift from a galaxy spectrum is one of the most fundamental parts of a spectroscopic survey
Each measurement has been divided by the statistical error expected for the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) survey, i.e. the square root of the diagonal of the covariance matrix derived from the mocks
These catalogues were used to explore the impact of the redshift dependence of the contamination fraction and the correlation function of the contaminants on the observed correlation functions

Summary

Introduction

The measurement of a redshift from a galaxy spectrum is one of the most fundamental parts of a spectroscopic survey. This is usually achieved by relying on features in the spectra such as emission and absorption lines and the shape of the continuum. The impact of interlopers on the correlation function and power spectrum of a galaxy sample has been studied in the literature It has been seen that the presence of interlopers in a sample changes the galaxies’ correlation function and power spectrum. If the interlopers are clustered, a signal from their correlation function is added into the sample. It has been shown that these spurious clustering signals can cause biases in the inferred cosmological parameters (e.g. Pullen et al 2016; Addison et al 2019; Grasshorn Gebhardt et al 2019)

Objectives

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Conclusion