Euclid Quick Data Release (Q1): XXII. An investigation of optically faint, red objects in the Euclid Deep Fields

Our understanding of cosmic star formation at z > 3 used to largely rely on rest-frame UV observations. However, these observations overlook dusty and massive sources, resulting in an incomplete census of early star-forming galaxies. Recent infrared data from Spitzer and the James Webb Space Telescope (JWST) have revealed a hidden population at z ∼ 3 − 6 with extreme red colours. Taking advantage of the overlap between imaging of the Euclid Deep Fields (EDFs), covering about 60 deg², and ancillary Spitzer observations, we identified 27 000 extremely red objects with H_E − IRAC2 > 2.25 (dubbed HIEROs) down to a 10σ completeness magnitude limit of IRAC2 = 22.5 AB. After a visual investigation to discard artefacts and any objects with troubling photometry, we were left with a final sample of 3900 candidates. We retrieved the physical parameter estimates for these objects from the spectral energy distribution-fitting tool CIGALE. Our results confirm that HIERO galaxies can populate the high-mass end of the stellar mass function at z > 3, with some sources reaching extreme stellar masses (M_* > 10¹¹ M_⊙) and exhibiting high dust attenuation values (A_V > 3). However, we consider the stellar mass estimates unreliable for sources at z > 3.5. For this reason, we favour a more conservative lower-z solution. The challenges faced by spectral energy distribution-fitting tools in accurately characterising these objects underscore the need for further studies that incorporate both observations at shorter wavelengths and spectroscopic data. Euclid spectra will help resolve degeneracies and better constrain the physical properties of the brightest galaxies. Given the extreme nature of this population, characterising these sources is crucial for building a comprehensive picture of galaxy evolution and stellar mass assembly across most of the history of the Universe. This work demonstrates Euclid’s potential to provide statistical samples of rare objects, such as massive, dust-obscured galaxies at z > 3, which will be prime targets for JWST and the Atacama Large Millimeter/submillimeter Array (ALMA).