Abstract

The formation of massive galaxies in the first billion years of cosmic time is one of the most remarkable findings of Extragalactic Astronomy and challenges the predictions of galaxy formation models. Finding the right candidates is the first step towards understanding how such efficient galaxy formation could have happened at early cosmic time. Another, a priori completely different, high-z galaxy population whose formation is also very puzzling are the so-called ‘Little Red Dots (LRDs)’. Discovered almost by chance in JWST galaxy surveys, most of these objects appear to host overgrown super-massive black holes. An intriguing issue is how bright and extreme these objects could be. In the first part of this talk, I will present the results of our search of massive galaxy candidates at the end of the Epoch of Reionization using the first set of Euclid images over several tens of square degrees of the sky. I will also discuss the presence of massive galaxies in JWST surveys at similar redshifts and the coherence of all results in a common context. In the second part of my talk, I will present our results on the study of Euclid sources with double-power-law spectral energy distributions at z>4 in the COSMOS field, where the Euclid data has already a depth very similar to the final expected depth of the Euclid Deep Surveys. I will discuss the presence of scaled-up LRD candidates, identified based on their compactness, and their derived properties in comparison to those of classical JWST -selected LRDs. Finally, I will conclude by discussing whether LRDs could also eventually give rise to massive galaxy formation.