Description
We present a search for $z\simeq7$ Lyman-break galaxies in the COSMOS field using the $1.72 \ \rm{deg}^2$ near-infrared UltraVISTA survey. We incorporate deep optical and Spitzer imaging for a full spectral energy distribution (SED) fitting analysis. We find 291 candidate galaxies at $6.5\leq z \leq 7.5$ covering $-22.6 \leq M_{\rm{UV}} \leq -20.2$, faint enough to overlap with Hubble studies. A separate selection including complementary Euclid performance verification imaging in the SED fitting yields 141 galaxies in $0.65 \ \rm{deg}^2$. In the higher resolution Euclid imaging, $\sim18$ per cent of the sample show irregular/clumpy features. We compute the rest-frame UV luminosity function (LF) from our samples, extending to below the knee ($M^*\simeq-21.1$). We find that the shape of the rest-UV LF is consistent with previous findings of a double-power law at $z\simeq7$. The UltraVISTA+Euclid sample provides a cleaner measurement of the LF due to the overlapping near-infrared filters identifying molecular absorption features in the SEDs of brown dwarf interlopers. 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 brown dwarfs as point sources will be possible at $J \lesssim 24.5$. Finally, we present a high-equivalent-width Lyman-$\alpha$ emitter candidate identified by combining HSC, VISTA, and \euclid broadband photometry, highlighting the synergistic power these instruments will have in the Euclid Auxiliary Fields for identifying extreme sources in the Epoch of Reionization.
