Recent observations of the diffuse Galactic gr emission (DGE) by the {it Fermi} Large Area Telescope ({it Fermi}-LAT) have shown significant deviations, above a few GeV until about 100 GeV, from DGE models that use the GALPROP code for the propagatio
n of cosmic ray (CR) particles outside their sources in the Galaxy and their interaction with the target distributions of the interstellar gas and radiation fields. The surplus of radiation observed is most pronounced in the inner Galaxy, where the concentration of CR sources is strongest. The present study investigates this {it Fermi}-LAT Galactic Plane Surplus by estimating the gr emission from the sources themselves, which is disregarded in the above DGE models. It is shown that indeed the expected hard spectrum of CRs, still confined in their sources (SCRs), can explain this surplus. The method is based on earlier studies regarding the so-called EGRET GeV excess which by now is generally interpreted as an instrumental effect. The contribution from SCRs is predicted to increasingly exceed the DGE models also above 100 GeV, up to gr energies of about ten TeV, where the corresponding surplus exceeds the hadronic part of the DGE by about one order of magnitude. Above such energies the emission surplus should decrease again with energy due to the finite life-time of the assumed supernova remnant sources. Observations of the DGE in the inner Galaxy at 15 TeV with the Milagro gr detector and, at TeV energies, with the ARGO-YBJ detector are interpreted to provide confirmation of a significant SCR contribution to the DGE.
All galaxies without a radio-loud AGN follow a tight correlation between their global FIR and radio synchrotron luminosities, which is believed to be ultimately the result of the formation of massive stars. Two colliding pairs of galaxies, UGC12914/5
and UGC 813/6 deviate from this correlation and show an excess of radio emission which in both cases originates to a large extent in a gas bridge connecting the two galactic disks. We are aiming to clarify the origin of the radio continuum emission from the bridge. The radio synchrotron emission expected from the bridge regions is calculated, assuming that the kinetic energy liberated in the predominantly gas dynamic interaction of the respective interstellar media (ISM) has produced shock waves that efficiently accelerate nuclei and electrons to relativistic energies. We present a model for this acceleration and calculate the resulting radio emission, its spectral index and the expected high-energy gamma-ray emission. It is found that the nonthermal energy produced in the collision is large enough to explain the radio emission from the bridge between the two galaxies. The calculated spectral index at the present time also agrees with the observed value. The deviation of these two interacting galaxy systems from the standard FIR-radio correlation is consistent with the acceleration of an additional population of electrons. This process is not related to star formation and therefore it is expected that the systems do not follow the FIR-radio correlation. The acceleration of relativistic electrons in shocks caused by an ISM collision, in the same way as described here, is likely to take place in other systems as well, as in galaxy clusters and groups or high-redshift systems.
