Star formation in luminous quasar host galaxies at z = 1-2

Abstract

We present deep Hubble Space Telescope (HST)/Wide-Field Planetary Camera 2 (WFPC2), rest-frame U images of 17 ∼L^⋆ quasars at z ≈ 1 and 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 Near Infrared Camera and Multi-Object Spectrometer 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 (RLQs) in our sample are more luminous than their radio-quiet quasar (RQQ) counterparts at each epoch, but have indistinguishable colours, confirming that the RLQs are drawn from only the most massive galaxies (10¹¹-10¹² M_⊙ even at z ≈ 2), while the RQQs are slightly less massive (∼10¹¹ M_⊙). 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_⊙ yr^-1 for the RLQs and ∼100 M_⊙ yr^-1 for the RQQs at z ≈ 2. By z ≈ 1, these rates have fallen to ∼150 M_⊙ yr^-1 for the RLQs and ∼50 M_⊙ yr^-1 for the RQQs. We conclude that while the host galaxies are extremely massive, they remain actively star forming at, or close to, the epoch of the quasar.