Euclid: The Early Release Observations -- NISP-only sources and the search for luminous $z=6-8$ galaxies

This paper presents a search for new z ≳ 6 galaxies from the Euclid Early Release Observation program “Magnifying Lens”. The 1 deg2 area covered by the twin Abell lensing cluster fields is comparable in size to the few other deep near-infrared surveys such as COSMOS, and so provides a new opportunity for significantly increasing known samples of rare UV-bright galaxies (MUV < −21 AB). Beyond their still uncertain role in reionization, these UV-bright galaxies are ideal laboratories from which to study galaxy formation and constrain the bright-end of the UV luminosity function. Of the 501 994 sources detected from a combined YE,JE,HE detection image, 148 do not have any appreciable VIS/IE flux. They span a range in spectral colours, separated into two classes: 119 extremely red sources and 29 Lyman-break galaxy candidates. Best-fit redshfits and spectral templates from eazy-py suggest the former is composed of both z ≳ 5 dusty star-forming galaxies and z ≈ 1–3 quiescent systems and the latter a more homogenous collection of Lyman break galaxies at z ≈ 6–8. In both cases, contamination by L- and T-type dwarfs cannot be ruled out. Additional contamination from persistence is investigated by a novel time series analysis. This work lays the foundation for future searches within the Euclid Deep Fields, where thousands more z ≳ 6 Lyman break systems will be identified. Spectroscopic follow-up will then reveal the astrophysical processes driving these nascent star-forming galaxies and thereby refine models of galaxy formation.