A little red dot at z = 7.3 within a large galaxy overdensity

Abstract The nature of ‘little red dots’ and their relation to other forms of accreting supermassive black holes remain an open question. Here we report the discovery of a little red dot at z = 7. 3. It is attenuated by moderate amounts of dust, A V = 2. 79 mag, and has an intrinsic bolometric luminosity of 10 46. 6 erg s −1 and a supermassive black hole mass of 5 × 10 8 M ⊙. Most notably, this object is embedded in an overdensity of eight nearby galaxies, allowing us to calculate a spectroscopic estimate of the clustering of galaxies around little red dots. We find a little red dot versus galaxy cross-correlation length of r 0 = 8 ± 2 h −1 cMpc, comparable to that of z ≈ 6 ultraviolet-luminous quasars. The resulting estimate of their minimum dark matter halo mass ₁₀ (M₇₀₋₎, /{{M}}) =12. 0-₁. ₀^+0. 8 log 10 (M halo, min / M ⊙) = 12. 0 − 1. 0 + 0. 8 indicates that nearly all haloes above this mass must host actively accreting supermassive black holes at z ≈ 7, in strong contrast with the far smaller duty cycle of luminous quasars (<1%). Our results, taken at face value, motivate a picture in which supermassive black holes in little red dot phases could serve as the obscured precursors of ultraviolet-luminous quasars, which provides a natural explanation for the short ultraviolet-luminous lifetimes inferred from both quasar clustering and quasar proximity zones.