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

TL;DR

This study leverages Euclid’s Quick Data Release (Q1) to extend the JWST-discovered population of little red dots (LRDs) to brighter magnitudes and lower redshifts (z<4). By fitting rest-frame UV and optical slopes with two power laws to broad multi-wavelength photometry (Euclid, IRAC, and ground-based data) and enforcing compactness and a v-shaped spectral energy distribution, the authors assemble 3341 LRD candidates spanning z≈0.33–3.6, including 29 IRAC-detected objects. The rest-frame UV luminosity function derived via a 1/$V_{\max}$ approach shows an increase in number density toward z≈1.5–2.5 and a decline at lower z, though the evolution weakens for the IRAC-detected subsample, which is statistically limited. Compared with QSO UV luminosity functions, LRDs remain sub-dominant at z<4, implying they are not the primary AGN population in this epoch. The work highlights the need for follow-up spectroscopy and higher-resolution imaging to confirm the nature of LRDs and to assess their connection to bright JWST sources, while demonstrating the valuable role of Euclid’s large area in constraining the bright end of the LRD luminosity function.

Abstract

Recent James Webb Space Telescope (JWST) observations have revealed a population of sources with a compact morphology and a characteristic `v-shaped' continuum, namely blue at rest-frame $λ<4000$A and red at longer wavelengths. The nature of these sources, called `little red dots' (LRDs), is still debated, as it is unclear if they host active galactic nuclei (AGN) and their number seems to drastically drop at z<4. We take advantage of the $63 °^2$ covered by the quick Euclid Quick Data Release (Q1) to extend the search for LRDs to brighter magnitudes and to lower redshifts than what has been possible with JWST. The selection is performed by fitting the available photometric data (Euclid, the Spitzer Infrared Array Camera (IRAC), and ground-based $griz$ data) with two power laws, to retrieve both the rest-frame optical and UV slopes consistently over a large redshift range (i.e, z<7.6). We exclude extended objects and possible line emitters, and perform a careful visual inspection to remove any imaging artefacts. The final selection includes 3341 LRD candidates at z=0.33-3.6, with 29 detected also in IRAC. The resulting rest-frame UV luminosity function, in contrast with previous JWST studies, shows that the number density of LRD candidates increases from high-z down to z=1.5-2.5 and decreases at lower z. However, less evolution is apparent focusing on the subsample of more robust LRD candidates having IRAC detections, which however has low statistics and limited by the IRAC resolution. The comparison with previous quasar (QSO) UV luminosity functions shows that LRDs are not the dominant AGN population at z<4 and $M_{\rm UV}<-21$. Follow-up studies of these LRD candidates are pivotal to confirm their nature, probe their physical properties and check for their compatibility with JWST sources, given the different spatial resolution and wavelength coverage of Euclid and JWST.

Figures (11)

• Figure 1: Redshifts in which the different , IRAC, and ground-based filters trace the optical (red bars) or UV (blue bars) continuum. The grey shaded area indicates the redshift range in which we have at least four filters to derive the slopes necessary to select LRDs. The hatched area indicates the redshift range covered without IRAC. The first two bars, separated by an horizontal black solid line, indicate the redshift range in which at least two JWST NIRCam broad-band filters trace the rest-frame optical or UV continuum.
• Figure 2: Two example LRD candidates, showing $4\arcsec \times4\arcsec$ cutouts in the four filters and the two shortest IRAC channels. From left to right: , , , , IRAC1, and IRAC2. We reported the size of the PSF in the top left and the physical scale on the bottom left of each panel.
• Figure 3: Fit with two power-laws of the photometric data of the two example LRD candidates shown in \ref{['fig:cutouts']}. We show fluxes with ${\rm S/N}>3$ as black squares, while $3\sigma$ upper limits are shown with empty triangles. The best fit is shown with a blue solid line, while the shaded region shown the 16% and 84% uncertainties. We report on the top left the $\chi^{2}$ and the output parameters.
• Figure 4: $M_{\rm UV}$ and redshift for all LRD candidates, show as contour lines, equally spaced from 10% to 90% with the last one representing 99% of the distribution. The remaining 1% of the sample is shown with stars, colour-coded based on their field. We also show three samples of LRDs selected with JWST observations Kocevski2024Labbe2023Kokorev2024Akins2024. The black dotted line show the 80% completeness expected once the EDFs are at their final depth.
• Figure 5: Top: Half-light ratio in the JWST/F444W filter vs. redshift for the sample of extended v-shape objects by Kocevski2024. The horizontal solid line indicate the band FWHM, scaled assuming a Gaussian function. Bottom: fraction of extended v-shape objects that is expected to be unresolved in the band, as a function of redshift.