Radio Galaxies: Supernova to Synchrotron

Abstract

One of the earliest surprises yielded by the science of radio astronomy was that galaxies emit radio waves. The processes that produce visible light the nuclear burning in stars were self-evident in galaxies. Radio waves hadn't been expected there. Yet some galaxies emit as much energy in radio as they do in light. Furthermore, the radio-emitting matter of galaxies generally extends a long distance beyond the visible portion of the galaxy. This extension raises rather serious problems of organization and dynamics. Centaurus A was the first radio galaxy discovered arid remains the nearest known radio galaxy, about 10 million light-years away. Its shape is typical of the breed. Visibly, Centaurus A is a bright, compact ellipse with a dark lane or band of dust across its middle. In radio Cen A has two teardrop-shaped lobes extending at right angles to the dust band and reaching far beyond the bright disk to a distance of 50 or 60 kiloparsecs (160,000 to 200,000 light-years). The question is what makes these lobes and what maintains them? David De Young of Kitt Peak National Observatory now presents a theory that answers those questions at least in part. Since the beginning of radio astronomy it has been known that the radio emissions of Cen A and other radio galaxies are synchrotron emission, the radiation of energetic electrons spiraling around in a magnetic field. So the question becomes how to ensure a continuous supply of energetic electrons. De Young begins with recent observations made at Cerro Tololo Inter-American Observatory in Chile that all is not dark in the radio lobes. Sensitive observation can see groups of massive young stars there. De Young postulates that in the core of Cen A is a large rotating mass of gas. Some of this gas continually spews out along the poles of rotation. As it moves through the inner part of the galaxy, it picks up material, debris ejected from stars, and drags it out into the lobes. In the lobes stars like the ones observed can be formed from this mixture of primordial and processed matter. Some of these stars will be so massive that they last only a short time, say 10 million years, before they explode as supernovas. Supernovas yield lots of energetic electrons. The sequence continues for eons: gas from the core, new stars, supernovas, energetic electrons. What remains to be explained is why the gas is ejected from the core in the first place. -D. E. Thomsen 0