Fermi LAT observes the core and giant lobes of the close-by radio galaxy Centaurus A

May 5, 2015

Centaurus A (Cen A) is one of the brightest radio sources in the sky: it is a giant elliptical Galaxy about 10 million light years away, making it the closest active galaxy we know. A remarkable feature of the radio image of this galaxy is that the bright central source is accompanied by a pair of giant radio "lobes," thought to be fuelled by relativistic jets generated in the dynamical process of gas accretion around the super-massive black hole residing at the galaxy's center. New Fermi observations provide direct evidence of electrons with energies beyond 100 GeV in these lobes - either they are being accelerated in situ, or the ride from the galaxy center was easier than previously thought...

The total angular extent of the Cen A radio lobes is ~10deg, corresponding to ~20 times the  apparent size of the full moon and making Cen A one of the largest discrete objects on the sky. The implied physical size of these structures is also enormous: it is around 2 million light years, many times the size of the Milky Way. Today, such double-lobed radio structures around galaxies have become the defining feature of radio or "active" galaxies in general. In the particular case of Cen A, the central black hole is millions of times more massive than the one in the center of our own Galaxy. There are indications that its jets might produce the highest energy cosmic-rays observed to be hitting Earth's atmosphere with energies a hundred million times larger than particles produced in the largest man-made particle accelerator.

The Fermi LAT has identified gamma-ray emission from both the core and the giant radio lobes of Cen A and measured the spectrum of the two components individually with high precision between 100 MeV and several tens of GeV. The analysis was conducted by a multi-national team with significant contributions from former KIPAC researchers Lukas Stawartz and Teddy Cheung. The measurements show that the lobes emit over ten times more energy in gamma rays than at radio wavelengths in which they were discovered. Furthermore it is the first direct evidence of electrons with energies beyond 100 GeV in these lobes, as such energies are required to produce the observed photon spectrum. Since high energy photons lose energy quickly by interaction with the magnetic field and background radiation, the observed electron energies in combination with the size of the lobes implies that either particles are accelerated in the giant lobes itself or very effectively transported from the core to large distances.

The core of Cen A has been a known gamma-ray emitter from EGRET and HESS observations, even up to TeV energies, however with the more accurate Fermi measurements and the possibility to separate the lobe and core emission regions, interesting new conclusions can be drawn about the nature of the particle acceleration occurring close to the central black hole and about the structure of the jet. The features of the radio-to-gamma-ray spectrum indicate that a single population of high-energy particles cannot explain simultaneously the emission up to 100 MeV and the emission at several hundred GeV. The speed of the jet also seems to be lower than the usual speeds observed in more distant active galaxies. Finally, the gamma-ray emission we see from this source is compatible with Cen A being a significant source of the ultra-high energy cosmic rays observed in the Earth's atmosphere. 
Centaurus A is a truly unique object on the sky, which can allows us to study many important aspects of particle acceleration in highly relativistic jets, and their propagation in the adjacent intergalactic medium. With the continuously ongoing observation by Fermi we will be able to further improve our knowledge of the spectral properties and morphology of this source in the near future.

Tidbit author: Markus Ackermann