Magnetisation of the IGM

Abstract

Magnetic fields are ubiquitous in the Cosmos. They are observed on all scales -- from our solar system to galaxy clusters millions of light years away. The dynamo theory and the theory of gravitational compression during structure formation in the early Universe explain how magnetic fields are generated. However, magnetohydrodynamic equations tell us that both these mechanisms require the presence of an initial seed field, which is then amplified. The origin of this seed field remains unclear. It could be either primordial, or it could have been generated during structure formation. For the latter case, one of the possibilities suggested by theory and tested by simulations is that the magnetic seed fields could have been generated in the very first stars, and then in the process of galactic evolution, amplified and driven out by galactic winds into the inter-galactic medium (IGM). According to the standard LCDM cosmological model, low-mass dwarf galaxies were large in number in the early Universe. Hence, it is possible that they could have played an important role in magnetising the IGM by this process.<br /> Low-frequency radio observation is one of the ways to find observational evidence for this theory, because synchrotron emission from cosmic sources, which is most easily detectable at low radio frequencies, is a tracer of total magnetic field strengths. Also, the Faraday effect, that is very significant at low frequencies, can be a diagnostic tool to detect weak magnetic fields. With this motivation, we observed two nearby star-forming dwarf galaxies NGC 1569 and NGC 4449 at a low frequency of 350 MHz, with the Westerbork Synthesis Radio Telescope, in full polarisation.<br /> We detect radio continuum emission from both galaxies and find that the extent of the synchrotron halo of NGC 1569 is larger than that found in all higher radio frequencies. When analysed using complementary images at higher frequencies, the spectra in the core of both galaxies are found to be very flat (-0.4), which might point towards non-linear diffusive shock-acceleration. The break in the spectra of both migrates towards lower frequencies with time. The equipartition magnetic field strength in both the halos reaches down to 4 muG, which is comparable to that predicted by simulations. We estimate the spectral ages of both the galaxies and find high wind velocities in both, which shows that winds from these galaxies can indeed drag magnetic fields and cosmic-ray electrons into the IGM. Rotation measure synthesis performed on NGC 1569 was unable to detect polarised emission.<br /> We conclude that our observational investigation gives reasonable evidence that starburst dwarfs may have played an important role in magnetising the IGM. The detection of IGM-scale magnetic fields and of weak polarised emission from dwarf galaxies are most probably limited by telescope sensitivity. Observations at even lower frequencies with new telescopes like LOFAR (or SKA in the near future), which will have far higher sensitivity, should be able to detect larger synchrotron halos and consequently better constrain the role of starburst dwarf galaxies in magnetising the IGM.