What do observations tell us about the highest-redshift supermassive
  black holes?
  I review the current understanding of some key properties of the earliest
growing supermassive black holes (SMBHs), as determined from the most
up-to-date observations of z>=5 quasars. This includes their accretion rates
and growth history, their host galaxies, and the large-scale environments that
enabled their emergence less than a billion years after the Big Bang. The
available multi-wavelength data show that these SMBHs are consistent with
Eddington-limited, radiatively efficient accretion that had to proceed almost
continuously since very early epochs. ALMA observations of the hosts' ISM
reveal gas-rich, well developed galaxies, with a wide range of SFRs that may
exceed ~1000 M_sol/yr. Moreover, ALMA uncovers a high fraction of companion,
interacting galaxies, separated by <100 kpc (projected). This supports the idea
that the first generation of high-mass, luminous SMBHs grew in over-dense
environments, and that major mergers may be important drivers for rapid SMBH
and host galaxy growth. Current X-ray surveys cannot access the lower-mass,
supposedly more abundant counterparts of these rare z>5 massive quasars, which
should be able to elucidate the earliest stages of BH formation and growth.
Such lower-mass nuclear BHs will be the prime targets of the deepest surveys
planned for the next generation of facilities, such as the upcoming Athena
mission and the future Lynx mission concept.
