Two researchers of the Universitat de València, Petar Mimica and Miguel Ángel Aloy, have participated in the observation of an eruption produced by a black hole that tore a star. The results have just been published in the magazine “Science”.
In January 2005, a twinkle was detected in the core of the galaxy in merging process Arp 299-B (which is located at 150 million light years from the Earth). First, they thought it was a supernova explosion. However, researchers have been observing it for ten years from different wave lengths and they have noticed that the bright region has spread. They concluded that it was actually a jet caused by a central supermassive black hole of the galaxy after tearing a star. The results of the study, which is led by the researchers Seppo Mattila (University of Turku, Finland) and Miguel Pérez Torres (Astrophysics Institute of Andalusia of the Spanish National Research Council, IAA-CSIC) are published in the magazine “Science”. The researchers Petar Mimica and Miguel Ángel Aloy of the Department of Astronomy and Astrophysics of the Universitat de València have participated in the study as well.
According to the theoretical models, in an event of disruption caused by a tidal force, in which the black hole tears a star, half of its mass is expelled to the space. The other half is absorbed by the supermassive black hole. The sudden injection of materials produces a twinkle (visible with X ray, gamma ray and optically) followed by transitory radio emissions and the formation of a material jet that initially moves to a similar velocity to the light speed.
“Never before have we been able to directly observe the formation and evolution of a jet from one of these events,” said Miguel Perez-Torres, of the Astrophysical Institute of Andalusia. He became a doctor in the Department of Astronomy and Astrophysics of the Universitat de València.
“As time passed, the new object stayed bright at infrared and radio wavelengths, but not in visible light and X-rays. The most likely explanation is that thick interstellar gas and dust near the galaxy’s centre absorbed the X-rays and visible light, then re-radiated it as infrared” said Seppo Mattila, of the University of Turku.
The researchers used the Nordic Optical Telescope in the Canary Islands and NASA’s Spitzer space telescope to follow the object’s infrared emission. Fortunately, the radio waves are not absorbed in the core of the galaxy, but find their way through it to reach the Earth. Only the presence of enough cold and dense gas can absorb totally or partially the emission of low radio frequency in the surroundings of the galactic centre of Arp 299-B. Taking advantage of this fact, they have been able to carry out constant observations with a network of international radio telescopes, including the European VLBI Network. “Collecting data patiently for over a decade gave us a great prize” stated Miguel Pérez Torres, the responsible of the interferometric array of radio observations that revealed the source of the radio emission that was expanding in one direction, as it was expected.
Petar Mimica, a researcher of the Universitat de València indicated that “These observations allowed researchers to determine that the materials within the jet moved at an average of 75,000 kilometres per second (a quarter of the speed of light) and that it was decelerating.” He added that the numerical simulations carried out in the Department of Astronomy and Astrophysics were crucial for determining the total energy of the jet and the density of the gas in the surrounding area.
The combination of the observations from different length waves during all this time permitted the group of researchers to dismiss scenarios such as a supernova explosion or a gamma-ray burst, determining that the most likely explanation was a supermassive black hole of Arp 299-B, containing approximately 20 million solar masses, had torn a star with a mass 2 or 6 times bigger than the one of the Sun. However, opting for this explanation instead of another more conventional in which the jet was produced by an active galactic nucleus (AGN) was not a simple process. “We had to use a modern numerical modelling that we created in the supercomputers of the Universitat de València,” indicated Mimica. “It was necessary to compose hundreds of different numerical models to estimate a coherent scenery that could explain how a jet could produce the radio emission that was being observed,” said Mimica.
The problem is that we cannot observe the jet, but its radiative track. “In reality, we could compare the jet with a river, and the matter that it emits in radio waves with an oil stain boiling on the surface. We won’t be able to see the water flowing at night, but the bright fire produced by the combustion of the oil,” explains Aloy. “Noticing that there is a river or even connecting the movement of the river with the combustion point can only be done by using a numerical simulation which is extremely accurate,” stated Aloy.
The best model that can be built requires two basic elements. On the one hand, the jet had to be extremely misaligned with regard to the rotation axis of the galaxy, which is very difficult to be compatible with a jet produced in an AGN. On the other hand, the jet needed to go through the cloud of dust and gases that surrounds the supermassive central black hole. Bringing together both elements, we have arrived to an explanation as convincing as surprising, because the very same cloud that toned down the jet in X rays within the visible range was the key to understand the relatively slow velocity of the jet, its deceleration and the partial absorption (but incomplete) of the radio waves in the first phases observed,” detailed Aloy.
Inactive black holes
Most galaxies contain supermassive black holes in their central regions that contain thousands of millions times the solar mass. It is the case of objects that have a very intense gravitational field so that not even light can go through it. They show a typical structure made by a disc of gas and dust (Accretion disk) that absorbs the materials in its surroundings and, in the case that it finds an active black hole, a couple of particle jets at relativistic speeds that merge in the poles. This ejection phenomenon is very common in radio-galaxies, quasars and AGNs.
“However, supermassive black holes remain inactive for a long time. The disruption activities caused by tidal forces, such as the one occurred in Arp299-B, give us a unique chance to study the vicinity of these powerful objects,” explains the scientific of CSIC. Mattila added that because of the fact that the central regions of the galaxies contain great quantities of gas, which absorb the light in X rays and optically, it is possible that this phenomenon is much more usual but have gone unnoticed.
It is believed that this phenomenon was more common in the early phases of the universe. Therefore, the study contributes to understand the environment in which the galaxies were born thousands of millions of years ago. Researchers of 26 international institutions have collaborated in this project, including the Universitat de València and the Centre of Astrobiology (Joint Centre of the CSIC and the Spanish Technical Aerospace Institute).
S. Mattila, M. Pérez-Torres, A. Efstathiou, P. Mimica, M. Fraser, E. Kankare, A. Alberdi, M. Á. Aloy, et al. A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger. Science. DOI: