We present a search for z ~= 7 Lyman-break galaxies using the 1.72 deg^2^ near-infrared UltraVISTA survey in the COSMOS field, reaching 5 {sigma} depths in Y of 26.2. We incorporate deep Euclid optical and Euclid + Spitzer NIR imaging for a full spectral energy distribution (SED) fitting analysis. We find 289 candidate galaxies at 6.5 <= z <= 7.5 covering -22.6 <= MUV <= -20.2, faint enough to overlap with Hubble Space Telescope studies. We conduct a separate selection by including complementary Euclid performance verification imaging (reaching 5 {sigma} depths of 26.3), yielding 140 galaxies in 0.65 deg^2^, with 38 sources unique to this sample. We compute the rest-frame UV luminosity function (UV LF) from our samples, extending below the knee (M*=-21.14). We find that the shape of the UV LF is consistent with both a Schechter function and double-power law (DPL) at the magnitudes probed by this sample, with a DPL preferred at MUV 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-{alpha} emitter candidate identified by combining HSC, VISTA, and Euclid broadband photometry, highlighting the synergistic power t hese instruments will have in the Euclid Auxiliary Fields for identifying extreme sources in the Epoch of Reionisation.

Cone search capability for table J/A+A/707/A239/uviez7 (UltraVISTA+Euclid selection of LGB candidates for SED fit with HSC+VISTA+Spitzer/IRAC+Euclid (table 3))

Cone search capability for table J/A+A/707/A239/uvistaz7 (UltraVISTA-only selection LGB candidates for SED fit with HSC+VISTA+Spitzer/IRAC without Euclid (table 3))