Abstract
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). Using simple stellar
population "frosting" models we estimate that ~1% of the mass of the host
galaxies is undergoing star formation at z~2 and ~0.1% at z~1. The mean star
formation rates are ~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.
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