Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales1,2,3. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas4. Recent theory5,6,7 and simulations8,9,10 instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds—a departure from the ‘hot mode’ accretion model—although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma11,12,13. Under the right conditions, thermal instabilities produce a rain of cold clouds that fall towards the galaxy’s centre14, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core15. The observations show that these cold clouds also fuel black hole accretion, revealing ‘shadows’ cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations16 of warmer atomic gas at extremely high spatial resolution17, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.
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ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. We are grateful to the European ALMA Regional Centres, particularly those in Garching and Manchester, for their dedicated end-to-end support of data associated with this paper. We thank R. Larson for discussions. G.R.T. acknowledges support from National Aeronautics and Space Administration (NASA) through Einstein Postdoctoral Fellowship Award Number PF-150128, issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. F.C. acknowledges the European Research Council (ERC) for the Advanced Grant Program no. 267399-Momentum. B.R.M. is supported by a grant from the Natural Sciences and Engineering Research Council of Canada. T.A.D. acknowledges support from a Science and Technology Facilities Council (STFC) Ernest Rutherford Fellowship. A.C.E. acknowledges support from STFC grant ST/L00075X/1. A.C.F. and H.R.R. acknowledge support from ERC Advanced Grant Program no. 340442-Feedback. M.N.B. acknowledges funding from the STFC. Basic research in radio astronomy at the Naval Research Laboratory is supported by 6.1 Base funding.
About this article
Nature Astronomy (2017)