Parsec-scale radio cores in early-type galaxies

Abstract

We find compact (<0.03 arcsec) radio-continuum cores in about 70 per cent of radio-emitting elliptical and S0 galaxies over a wide range in total radio power (1021–1026 WHz–1 at 5 GHz). The cores usually have a flat or rising spectrum between 2.3 and 8.4 GHz, with a median spectral index of + 0.3. Even at low luminosities, the radio emission from most elliptical galaxies appears to be powered by a parsec-scale ‘engine’ like those in classical radio galaxies and quasars. The core and total radio power are related (⁠|${P_{{\rm{core}}}}\, \propto \,P_{{\rm{total}}}^{0.7}$| on average), and the parsec-scale cores of radio galaxies are typically one hundred times more powerful than those in ‘normal’ giant elliptical galaxies.