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Abstract

We resolve the host galaxies of seven gravitationally lensed quasars at redshift 1.5–2.8 using observations with the Atacama Large (sub)Millimetre Array. Using a visibility plane lens modelling technique, we create pixellated reconstructions of the dust morphology, and CO line morphology and kinematics. We find that the quasar hosts in our sample can be distinguished into two types: (1) galaxies characterized by clumpy, extended dust distributions (R_eff ∼ 2 kpc) and mean star formation rate (SFR) surface densities comparable to sub-mm-selected dusty star-forming galaxies (Σ_SFR ∼ 3  M_⊙ yr⁻¹ kpc⁻² ) and (2) galaxies that have sizes in dust emission similar to coeval passive galaxies and compact starbursts (R_eff ∼ 0.5 kpc), with high mean SFR surface   densities (Σ_SFR = 400–4500  M_⊙ yr⁻¹ kpc⁻² ) that may be Eddington-limited or super-Eddington. The small sizes of some quasar hosts suggest that we observe them at a stage in their transformation into compact spheroids via dissipative contraction, where a high density of dynamically unstable gas leads to efficient star formation and black hole accretion. For the one system where we probe the bulk of the gas reservoir, we find a gas fraction of just 0.06 ± 0.04 and a depletion time-scale of 50 ± 40 Myr, suggesting it is transitioning into quiescence. In general, we expect that the extreme level of star formation in the compact quasar host galaxies will rapidly exhaust their gas reservoirs and could quench with or without help from active galactic nucleus feedback.