[Picture: AUGER Collaboration, click to enlarge]
The above plot by the AUGER collaboration shows the celestial sphere in galactic coordinates. This is essentially a map of the sky much like a map of the earth, where the equator is the Milky Way's galactic plane and the sun is in the center. The black circles with a radius of 3.1° show the arrival directions of the 27 highest energy cosmic rays detected by AUGER with energies greater than 57 x 1018 eV. The red stars show the positions of 472 active galactic nuclei (AGN) within 75 megaparsecs (Mpc) distance to the earth. The blue region defines the field of view of Auger; deeper blue indicates larger exposure, and thus more expected events. The solid curve marks the boundary of AUGER's field of view.
The white * is Centaurus A, the closest AGN. Two of the 27 cosmic rays have arrival directions within 3° of this galaxy.
The data analysis depicted here has been fairly new, and we reported on it in October. The AUGER Collaboration finds correlations between the events of highest energies and AGNs and they are able to reject the hypothesis of an isotropic distribution of these cosmic rays at a confidence level of 99%. This is interesting for two reasons.
First, it is the first time that the sources of these cosmic ray events could be shown to be correlated with the AGN. This reliably rules out speculations about the origin of these UHECRs in local, galactic sources. Though it has been expected, until now there was no experimental confirmation that they originate outside our galaxy. Though one should note that this correlation does not necessarily mean the AGNs themselves are the sources, as the sources could just also be correlated with the AGNs.
Second, this correlation vanishes if one includes AGNs further away than ~90 Mpc, which is what one would expect from the GZK cutoff: at this high energy, a proton's mean free path is below ~ 90 Mpc because the protons will scatter at the CMB background and form pions. The vanishing correlation is thus an independent confirmation for the presence of the cut-off.
For me this is one of the most important experimental results of the year.
This post is part of our 2007 advent calendar A Plottl A Day.