SAMI observes shifty black holes as loners, not pairs
After two galaxies merge, their two supermassive black holes will fall to the centre of the merged galaxy, orbit each other in a binary system, and finally coalesce into a single black hole. It has been suggested that galaxies near the end of this process could be spotted thanks to the orbital motions of the black holes. In particular, if one of the black holes is accreting material, and consequently is lit up as an active galactic nucleus (AGN), its orbit with the other black hole means that the light it emits will be slightly blueshifted or redshifted relative to its host galaxy. However, there are other ways this shift can be produced, so more information is needed to know if a shifted, or offset, AGN is in a binary orbit or not.
Two galaxies with offset AGN were recently observed as part of the SAMI Galaxy Survey. The survey obtains spatially resolved spectroscopy for each galaxy, giving a highly detailed picture of the stars and ionised gas. This allowed the SAMI team, led by CAASTRO Affiliate Dr James Allen (University of Sydney), to get a much better understanding of the histories of these galaxies than had previously been possible. Each is an interesting object in its own right, but in both cases the evidence points to other explanations for the offset AGN, not a binary orbit. The first galaxy appears to have undergone a merger with a smaller galaxy, about 100 million years ago. The gas that was brought in by the smaller progenitor has not yet settled down into a stable configuration, but is moving through the galaxy with a very different velocity to the stars, causing the observed shift. In the other case, the AGN is powering an outflow of gas pointed towards us, and the shift is simply due to observing this outflow. The fact that no evidence was found for either of these galaxies containing binary supermassive black hole systems suggests that offset AGN may not, after all, be an efficient way of finding such systems.
J. T. Allen, A. L. Schaefer, N. Scott, L. M. R. Fogarty, I.-T. Ho, A. M. Medling, S. K. Leslie, J. Bland-Hawthorn, J. J. Bryant, S. M. Croom, M. Goodwin, A. W. Green, I. S. Konstantopoulos, J. S. Lawrence, M. S. Owers, S. N. Richards, R. Sharp in MNRAS (2015) “The SAMI Galaxy Survey: Unveiling the nature of kinematically offset active galactic nuclei”