A Little Red Dot at $\mathbf{z}=\mathbf{7.3}$ 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, $A_V=2.79\text{mag}$, with an intrinsic bolometric luminosity of ${10}^{46.6}\text{erg}{\text{s}}^{-1}$ and a SMBH mass of $5\times {10}^8{\text{M}}_{\odot }$. 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-galaxy cross-correlation length of $r_0\text{\hspace{0.17em}⁣}=\text{\hspace{0.17em}⁣}8\pm 2{\text{h}}^{-1}\text{cMpc}$, comparable to that of $z\text{\hspace{0.17em}⁣}\sim \text{\hspace{0.17em}⁣}6$ UV-luminous quasars. The resulting estimate of their minimum dark matter halo mass of ${\log }_{10}(M_{\text{halo, min}}\mathrm{/}{\text{M}}_{\odot })=12.0_{-1.0}^{+0.8}$ indicates that nearly all halos above this mass must host actively accreting SMBHs at $z\approx 7$, in strong contrast with the far smaller duty cycle of luminous quasars ($<1\mathrm{\%}$). Our results, taken at face value, motivate a picture in which SMBHs in Little Red Dot phases could serve as the obscured precursors of UV-luminous quasars, which provides a natural explanation for the short UV-luminous lifetimes inferred from both quasar clustering and quasar proximity zones.