A Little Red Dot at $\mathbf{z=7.3}$ within a Large Galaxy Overdensity

TL;DR

This paper reports the discovery of a high-redshift Little Red Dot galaxy with an active supermassive black hole, embedded in a galaxy overdensity, providing insights into early SMBH growth and galaxy clustering at z=7.3.

Contribution

It presents the first detailed characterization of a Little Red Dot at z=7.3, linking SMBH activity to galaxy overdensities and dark matter halos in the early universe.

Findings

The Little Red Dot has a bolometric luminosity of 10^{46.6} erg/s.

It is located in an overdense region with eight nearby galaxies.

The clustering analysis suggests halos above 10^{12} M_sun host active SMBHs at z=7.

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, $A_V = {2.79}\,\textrm{mag}$, with an intrinsic bolometric luminosity of $10^{46.6}\,\textrm{erg}\,\textrm{s}^{-1}$ and a SMBH mass of $5\times10^8\,\textrm{M}_\odot$. 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 $r_0\!=\!8\pm2\,\textrm{h}^{-1}\,\textrm{cMpc}$, comparable to that of $z\!\sim\!6$ UV-luminous quasars. The resulting estimate of their minimum dark matter halo mass of $\log_{10}(M_{\textrm{halo, min}}/\textrm{M}_{\odot})= 12.0_{-1.0}^{+0.8}$ indicates that nearly all halos above this mass must host actively accreting SMBHs at $z\approx7$, in strong contrast with the far smaller duty cycle of luminous quasars ($<1\%$). 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.