  The ultraviolet/optical continuum of quasars is thought to be from an
accretion flow around a supermassive black hole, and it dominates the radiative
output of quasars. However, the nature of this emission, often called the Big
Blue Bump, has not been well understood. Robust evidence for its thermal nature
would be a continuum opacity edge feature intrinsic to this component, but this
has not been clearly confirmed despite the predictions by many models. We are
now developing and exploiting a new method to detect the Balmer edge of
hydrogen opacity. The method overcomes for certain objects the major obstacle
of the heavy contamination from emissions outside the nucleus by taking the
polarized flux spectrum. If our interpretation of the polarized flux is
correct, our data show that the Big Blue Bump emission has a Balmer edge in
absorption, indicating that the emission is indeed thermal, and the emitter is
optically thick.
