Euclid: Discovery of bright z~7 Lyman-break galaxies in UltraVISTA and Euclid COSMOS

We present a search for z ^2 near-infrared (NIR) UltraVISTA survey in the COSMOS field, reaching 5,σ depths in Y of 26.2. We incorporated deep Lyman-break galaxies using the 1.72 , deg Euclid optical and Euclid + NIR imaging Spitzer for a full spectral energy distribution (SED) fitting analysis. We found 289 candidate galaxies at 6.5łeq z łeq 7.5 covering -22.6 łeq Space Telescope studies. We conducted a separate selection by including complementary Euclid performance verification imaging (reaching 5,σ depths of 26.3), yielding 140 galaxies in 0.65 , łeq -20.2, faint enough to overlap with Hubble deg ^2, with 38 sources unique to this sample. We computed the rest-frame UV luminosity function (UV LF) from our samples, extending below the knee (M^*=-21.14^ +0.28 _ -0.25 ). We find that the shape of the UV LF is consistent with both a Schechter function and a double power law (DPL) at the magnitudes probed by this sample, with a DPL preferred at Muv<-22.5 when bright-end results are included. The UltraVISTA+Euclid sample provides a clean measurement of the LF due to the overlapping NIR filters identifying molecular absorption features in the SEDs of ultra-cool dwarf interlopers, and additional faint galaxies were recovered. A comparison with JWST LFs at z>7 suggests a gentle evolution in the bright-end slope, although this is limited by a lack of robust bright-end measurements at z>9. We forecast that in the Euclid Deep Fields, the removal of contaminant ultra-cool dwarfs as point sources will be possible at JE < 24.5. Finally, we present a high-equivalent-width Lyman-α emitter candidate identified by combining HSC, VISTA, and Euclid broadband photometry, highlighting the synergistic power these instruments will have in the Euclid Auxiliary Fields for identifying extreme sources in the epoch of reionisation.