Now – for the first time – astronomers have pictures of the formation and expansion of the fast-moving jet of material which was ejected when the powerful gravity of a supermassive black hole tore apart the unlucky star.
Only a small number of events of this kind – so called Tidal Disruption Events (TDEs) – have ever been detected.
Previously, theorists had suggested that material pulled from a doomed star such as this formed a rotating disk around the black hole, emitting intense X-rays and visible light. It was also thought that it would launch jets of material outward from the poles of the disk at nearly the speed of light. Theories which have proved to be correct.
The first indication of this material pulled from doomed star came in January 2005 when astronomers discovered a bright burst of infrared emissions coming from the nucleus of one of the colliding galaxies in Arp 299, nearly 150m light-years from Earth. Later that year radio observations revealed a new, distinct source of emission from the same location as the new bright infrared source, which they were able to follow as it evolved.
For the next few years the team watched this new object using many of the most powerful telescopes in the world. While it remained bright at infrared and radio wavelengths, no visible or X-ray light was detectable. The location of this source of infrared emissions was buried deep within the dust enshrouded heart of one of the colliding galaxies. It meant all of the X-ray and visible light expected to come from this source was absorbed by the dust before being re-emitted at longer wavelengths, in the infrared.
The researchers used the Nordic Optical Telescope on the Canary Islands and Nasa's Spitzer space telescope to follow the object's infrared emission.
Over the course of nearly 10 years, the radio emissions were tracked using a technique called Very Long Baseline Interferometry. This involves remotely connecting many of the world's most powerful radio telescopes – including the European VLBI Network along with several e-MERLIN telescopes in the UK – in a series of simultaneous and coordinated observing campaigns.
This technique yields extremely high-resolution imaging and provided the only method available to image the evolution of this new phenomena. The results showed that the source of radio emission was expanding in one direction – just as expected for a jet. The measured expansion indicated that the material in the jet moved at an average of a quarter the speed of light.
Tidal Disruption Events like this are important episodes in the life of black holes and are thought to have been much more common in the distant universe. Disruption events, especially those witnessed in relatively local systems such as this, are important and unique opportunities to understand the consequences of such events, including how super-fast jets of material are formed and how they evolve.
Only a few of these types of events have previously been witnessed with most seen in visible light in optical surveys. This event is different because it's location was shrouded by thick layers of dust meaning that no visible light could be seen at all. This location, of course, is where we may expect such events to occur – in the centres of galaxies containing a black hole and where large numbers of stars are forming. As such this discovery may be the tip of the iceberg of a hitherto hidden population of similar dramatic events.
