Feeding the Supermassive Black Holes in Galaxies
An optical, Hubble image of the galaxy cluster Abell 2390. (The large arcs seen around the central cluster are distortions of light from galaxies behind the cluster and farther away.) Astronomers have observed molecular absorption lines along the line-of-sights to the black hole nuclei in eighteen massive galaxies in clusters like Abell 2390, and shown that the molecular gas is in cold clouds that are gradually acceting onto the nucleus, triggering activity.
When cold gas and dust accretes onto the supermassive black hole at a galaxy's core, the galactic nucleus can become activated and eject jets of rapidly moving charged particles. These particles in turn radiate strongly across the electromagnetic spectrum. What prompts the material to accrete in the first place, and the paths it follows as it descends into the accretion maelstrom, are poorly understood physical processes.
Theory, and some simulations, have attempted to model the accretion processes in clusters of galaxies based on limited numbers of emission line studies. They predict, for example, that there are large reservoirs of cold gas in small clouds orbiting a few hundreds of light-years from the black hole, with collisions prompting them to release material for accretion. CfA astronomer Grant Tremblay and a team of colleagues have undertaken a study of the absorption lines in these clouds in submillimeter and radio molecular lines. The advantages of absorption versus emission lines are that absorption line studies probe only the narrow line-of-sight to bright nuclear background whereas emission usually comes from a much larger region because the telescope beam sizes are relatively large; moreover, absorption lines provide key kinematic information about the gas motions along this constrained line-of-sight.
The scientists used the ALMA facility to study absorption in fifteen molecular transitions from carbon monoxide and two other simple molecules (and also atomic hydrogen gas) in what are thought to be cold gas clouds near the nuclei of eighteen of the brightest galaxies in clusters. They find that the gas temperatures vary between about twenty and eighty degrees Kelvin, and that the gas velocities imply the material is falling towards the respective nuclear black holes. The new results are consistent with models of chaotic, cold accretion. The sources are not all identical in behavior, however, but as a group they support the model of molecular clouds drifting and infalling onto the black hole's environment, thereby triggering nuclear activity.
"Constraining Cold Accretion On to Supermassive Black Holes: Molecular Gas in the Cores of Eight Brightest Cluster Galaxies Revealed by Joint CO and CN Absorption," Tom Rose, A. C. Edge, F. Combes, M. Gaspari, S. Hamer, N. Nesvadba, A. B. Peck, C. Sarazin, G. R. Tremblay, S. A. Baum, M. N. Bremer, B. R. McNamara, C. O'Dea, J. B. R. Oonk, H. Russell, P. Salome, M. Donahue, A. C. Fabian, G. Ferland, R. Mittal, and A. Vantyghem, MNRAS 489, 349, 2019.