Supermassive black hole inflates giant bubble

Observations of the Radio Galaxy Messier 87 with the European Low-frequency LOFAR Telescope

October 29, 2012
Using the brand-new radio telescope LOFAR, an international team of astronomers led by the Max Planck Institute for Astrophysics have taken one of the best images ever of giant bubbles produced by a super-massive black hole. The picture - taken in a frequency range normally used by airplane pilots - shows what looks like a giant balloon filled with plasma. This bubble, which exceeds the size of an entire galaxy, was slowly inflated by one of the most massive black holes in our cosmic neighbourhood.

Surprisingly, black holes do not only swallow matter, never to be seen again, but they can also eject a fraction of the doomed particles. The expelled matter forms a hot plasma stream and leaves the black holes' host galaxy at a velocity close to the speed of light. As the plasma gradually slows down, it creates a large and extremely tenuous bubble engulfing the entire galaxy as well as its surroundings. This plasma bubble is invisible for optical telescopes, but is very prominent at low radio frequencies, as has now been shown by recent observations with the LOFAR telescope array.

"Our detection is of great importance because it shows the great potential of LOFAR so get a detailed view of galaxies and their surroundings", says Francesco de Gasperin from the Max Planck Institute for Astrophysics, first author of the study that will be published in the journal Astronomy & Astrophysics. "In terms of galaxy evolution, it provides compelling evidence of the strong interaction between the galaxy's super-massive black hole and the galaxy's surroundings", he adds. "Like symbiotic species, the galaxy and its super-massive black hole have intimately related lives, the galaxy providing matter to feed the black hole, and the black hole returning energy to the galaxy."

The scientists observed a huge elliptical galaxy, Messier 87 (M87), at the centre of a galaxy cluster in the constellation of Virgo. This galaxy is 2000 times more massive than our Milky Way and hosts in its centre one of the most massive black hole discovered so far, with 6 billion times more mass than our Sun. Far from being quiet, every few minutes this black hole swallows an amount of matter similar to that of the whole Earth, converting part of it into radiation and a larger part into powerful jets of ultra-fast particles, which are responsible for the observed radio emission. The image of the bubble produced by the black hole activity was made during the test-phase of the new International LOFAR Telescope (ILT) at radio frequencies between 25 and 160 MHz.

"This is the first time that such high-quality images have been possible at such low frequencies", says, Prof. Heino Falcke, Radboud University Nijmegen & Max Planck Institute for Radio Astronomy, chairman of the board of the ILT and co-author of the study. "This is one of the most difficult regions in the sky for a radio telescope - we would not have expected to get such high-quality results so early in the commissioning phase of LOFAR."

The information contained in the spectrum of the radio waves provides a track record of the activity of supermassive black holes. The team found that the bubbles formed by jets are surprisingly young, just about 40 million years, which is a mere instant on cosmic time scales. "What is particularly fascinating", says Andrea Merloni from the Max-Planck Institute of Extraterrestrial Physics in Garching, who supervised de Gasperin's doctoral work, "is that, by measuring the power of the large scale outflow observed by LOFAR, we learn a great deal about the violent processes of matter-to-energy conversion taking place very close to a black hole. In this particular case, the black hole seems to be much more efficient in accelerating the jet than producing visible radiation."

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