A search for steep low-frequency radio spectra among quasars and clusters of galaxies

Abstract

Using published flux densitiesS at low frequenciesv, radio spectra were constructed for 3C, 4C, and 4CT radio sources in Abell clusters of galaxies, radio galaxies outside Abell clusters, and quasars with known redshifts. About half the sources in rich Abell clusters (richness classesR>-2) have steep spectra between 38 and 178 MHz with spectral indices α ¹⁷⁸₃₈ > whereS∝v ^−α. However, radio galaxies outside clusters have values of α ¹⁷⁸₃₈ ⩽ 1.2, and no steep spectra were found among 170 quasars. The radio sources in rich clusters are probably confined by intergalactic gas, and the steep spectra develop over a period of ∼10⁹ yr as relativistic electrons lose energy. The absence of steep spectra among quasars does not necessarily mean that quasars never occur in rich clusters of galaxies, since quasars are probably being observed only in their early high-luminosity phases. The possibility that some quasar events occur in the nuclei of the dominant cD galaxies in clusters is discussed, but quasar events may occur in more than one type of galaxy.