The Future of Direct Supermassive Black Hole Mass Estimates
  (Abridged) The repeated discovery of supermassive black holes (SMBHs) at the
centers of galactic bulges, and the discovery of relations between the SMBH
mass (M) and the properties of these bulges, has been fundamental in directing
our understanding of both galaxy and SMBH formation and evolution. However,
there are still many questions surrounding the SMBH - galaxy relations. For
example, are the scaling relations linear and constant throughout cosmic
history, and do all SMBHs lie on the scaling relations? These questions can
only be answered by further high quality direct M estimates from a wide range
in redshift. In this paper we determine the observational requirements
necessary to directly determine SMBH masses, across cosmological distances,
using current M modeling techniques. We also discuss the SMBH detection
abilities of future facilities. We find that if different M modeling
techniques, using different spectral features, can be shown to be consistent,
then both 30 m ground- and 16 m space-based telescopes will be able to sample M
1e9Msol across ~95% of cosmic history. However, we find that the abilities of
ground-based telescopes critically depend on future advancements in adaptive
optics systems; more limited AO systems will result in limited effective
spatial resolutions, and forces observations towards the near-infrared where
spectral features are weaker and more susceptible to sky features. Ground-based
AO systems will always be constrained by relatively bright sky backgrounds and
atmospheric transmission. The latter forces the use of multiple spectral
features and dramatically impacts the SMBH detection efficiency. The most
efficient way to advance our database of direct SMBH masses is therefore
through the use of a large (16 m) space-based UVOIR telescope.
