  We study the environments of a sample of 61 extremely rare z~1.6
Ultra-Massive Passively Evolving Galaxies (UMPEGs: stellar masses M_stars
>10^11.5 M_sun) which -- based on clustering analysis presented in Cheema et
al. (2020) -- appear to be associated with very massive (M_halo ~ 10^14.1 h^-1
M_sun) dark matter halos that are likely to be the progenitors of z~0 massive
(Coma- and Virgo-like) galaxy clusters. We find that UMPEGs on average have
fewer than one satellite galaxy with mass ratio M_sat : M_UMPEG >~ 1:5 (i.e.,
M_sat >~ 10^10.8 M_sun) within 0.5 Mpc; the large mass gap that we observe
between the typical UMPEG and its most massive satellite implies that the z~1.6
UMPEGs assembled through major mergers. Using observed satellite counts with
merger timescales from the literature, we estimate the growth rate due to
mergers with mass ratio of >~ 1:4 to be ~13% Gyr^-1 (with a ~2x systematic
uncertainty). This relatively low growth rate is unlikely to significantly
affect the shape of the massive end of the stellar mass function, whose
evolution must instead be driven by the quenching of new cohorts of
ultra-massive star-forming galaxies. However, this growth rate is high enough
that, if sustained to z~0, the typical z~1.6 M_UMPEG=10^11.6 M_sun UMPEG can
grow into a M_stars~10^12 M_sun brightest cluster galaxy (BCG) of a present-day
massive galaxy cluster. Our observations favour a scenario in which our UMPEGs
are main-branch progenitors of some of the present-day BCGs that have first
assembled through major mergers at high redshifts and grown further through
(likely minor) merging at later times.
