Euclid Quick Data Release (Q1) Extending the quest for little red dots to z 4

Context. Recent James Webb Space Telescope (JWST) observations have revealed an interesting population of sources with a compact morphology and a characteristic ‘v-shaped’ continuum, i.e., blue at λ 4000 Å and red at longer wavelengths. The nature of these sources, called ‘little red dots’ (LRDs), is still highly debated, as it is unclear if they host an active galactic nuclei (AGN) and their number seems to drastically drop at z 4. Aims. In this work we take advantage of the large area covered by the quick Euclid data release (Q1) to extend the search for LRDs to brighter magnitudes and to lower redshift than what has been possible with JWST. This is fundamental to have a broader view of the evolution of this peculiar galaxy population. Methods. The selection is performed by fitting the available photometric data with two power-laws, to retrieve both the rest-frame optical and UV slope consistently over a large redshift range. We then exclude extended objects, possible line-emitters, and perform a careful visual inspection to remove any remaining artifact. The combination of Euclid, IRAC, and ground-based griz data, allows us to select 16 700 LRD candidates from z = 0.12 to z = 6. Results. The resulting rest-frame UV luminosity function, in contrast with previous JWST studies, shows that the number density of LRD candidates increases from high-redshift down to z = 1.5–2.5 and decreases at even lower redshifts. The comparison with previous QSO UV luminosity functions showed that LRDs are not the dominant AGN population at z 4 and MUV −21. Follow-ups of these LRD candidates are pivotal to probe their structure and check for their compatibility with JWST sources, given that the different spatial resolution of Euclid and JWST could select different sample of compact sources. Key words. Galaxies: active; Galaxies: luminosity function; Galaxies: evolution