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 [1][2][3] . The location and evolution of this obscuring material have been the subject of intense research in the past decades 4,5 , 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 1,[6][7][8] . 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 9 ; by the gravitational potential of the black hole 10 ; by radiative feedback [11][12][13][14] ; or by the interplay between outflows and inflows 15 . 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.Our group has carried out a large multi-wavelength study of the 836 accreting supermassive black holes (i.e., active galactic nuclei or AGN) detected by the all-sky hard X-ray (14-195 keV) Swift Burst Alert Telescope survey 16, 17 (see §1 of the Methods). The energy range covered by Swift/BAT makes it ideal for studying the characteristics and evolu- Figure 1: Relation between the fraction of obscured AGN and the Eddington ratio. The fraction of obscured Compton-thin [10 22 ≤ (NH/cm −2 ) < 10 24 ] sources shown as a function of the Eddington ratio λ Edd (i.e. the AGN luminosity normalized by the maximum value for solar-metalicity, fully-ionized, dust-free gas in a spherical geometry) for our hard X-ray selected sample in the 10 −5.6 ≤ λ Edd < 1 range. The values are normalized to unity in the 10 20 ≤ (NH/cm −2 ) < 10 24 interval. The shaded area represents the 16th and 84th quantiles of a binomial distribution 20 . The vertical red dashed line represents the effective Eddington limit for a dusty gas 14 with NH = 10 22 cm −2 (see §2). The figure shows that the covering factor of the obscuring material with 10 22 ≤ (NH/cm −2 ) < 10 24 decreases sharply around the Eddington limit for dusty gas, highlighting the fact that radiation pressure strongly affects obscuration in AGN.tion of the absorbing material surrounding the AGN, being unaffected by obscuration up to column densities NH ≃ 10 24 cm −2 . More...

BAT AGN Spectroscopic Survey. VIII. Type 1 AGN with Massive Absorbing Columns

“…For Type 1 AGNs, we do not observe a similar radical decrease in the X-ray absorbed fraction that Ricci et al (2017b) find for primarily Type 2 AGNs. Using SMBH masses from the BASS DR1 and a cutoff of log λ Edd =−1.5, we find an X-ray absorbed fraction of 28 13 16 -+ % and 18 5 7 -+ % for low and high Eddington Type 1 AGNs.…”

“…If we suppose our measurements, completely determined from both optical and X-ray spectra, are correct, then we can put them into the context of the recent work of Ricci et al (2017b). They studied the general X-ray obscured fraction in the entire BASS sample, finding a significant decrease of the total X-ray absorbed fraction at a high Eddington ratio (λ Edd ).…”

“…They studied the general X-ray obscured fraction in the entire BASS sample, finding a significant decrease of the total X-ray absorbed fraction at a high Eddington ratio (λ Edd ). The explanation in Ricci et al (2017b) is that radiative feedback from the AGN shapes the obscuring structure. At a low Eddington ratio, gas and dust are able to build up around the SMBH, increasing the covering factor of the "torus," while at high Eddington, the AGN has cleared away large amounts of gas and dust.…”

Luminous WISE-selected Obscured, Unobscured, and Red Quasars in Stripe 82∗

“…high redshifts and high luminosities, red Type-1 AGNs begin to dominate the space density. If the red quasar phenomenon is an evolutionary phase, it may not have a strong luminosity dependence-other than being a high-luminosity phenomenon -resulting in the flat luminosity dependence seen in the magenta points (this was shown to be true for high Eddington ratio AGNs studied by Ricci et al 2017b). In the highestluminosity bin our survey volume is still too small to include the most luminous sources at low redshift.…”

BAT AGN Spectroscopic Survey. V. X-Ray Properties of the Swift /BAT 70-month AGN Catalog

“…For the 12 objects for which we could constrain OA q we find a median value of 58°±3°. A detailed study of the obscuration properties of Swift/BAT AGNs is Ricci et al (2017c), where we discuss how absorption is related to the physical characteristics of the accreting SMBH.…”