Massive , quiescent galaxies are already observed at redshift z \sim 4 , 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 [ C II ] – 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 _ { 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 .