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