A galaxy as particle accelerator

Related images
(click to enlarge)

Image: NASA and the Hubble Heritage Team STScI/AURA

Image: MAGIC/HESS/VERITAS/VLBA-Team

It is one of the largest among the giants: With two to three billion times the mass of our sun, the galaxy Messier 87 dominates the Virgo cluster. A supermassive black hole exists in the centre of this galaxy. So called jets (gigantic plasma flows) shoot out from the vicinity of the black hole at close to light speed. Scientists - among others from the Max Planck Institutes for Nuclear Physics and Physics - have observed, simultaneously in gamma and radio frequencies, this active galactic core region. Thereby they discovered that the elementary particles are accelerated to extremely high energy levels in closest proximity to the black hole (Science Express, July 2, 2009). Messier 87 is a giant elliptical radio galaxy in our immediate cosmic vicinity, just 55 million light years away. In its centre, it hosts a super-massive black hole, about six thousand million times more massive than our own Sun. In a "jet", a giant outflow from the central engine of this galaxy, charged particles (electrons and protons) may be accelerated to velocities close to the speed of light. Inevitable witnesses and messengers of these acceleration processes are very-high-energy gamma rays, photons a thousand billion times more energetic than optical light. They are produced when the accelerated particles interact with their environment.

High-energy gamma rays constitute the highest energy electromagnetic radiation observable, and are generated by the most violent cosmic objects such as supernovae, active galactic nuclei, and gamma ray bursts. They allow us to study a realm of extreme physical conditions, far beyond what can be studied in laboratories here on Earth.

First indications for very-high energy gamma radiation from Messier 87 were found as early as 1998 with the HEGRA telescopes, which was the predecessor experiment of H.E.S.S. and MAGIC. This detection could be confirmed with the H.E.S.S. telescopes in 2006. These observations also revealed a fast variability of the gamma-ray flux within few days, as seen again in the 2008 campaign. This implies that the extension of the gamma-ray source must be exceptionally compact, and is located presumably in the immediate vicinity of the supermassive black hole in Messier 87.

In early 2008, the three world-leading instruments sensitive to gamma rays in this energy domain, H.E.S.S., MAGIC and VERITAS, jointly observed Messier 87 from January to May 2008, collecting 120 hours' worth of data. During this campaign, Messier 87 underwent two major outbursts of gamma-ray emission. Simultaneous high resolution radio observations of the activity of Messier 87 using the VLBA, a system of ratio telescopes spanning the United States, indicate a persistent increase of the radio flux from the innermost "core" of Messier 87, which is associated with the immediate vicinity of the central black hole. The collaboration of observatories sensitive to the lowest (radio) and highest (gamma-ray) parts of the electromagnetic spectrum made it possible for the first time to pin down the location of activity during the gamma-ray outbursts and thus the site of the particle accelerator in Messier 87.

Source: Max-Planck-Gesellschaft