2025A&A...703A..36L

We report the discovery of a little red dot (LRD), dubbed BiRD ('big red dot'), at z ∼ 2.33 in the field around the z ∼ 6.3 quasar SDSS J1030+0524. Using JWST/NIRCam images, we identified it as a bright outlier in the F200W - F356W color versus F356W magnitude diagram of point sources in the field. The NIRCam/WFSS spectrum reveals the emission from He Iλ 10830 and Paγ line, both displaying a narrow and a broad (FWHM ≳ 2000 km s^–1) component. The He I line is affected by an absorption feature, tracing dense gas with He I column density in the 2³S level N(He I) ∼ 0.5 - 1.2 × 10¹⁴ cm^–2, depending on the location of the absorber, which is outflowing at a speed of $ \Delta v =-830_{-148}^{+131} $ km s^–1. As observed in the majority of LRDs, BiRD does not exhibit any X-ray or radio emission down to 3.7 × 10⁴² erg s^–1 and 3 × 10³⁹ erg s^–1, respectively. The black hole mass and the bolometric luminosity, both inferred from the Paγ broad component, amount to M_BH ∼ 10⁸ M_⊙ and L_bol ∼ 3 × 10⁴⁵ erg s^–1, respectively. Intriguingly, BiRD presents strict analogies with other two LRDs spectroscopically confirmed at cosmic noon, namely, GN-28074 (nicknamed Rosetta Stone) at z ∼ 2.26 and RUBIES-BLAGN-1 at z ∼ 3.1. The blueshifted He I absorption detected for all three sources suggests that gas outflows could be common in LRDs. We derived a first estimate of the space density of LRDs at z ∼ 2 - 3 based on JWST data, as a function of the bolometric luminosity and black hole mass. The space density $ \Phi(L) = 4.0^{+4.0}_{-2.4} \times 10^{-6} $ Mpc^–3 dex^–1 is only a factor of ∼2 - 3 lower than that of UV-selected quasars with comparable bolometric luminosity and redshift, meaning that the contribution of LRDs to the broader AGN population is also relevant at cosmic noon. A similar trend has also been observed with respect to black hole masses. As suggested by recent theories, if LRDs can indeed serves as probes of the very first and rapid growth of black hole seeds, our finding suggests that the formation of black hole seeds continues to be efficient at least up to cosmic noon.