Star formation in luminous quasar host galaxies at z = 1-2
Marek Kukula2013, Monthly Notices of the Royal Astronomical Society
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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 radioloud 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 ⊙ even at z ≈ 2), while the RQQ's are slightly less massive (∼ 10 11 M ⊙ ). This is consistent with the well-known anti-correlation 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 RLQ's and ∼ 100 M ⊙ yr −1 for the RQQ's at z ≈ 2. By z ≈ 1, these rates have fallen to ∼ 150 M ⊙ yr −1 for the RLQ's and ∼ 50 M ⊙ yr −1 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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