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

Abstract

In the standard quasar model, the accretion disk 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 ALMA submillimeter host galaxy sizes to investigate the ISM contribution to the quasar obscuration. We calculate SFR and ISM column densities for the IR quasars and a control sample of submillimeter galaxies (SMGs) not hosting quasar activity and show that: (1) the quasar obscured fraction is constant up to SFR≈300M⊙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≳300M⊙yr−1, the SMGs and IR quasars have similarly compact submillimeter sizes (Re≈0.5−3kpc) and, consequently, the ISM can heavily obscure the quasar, even reaching Compton-thick (NH>1024cm−2) levels in extreme cases. Based on our results, we infer that ≈10−30% of the IR quasars with SFR≳300M⊙yr−1 are obscured solely by the ISM.

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ORCID for Claire Greenwell: orcid.org/0000-0002-7719-5809

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