Spectral Energy Distributions of Companion Galaxies to z$\sim$6 Quasars
  Massive, quiescent galaxies are already observed at redshift $z\sim4$, i.e.
$\sim$1.5 Gyr after the Big Bang. Current models predict them to be formed via
massive, gas-rich mergers at $z>6$. Recent ALMA observations of the cool gas
and dust in $z\gtrsim$6 quasars have discovered [CII]- and far infrared-bright
galaxies adjacent to several quasars. In this work, we present sensitive
imaging and spectroscopic follow-up observations, with HST/WFC3, Spitzer/IRAC,
VLT/MUSE, Magellan/FIRE and LBT/LUCI-MODS, of ALMA-detected, dust-rich
companion galaxies of four quasars at $z\gtrsim 6$, specifically acquired to
probe their stellar content and unobscured star formation rate. Three companion
galaxies do not show significant emission in the observed optical/IR wavelength
range. The photometric limits suggest that these galaxies are highly
dust-enshrouded, with unobscured star formation rates SFR$_{\rm UV}<$few
M$_{\odot}$ yr$^{-1}$, and a stellar content of $M_{*}<$10$^{10}$ M$_{\odot}$
yr$^{-1}$. However, the companion to PJ167-13 shows bright rest-frame UV
emission (F140W AB = 25.48). Its SED resembles that of a star-forming galaxy
with a total SFR$\sim$50 M$_{\odot}$ yr$^{-1}$ and $M_{*}\sim 9 \times 10^{9}$
M$_{\odot}$. All the companion sources are consistent with residing on the
galaxy main sequence at $z\sim$6. Additional, deeper data from future
facilities, such as JWST, are needed in order to characterize these gas-rich
sources in the first Gyr of cosmic history.
