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

Abstract

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 $\rm SFR\approx 300 \: {\rm M}_{\odot } \: yr^{-1}$, and then increases towards higher SFR, suggesting that the ISM obscuration plays a significant role in starburst host galaxies, and (2) at $\rm SFR\gtrsim 300 \: {\rm M}_{\odot } \: yr^{-1}$, the SMGs and IR quasars have similarly compact submillimetre sizes ($R_{\rm e}\approx 0.5{\!-\!}3\,\mathrm{ kpc}$) and consequently, the ISM can heavily obscure the quasar, even reaching Compton-thick ($N_{\rm H}\gt 10^{24} \rm \: cm^{-2}$) levels in extreme cases. Based on our results, we infer that ${\approx} 10{\!-\!}30~{{ \rm per\ cent}}$ of the IR quasars with $\rm SFR\gtrsim 300 \: {\rm M}_{\odot } \: yr^{-1}$ are obscured solely by the ISM.