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). 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.