There may be fewer supermassive black hole binaries at the cores of galaxies than previously thought, says a team of scientists from Brandeis University in Waltham, Massachusetts, and Raman Research Institute in Bangalore, India.
Two black holes are entwined in a gravitational tango in this artist’s conception. Image credit: NASA.
Most massive galaxies in the Universe are thought to harbor at least one supermassive black hole at their centers.
When two such galaxies collide, their black holes join in a close orbital dance that results in the pair combining. That process is the strongest source of the long-sought gravitational waves, still yet to be directly detected.
“Gravitational waves represent the next great frontier in astrophysics, and their detection will lead to new insights on the Universe. It’s important to have as much information as possible about the sources of these waves,” said team member Dr David Roberts of Brandeis University.
Dr Roberts and co-authors studied a sample of galaxies called ‘X-shaped radio galaxies,’ whose peculiar structure indicated the possibility that the radio-emitting jets of superfast particles ejected by disks of material swirling around the central black holes of these galaxies have changed directions. The change was caused by an earlier merger with another galaxy, causing the spin axis of the black hole as well as the jet axis to shift direction.
Working from an earlier list of 100 such objects, they collected archival data from the Karl G. Jansky Very Large Array (VLA) radio telescope to make detailed images of 52 of them.
Their analysis of the new images led them to conclude that only 11 are ‘genuine’ candidates for galaxies that have merged, causing their radio jets to change direction. The jet changes in the other galaxies came from other causes.
Extrapolating from this result, the astronomers estimated that fewer than 1.3% of galaxies with extended radio emission have experienced mergers. This rate is 5 times lower than previous estimates.
“This could significantly lower the level of very-long-wave gravitational waves coming from X-shaped radio galaxies, compared to earlier estimates,” Dr Roberts said.
“It will be very important to relate gravitational waves to objects we see through electromagnetic radiation, such as radio waves, in order to advance our understanding of fundamental physics.”
Dr Roberts and his colleagues reported their results in a pair of papers in the Astrophysical Journal Letters (arXiv.org preprint) and the Astrophysical Journal Supplements (arXiv.org preprint).
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David H. Roberts et al. 2015. The Abundance of X-Shaped Radio Sources: Implications for the Gravitational Wave Background. ApJ 810, L6; doi: 10.1088/2041-8205/810/1/L6
David H. Roberts et al. 2015. The Abundance of X-Shaped Radio Sources I. VLA Survey of 52 Sources With Off-Axis Distortions. ApJS 220, 7; doi: 10.1088/0067-0049/220/1/7
