Cross-Correlation Studies for Detecting Interlopers in the Euclid Survey

To probe the nature of dark energy, Euclid is carrying out imaging and spectroscopy over 14,000 square degrees of extragalactic sky for galaxy clustering and weak lensing measurements. Euclid’s near infrared spectrometer and photometer (NISP) is a slitless grism spectrograph that records spectra of all sources in the field of view. The principal targets of the spectroscopic campaign are α emission line galaxies from redshift 0.9 to 1.8. However, slitless spectroscopy poses a challenge due to the superposition of spectra on the detector, which can lead to source confusion and spurious detections. In addition, only a single strong line (Hα) is in the wavelength range of the spectrograph from redshift 0.9 to 1.5. Therefore, if a different line is identified as α, a wrong redshift will be assigned. This leads to galaxy interlopers in the catalog, which originate from different redshifts. Our method aims to infer the fraction of galaxy interlopers in redshift bins using angular cross-correlation statistics between the Euclid catalog and a reference galaxy catalog that has secure redshift determinations. We present preliminary results on low redshift interlopers based on the Euclid Flagship mock galaxy catalog. We find that using a DESI-like catalog covering 5,000 square degrees, we can infer the interloper fraction originating from galaxies at redshift 0 to 0.5 with a typical error of 1%, with a systematic uncertainty related to the galaxy bias. Thus, if spectroscopic surveys are available that overlap on the sky, the angular cross-correlations are a promising tool to assess the interloper fractions.