News, November 2023
Obscuration beyond the nucleus: infrared quasars can be buried in extreme compact starbursts
New study suggests interstellar medium plays a significant role in obscuration of quasars
The figure illustrates the potential sources of quasar obscuration. Panels (a), (b), and (c) show different relative orientations between the traditional quasar torus (orange flared disc) and the starburst (blue flared disc with stars). The dashed and continuous arrows represent obscured and unobscured l.o.s, respectively. The cartoons are illustrative, and are not drawn to scale.
A recent study conducted by Carolina Andonie from the Centre for Extragalactic Astronomy challenges the traditional model of quasar obscuration. Quasars are extremely luminous, active galactic nuclei powered by supermassive black holes. According to the standard model, their obscuration is primarily due to a dusty torus of gas and dust surrounding the black hole. However, Andonie argues that a substantial portion of the obscuration may actually be caused by the interstellar medium (ISM) when quasars are embedded in compact starbursts. The research team analyzed a sample of 578 infrared quasars at redshifts ranging from 1 to 3, and studied the sizes of their host galaxies using data from the Atacama Large Millimeter/submillimeter Array (ALMA). They compared these quasars to a control sample of submillimeter galaxies not hosting quasar activity. By calculating the star formation rates (SFR) and interstellar medium (ISM) column densities, the team investigated the contribution of ISM to quasar obscuration. The results of the study revealed two key findings. Firstly, the fraction of obscured quasars remained constant up to an SFR of approximately 300 solar masses per year, but increased at higher SFRs. This suggests that the ISM obscuration plays a significant role in starburst host galaxies where star formation is particularly intense. Secondly, at SFRs above 300 solar masses per year, the submillimeter galaxies and infrared quasars exhibited similar compact sizes, indicating that the ISM can heavily obscure the quasar. In extreme cases, the obscuration levels could reach Compton-thick levels, where the column density of hydrogen atoms exceeds 10^24 cm^-2. Based on these findings, the researchers estimate that around 10-30% of the infrared quasars with SFRs exceeding 300 solar masses per year are solely obscured by the interstellar medium. This challenges the long-standing assumption that the dusty torus alone is responsible for quasar obscuration. Understanding the mechanisms behind quasar obscuration is crucial for improving our knowledge of these powerful cosmic objects. By identifying the contribution of the interstellar medium in obscuring quasars, this study sheds light on the complex relationship between quasars, starbursts, and the surrounding galactic environment. Further research in this field may lead to a more comprehensive understanding of quasar physics and their impact on galaxy evolution.