  Observations of luminous quasars and their supermassive black holes at z~6
suggest that they formed at dense matter peaks in the early universe. However,
few studies have found definitive evidence that the quasars lie at cosmic
density peaks, in clear contrast with theory predictions. Here we present new
evidence that the radio-loud quasar SDSS J0836+0054 at z=5.8 could be part of a
surprisingly rich structure of galaxies. This conclusion is reached by
combining a number of findings previously reported in the literature: Bosman et
al. (2020) obtained the redshifts of three companion galaxies, confirming an
overdensity of i-dropouts found by Zheng et al. (2006). By comparing this
structure with those found near other quasars and large overdense regions in
the field at z~6-7, we show that the SDSS J0836+0054 field is among the densest
structures known at these redshifts. One of the spectroscopic companions is a
very massive star-forming galaxy (log M_*/M_sun ~ 10.3) based on its
unambiguous detection in a Spitzer 3.6 um image. This suggests that the quasar
field hosts not one, but at least two rare, massive dark matter halos (log
M_halo/M_sun > 12), corresponding to a galaxy overdensity of at least 20. We
discuss the properties of the young radio source. We conclude that the
environment of SDSS J0836+0054 resembles, at least qualitatively, the type of
conditions that may have spurred the direct collapse of a massive black hole
seed according to recent theory.
