  We demonstrate that the UV brightest quasars at z=1-2 live in overdense
environments. This is based on an analysis of deep Hubble Space Telescope WFC3
G141 grism spectroscopy of the galaxies along the lines-of-sight to UV luminous
quasars in the redshift range z=1-2. This constitutes some of the deepest grism
spectroscopy performed by WFC3, with 4 roll angles spread over a year of
observations to mitigate the effect of overlapping spectra. Of the 12 quasar
fields studied, 8 display evidence for a galaxy overdensity at the redshift of
the quasar. One of the overdensities, PG0117+213 at z=1.50, has potentially 36
spectroscopically confirmed members, consisting of 19 with secure redshifts and
17 with single-line redshifts, within a cylinder of radius ~700 kpc. Its halo
mass is estimated to be log (M/Msol)=14.7. This demonstrates that spectroscopic
and narrow-band observations around distant UV bright quasars may be an
excellent route for discovering protoclusters. Our findings agree with previous
hints from statistical observations of the quasar population and theoretical
works, as feedback regulated black hole growth predicts a correlation between
quasar luminosity and halo mass. We also present the high signal-to-noise
rest-frame optical spectral and photometric properties of the quasars
themselves.
