Radio emission from intergalactic gas, and its implications for low frequency astronomy in space

Abstract

Synthesis mapping of extragalactic systems at frequencies below 1.4 GHz has revealed important new clues about what we might discover at much lower frequencies in intergalactic space. State-of-the-art images at frequencies between 1420 MHz and 300 MHz can be considered important precursors to even lower frequency images such as might be generated by spaceor ground-based antenna arrays. In this talk I shall discuss how recent images at 327 and 408 MHz provide us with new information and some interesting clues about the physical phenomena which could be explored if we had sensitive, even lower frequency images at comparable or better resolution. The recent outfitting of both the NRAO Very Large Array and the Westerbork Synthesis Telescope (WSRT) with 327 MHz receivers has permitted us to generate images which combine the virtues of high dynamic range, good resolution and high sensitivity at a relatively low frequency. The DRAO Synthesis Telescope at 408 MHz, and the Cambridge 151 MHz array (also described at this meeting) though less sensitive, have wide field capability which is also important for some types of low frequency imaging which we would like to attempt at lower frequencies. In the following, I shall describe results of detailed mapping of intergalactic radio emission at 1.4GHz, 408MHz, and 326 MHz in the region of the Coma cluster of galaxies. The results provide some new information on intergalactic magnetic fields, and reveal large scale synchrotron emission at 327 MHz on intercluster scales for the first time. They point to potentially interesting discoveries that could be made in future at lower frequencies, where the diffuse intergalactic synchrotron emission will be even stronger. The Coma cluster, due both to its richness and relative proximity, is the most accessible "passive laboratory" for studying the intracluster medium of a cluster of galaxies. However, even for the Coma cluster the properties of its halo have been known in less than adequate detail. This is partly because most radio telescopes tuned to relatively low radio frequencies (at which radio halos are most prominent) have inadequate resolution, poorly known beam characteristics, or are insensitive to the faint, large scale radio emission.