The new ALMA observation is the first direct evidence that cold dense clouds can coalesce out of hot intergalactic gas and plunge into the heart of a galaxy to feed its central supermassive black hole. It also reshapes astronomers' views on how supermassive black holes feed, in a process known as accretion.
Previously, astronomers believed that, in the largest galaxies, supermassive black holes fed on a slow and steady diet of hot ionised gas from the galaxy's halo. The new ALMA observations show that, when the intergalactic weather conditions are right, black holes can also gorge on a clumpy, chaotic downpour of giant clouds of very cold molecular gas.
"Although it has been a major theoretical prediction in recent years, this is one of the first unambiguous pieces of observational evidence for a chaotic, cold rain feeding a supermassive black hole," said Grant Tremblay, an astronomer with Yale University in New Haven, Connecticut, USA, former ESO Fellow, and lead author on the new paper. "It's exciting to think we might actually be observing this galaxy-spanning rainstorm feeding a black hole whose mass is about 300 million times that of the Sun."
Tremblay and his team used ALMA to peer into an unusually bright cluster of about 50 galaxies, collectively known as Abell 2597. At its core is a massive elliptical galaxy, descriptively named the Abell 2597 Brightest Cluster Galaxy. Suffusing the space between these galaxies is a diffuse atmosphere of hot ionised gas, which was previously observed with NASA's Chandra X-ray Observatory.
Near the centre of this galaxy the researchers discovered just this scenario: three massive clumps of cold gas are careening toward the supermassive black hole in the galaxy's core at about a million kilometres per hour. Each cloud contains as much material as a million Suns and is tens of light-years across.
Normally, objects on that scale would be difficult to distinguish at these cosmic distances, even with ALMA's amazing resolution. They were revealed, however, by the billion-light-year-long "shadows" they cast toward Earth .
Additional data from the National Science Foundation's Very Long Baseline Array indicate that the gas clouds observed by ALMA are only about 300 light-years from the central black hole, essentially teetering on the edge of being devoured, in astronomical terms.
 The shadows are formed when the in-falling opaque gas clouds block out a portion of the bright background millimetre-wavelength light emitted by electrons spiraIling around magnetic fields very near the central supermassive black hole.
This research was presented in a paper entitled "Cold, clumpy accretion onto an active supermassive black hole", by Grant R. Tremblay et al., to appear in the journal Nature on 9 June 2016.
The team is composed of Grant R. Tremblay (Yale University, New Haven, Connecticut, USA; ESO, Garching, Germany), J. B. Raymond Oonk (ASTRON, Netherlands Institute for Radio Astronomy, Dwingeloo, the Netherlands; Leiden Observatory, Leiden University, Leiden, the Netherlands), Françoise Combes (LERMA, Observatoire de Paris, PSL Research University, College de France, CNRS, Sorbonne University, Paris, France), Philippe Salomé (LERMA, Observatoire de Paris, PSL Research University, College de France, CNRS, Sorbonne University, Paris, France), Christopher O'Dea (University of Manitoba, Winnipeg, Canada; Rochester Institute of Technology, Rochester, New York, USA), Stefi A. Baum (University of Manitoba, Winnipeg, Canada; Rochester Institute of Technology, Rochester, New York, USA), G. Mark Voit (Michigan State University, East Lansing, Michigan, USA), Megan Donahue (Michigan State University, East Lansing, Michigan, USA), Brian R. McNamara (Waterloo University, Waterloo, Ontario, Canada), Timothy A. Davis (Cardiff University, Cardiff, United Kingdom; ESO, Garching, Germany), Michael A. McDonald (Kavli Institute for Astrophysics & Space Research, MIT, Cambridge, Massachusetts, USA), Alastair C. Edge (Durham University, Durham, United Kingdom), Tracy E. Clarke (Naval Research Laboratory Remote Sensing Division, Washington DC, USA), Roberto Galván-Madrid (Instituto de Radioastronomía y Astrofísica, UNAM, Morelia, Michoacan, Mexico; ESO, Garching, Germany), Malcolm N. Bremer (University of Bristol, Bristol, United Kingdom), Louise O. V. Edwards (Yale University, New Haven, Connecticut, USA), Andrew C. Fabian (Institute of Astronomy, Cambridge University, Cambridge, United Kingdom), Stephen Hamer (LERMA, Observatoire de Paris, PSL Research University, College de France, CNRS, Sorbonne University, Paris, France) , Yuan Li (University of Michigan, Ann Arbor, Michigan, USA ), Anaëlle Maury (Laboratoire AIMParis-Saclay, CEA/DSM/Irfu CNRS, University Paris Diderot, CE-Saclay, Gif-sur-Yvette, France), Helen Russell (Institute of Astronomy, Cambridge University, Cambridge, United Kingdom), Alice C. Quillen (University of Rochester, Rochester, New York, USA), C. Megan Urry (Yale University, New Haven, Connecticut, USA), Jeremy S. Sanders (Max-Planck-Institut für extraterrestrische Physik, Garching bei München, Germany), and Michael Wise (ASTRON, Netherlands Institute for Radio Astronomy, Dwingeloo, the Netherlands).