  We combine Hubble Space Telescope images from the Great Observatories Origins
Deep Survey with archival Very Large Telescope and Keck spectra of a sample of
11 X-ray selected broad-line active galactic nuclei in the redshift range 1<z<2
to study the black hole mass - stellar mass relation out to a lookback time of
10 Gyrs. Stellar masses of the spheroidal component are derived from
multi-filter surface photometry. Black hole masses are estimated from the width
of the broad MgII emission line and the 3000A nuclear luminosity. Comparing
with a uniformly measured local sample and taking into account selection
effects, we find evolution in the form M_BH/M_spheroid ~ (1+z)^(1.96+/-0.55),
in agreement with our earlier studies based on spheroid luminosity. However,
this result is more accurate because it does not require a correction for
luminosity evolution and therefore avoids the related and dominant systematic
uncertainty. We also measure total stellar masses. Combining our sample with
data from the literature, we find M_BH/M_host ~ (1+z)^(1.15+/-0.15), consistent
with the hypothesis that black holes (in the range M_BH ~ 10^8-9 M_sun) predate
the formation of their host galaxies. Roughly one third of our objects reside
in spiral galaxies; none of the host galaxies reveal signs of interaction or
major merger activity. Combined with the slower evolution in host stellar
masses compared to spheroid stellar masses, our results indicate that secular
evolution or minor mergers play a non-negligible role in growing both BHs and
spheroids.
