Euclid: The first statistical census of dusty and massive objects in the ERO/Perseus field

Our comprehension of the history of star formation at z > 3 strongly relies on rest-frame ultraviolet observations. However, this selection systematically misses the dustiest and most massive sources, resulting in an incomplete census at earlier times. Infrared facilities such as Spitzer and the James Webb Space Telescope have shed light on a hidden population lying at z = 3 − 6 characterised by extreme red colours named HIEROs (HST-to-IRAC extremely red objects), identified by the colour criterion H_E − ch2 > 2.25. Recently, Euclid Early Release Observations (EROs) have opened the possibility to further study such objects, exploiting the comparison between Euclid and ancillary Spitzer/IRAC observations. The aim of this study was to investigate the effectiveness of this synergy in characterising the population of a small test area of 232 arcmin². We utilised catalogues in the Perseus field across the VIS and NISP bands, supplemented by data from the four Spitzer channels and several ground-based MegaCam bands (u, g, r, Hα, i, and z) already included in the ERO catalogue. We selected 121 HIEROs by applying the H_E − ch2 > 2.25 colour cut, cleaned this sample of globular clusters and brown dwarfs, and then inspected by eye the multi-band cutouts of each source, ending with 42 reliable HIEROs. Photometric redshifts and other physical properties of the final sample were estimated using the spectral-energy-distribution-fitting software Bagpipes. From the z_phot and M_* values, we computed the galaxy stellar mass function at 3.5 < z < 5.5. When we exclude all galaxies that could host an active galactic nucleus, or whose stellar masses might be overestimated, we still find that the high-mass end of the galaxy stellar mass function is similar to previous estimates, indicating that the true value could be even higher. This investigation highlights the importance of a deeper study of this still mysterious population, in particular to assess its contribution to the cosmic star-formation rate density and its agreement with current galaxy evolution and formation models. These early results demonstrate Euclid’s capabilities to push the boundaries of our understanding of obscured star formation across a wide range of epochs.