Star formation in luminous quasar host galaxies at z=1-2
  We present deep HST/WFPC2, rest-frame U images of 17 ~L* quasars at z=1 and
z=2 (V and I bands respectively), designed to explore the host galaxies. We fit
the images with simple axisymmetric galaxy models, including a point-source, in
order to separate nuclear and host-galaxy emission. We successfully model all
of the host galaxies, with luminosities stable to within 0.3 mag. Combining
with our earlier NICMOS rest-frame optical study of the same sample, we provide
the first rest-frame U-V colours for a sample of quasar host galaxies. While
the optical luminosities of their host galaxies indicate that they are drawn
purely from the most massive (>~L*) early-type galaxy population, their colours
are systematically bluer than those of comparably massive galaxies at the same
redshift. The host galaxies of the radio-loud quasars (RLQ) in our sample are
more luminous than their radio-quiet quasar (RQQ) counterparts at each epoch,
but have indistinguishable colours, confirming that the RLQ's are drawn from
only the most massive galaxies (10^{11}-10^{12} M_sun, even at z~2), while the
RQQ's are slightly less massive (~10^{11} M_sun). This is consistent with the
well-known anticorrelation between radio-loudness and accretion rate. Using
simple stellar population "frosting" models we estimate mean star formation
rates of ~350 M_sun/yr for the RLQ's and ~100 M_sun/yr for the RQQ's at z~2. By
z~1, these rates have fallen to ~150 M_sun/yr for the RLQ's and ~50 M_sun/yr
for the RQQ's. We conclude that while the host galaxies are extremely massive,
they remain actively star-forming at, or close to, the epoch of the quasar.
