Obscuration beyond the nucleus: infrared quasars can be buried in extreme compact starbursts

Abstract

In the standard quasar model, the accretion disc obscuration is due to the canonical dusty torus. Here, we argue that a substantial part of the quasar obscuration can come from the interstellar medium (ISM) when the quasars are embedded in compact starbursts. We use an obscuration-unbiased sample of 578 infrared (IR) quasars at z ≈ 1–3 and archival Atacama Large Millimetre/submillimetre Array submillimetre host galaxy sizes to investigate the ISM contribution to the quasar obscuration. We calculate star formation rates (SFR) and ISM column densities for the IR quasars and a control sample of submillimetre galaxies (SMGs) not hosting quasar activity and show that: (1) the quasar obscured fraction is constant up to SFR ≈ 300 M _☉ yr ⁻¹, and then increases towards higher SFR, suggesting that the ISM obscuration plays a significant role in starburst host galaxies, and (2) at SFR ≳ 300 M _☉ yr ⁻¹, the SMGs and IR quasars have similarly compact submillimetre sizes (R _e ≈ 0.5–3 kpc) and consequently, the ISM can heavily obscure the quasar, even reaching Compton-thick (N _H > 10 ²⁴ cm ⁻²) levels in extreme cases. Based on our results, we infer that ≈10–30 per cent of the IR quasars with SFR ≳ 300 M _☉ yr ⁻¹ are obscured solely by the ISM.

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