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

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 × 10⁸ 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 r₀ = 8 ± 2 h⁻¹ cMpc, comparable to that of z ≈ 6 ultraviolet-luminous quasars. The resulting estimate of their minimum dark matter halo mass log10(Mhalo,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.