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

Accepted for publication in MNRAS Letters