The close environments of accreting massive black holes are shaped by radiative feedback

Abstract: The large majority of the accreting supermassive black holes in the Universe are obscured by large columns of gas and dust. The location and evolution of this obscuring material have been the subject of intense research in the past decades, and are still highly debated. A decrease in the covering factor of the circumnuclear material with increasing accretion rates has been found by studies carried out across the electromagnetic spectrum. The origin of this trend has been suggested to be driven either by the increase in the inner radius of the obscuring material with incident luminosity due to the sublimation of dust; by the gravitational potential of the black hole; by radiative feedback; or by the interplay between outflows and inflows. However, the lack of a large, unbiased and complete sample of accreting black holes, with reliable information on gas column density, luminosity and mass, has left the main physical mechanism regulating obscuration unclear. Using a systematic multi-wavelength survey of hard X-ray-selected black holes, here we show that radiation pressure on dusty gas is indeed the main physical mechanism regulating the distribution of the circumnuclear material. Our results imply that the bulk of the obscuring dust and gas in these objects is located within the sphere of influence of the black hole (i.e., a few to tens of parsecs), and that it can be swept away even at low radiative output rates. The main physical driver of the differences between obscured and unobscured accreting black holes is therefore their mass-normalized accretion rate.

The Role of Radiative Feedback in Shaping the Environments of Accreting Massive Black Holes

The paper titled "The close environments of accreting massive black holes are shaped by radiative feedback" investigates the underlying mechanisms that regulate the distribution of material around accreting supermassive black holes (SMBHs). Leveraging a comprehensive multi-wavelength survey of hard X-ray-selected active galactic nuclei (AGN), the study provides new insights into how radiative feedback influences the obscuring material in these systems.

Overview of Research

The authors undertake a systematic study using 836 AGN detected via the hard X-ray (14--195 keV) band from the all-sky survey of the Swift Burst Alert Telescope (BAT). By focusing on this energy range, the research effectively bypasses issues related to line-of-sight obscuration up to very high column densities ($N_{H} \approx 10^{24} \rm cm^{-2}$). Such an approach allows for precise characterization of the accretion properties and the obscuring material around local AGN.

Key Findings

The central finding of this study is that radiation pressure on dusty gas is the dominant physical mechanism regulating the obscuration of accreting SMBHs. Analyzing spectroscopic data in both X-ray and optical bands, the study determines key parameters such as the column densities, intrinsic X-ray luminosities, and black hole masses. The analysis reveals that as the Eddington ratio, which normalizes AGN luminosity by black hole mass, increases, the covering factor of the circumnuclear material sharply decreases. This pattern is observed notably around the effective Eddington limit for dusty gas, implying that radiative feedback significantly influences the obscuring material.

The study highlights that the majority of the obscuring material exists within the sphere of influence of the SMBH, generally extending only a few to tens of parsecs from the black hole. Moreover, the paper demonstrates that the distinction between obscured and unobscured AGN is largely driven by their mass-normalized accretion rates. Systems with higher Eddington ratios are less likely to exhibit significant nuclear obscuration due to radiative feedback effectively clearing the surrounding environment.

Implications and Future Prospects

The implications of this research are twofold: practical and theoretical. On a practical level, the findings challenge traditional models of AGN unification predicated solely on orientation-dependent obscuration, suggesting instead a radiation-regulated model that incorporates intrinsic differences in mass accretion rates. Theoretically, this work underscores the delicate interplay between SMBH growth and their host environments, mediated by radiative feedback processes.

The results pave the way for future explorations into the role of AGN in galaxy evolution, particularly in the context of feedback mechanisms affecting star formation rates and influencing host galaxy properties. These findings may guide observational strategies for distinguishing between different AGN types based on accretion characteristics and contribute to refining cosmological models of galaxy-AGN co-evolution.

Overall, this study provides substantial evidence advancing the understanding of how radiative mechanisms shape the environments around accreting massive black holes, with significant ramifications for our broader comprehension of galaxy and black hole evolution in the cosmos.