An article published in the journal “Monthly Notices of the Royal Astronomical Society: Letters” reports a study on a supermassive black hole surrounded by a disk of materials that revealed unexpected features. A team of researchers led by Stefano Bianchi, of the University of Roma Tre, Italy, used the Hubble Space Telescope to observe the nucleus of the galaxy NGC 3147 discovering a small and soft disk, a reduced version of the large disks typical of active galactic nuclei, a structure that shouldn’t exist. The discovery represents a new opportunity to test some relativistic effects but could force astronomers to review certain models on active galactic nuclei.
Approximately 130 million light years from Earth for an estimated diameter of about 140,000 light years, the galaxy NGC 3147 is of the spiral type, like the Milky Way, but it seems one of its few common features. It’s a dim galaxy that was studied several years ago as a type 2 Seyfert galaxy candidate, which is a type of galaxy with an active galactic nucleus that presents spectral lines generated by strongly ionized gas and at the same time has an innermost region, called Broad Line Region (BLR), not visible. The supermassive black hole at the center has a mass estimated at 250 million times the Sun’s.
Because of its characteristics, the galaxy NGC 3147 seemed an excellent candidate to obtain confirmation that below a certain brightness there are no longer any accretion disks around supermassive black holes at the center of a galaxy. The result was the opposite as they found a tenuous structure that seems a reduced version of those typical of quasars, where, however, the materials are heated to the point of generating strong electromagnetic emissions. Such a tenuous structure is a surprise and it’s the first time it’s been discovered.
Another unique feature of that accretion disk is that it’s close enough to the black hole that the speed of the materials and the gravitational force of the supermassive black hole have a strong influence on the way we see the light emitted. Hubble’s Space Telescope Imaging Spectrograph (STIS) instrument made it possible to study those materials, which rotate around the black hole at a speed faster than 10% of the speed of light. The gas seems brighter when it moves towards the Earth and dimmer when it moves away from it, a phenomenon called Doppler boosting or relativistic beaming.
This situation is useful to perform new tests of the general and special theory of relativity. The models that didn’t predict the possibility that there could be an accretion disk similar to that of a quasar but very small will have to be reviewed because they’re not correct in the case of weak active galaxies such as NGC 3147. In essence, it was a very good discovery to investigate those extreme phenomena.