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...

We have produced the next generation of quasar spectral energy distributions (SEDs), essentially updating the work of Elvis et al. (1994) by using high-quality data obtained with several space and ground-based telescopes, including NASA's Great Observatories. We present an atlas of SEDs of 85 optically bright, non-blazar quasars over the electromagnetic spectrum from radio to X-rays. The heterogeneous sample includes 27 radio-quiet and 58 radio-loud quasars. Most objects have quasi-simultaneous ultraviolet-optical spectroscopic data, supplemented with some far-ultraviolet spectra, and more than half also have Spitzer mid-infrared IRS spectra. The X-ray spectral parameters are collected from the literature where available. The radio, far-infrared, and near-infrared photometric data are also obtained from either the literature or new observations. We construct composite spectral energy distributions for radio-loud and radio-quiet objects and compare these to those of Elvis et al., finding that ours have similar overall shapes, but our improved spectral resolution reveals more detailed features, especially in the mid and near-infrared.

The interstellar medium is crucial to understanding the physics of active galaxies and the coevolution between supermassive black holes and their host galaxies. However, direct gas measurements are limited by sensitivity and other uncertainties. Dust provides an efficient indirect probe of the total gas. We apply this technique to a large sample of quasars, whose total gas content would be prohibitively expensive to measure. We present a comprehensive study of the full (1 to 500 µm) infrared spectral energy distributions of 87 redshift < 0.5 quasars selected from the Palomar-Green sample, using photometric measurements from 2MASS, WISE, and Herschel, combined with Spitzer mid-infrared (5-40 µm) spectra. With a newly developed Bayesian Markov Chain Monte Carlo fitting method, we decompose various overlapping contributions to the integrated spectral energy distribution, including starlight, warm dust from the torus, and cooler dust on galaxy scales. This procedure yields a robust dust mass, which we use to infer the gas mass, using a gas-to-dust ratio constrained by the host galaxy stellar mass. Most (90%) quasar hosts have gas fractions similar to those of massive, star-forming galaxies, although a minority (10%) seem genuinely gas-deficient, resembling present-day massive early-type galaxies. This result indicates that "quasar mode" feedback does not occur or is ineffective in the host galaxies of low-redshift quasars. We also find that quasars can boost the interstellar radiation field and heat dust on galactic scales. This cautions against the common practice of using the far-infrared luminosity to estimate the host galaxy star formation rate.