What would the sky look like at long radio wavelengths?

Abstract

At wavelengths of about 10 meters the appearance of the sky is dominated by the intense synchrotron background radiation of the Galaxy. Several ionized-Hydrogen regions in the Galaxy make their appearance as discrete absorption features seen against this intense background radiation. Some of the Galactic supernova remnants are resolved even at moderate resolutions (∼0.°5) of most of the existing long-wavelength sky surveys. Away from the Galactic plane a large number of extragalactic sources are seen, most of which are unresolved at this resolution and are identified with radio galaxies and quasars while a very small fraction of them are due to clusters of galaxies. Only a couple of these sources are millisecond pulsars seen as continuum sources in the confusion-limited long wavelength surveys. At the resolution (∼ 20) and sensitivity (∼ 1 mJy/beam) that the Low Frequency Space Array is expected to achieve at wavelengths of about 10 meters, most of the supernova remnants, ionized-Hydrogen regions, halos in near-by clusters and halos around galaxies should be resolved. An all-sky survey carried out by the Space Array is expected to be at least two orders of magnitude more sensitive compared to the existing large scale sky surveys. A large population of millisecond pulsars might be detectable as continuum sources in such a survey. At wavelengths much longer than 10 meters, the Galaxy is no longer optically thin and at about 100 meters wavelength even the Warm Ionized Medium becomes optically thick for a path length of about 2 kpc. Hence, at such long wavelengths, the Galactic radio emission from only the solar neighborhood would be detectable. Observations in the wavelength range of 10 to 100 meters can thus lead to a 'tomographic' study of the Galaxy.