Acceleration in radio galaxy lobes.

Acceleration in radio galaxy lobes

Viewed by their radio emmissions, galaxies appear a good deal larger than they do in visible light. The visible part of the galaxy usually lies in the center between two huge lobes of radio-emitting material, while are many times the size of the visible portion. Astrophysiciss believe that extremely energetic processes produce these radio lobes. Now, in the Feb. 6 NATURE, two astronomers from the Max Planck Institute for Astronomy at Heidelberg, West Germany, report an important piece of direct evidence for such processes.

The scientists, K. Meisenheimer and H.-J. Roser, have measured the polarization of the light from a visible hotspot on the outer edge of one such radio lobe, the southern lobe of the galaxy 3C33. This is the first time the optical polarization of such a hotspot has been measured, they say, and the optical polarization exactly matches the polarization of the radio waves from the lobe. Such a match is evidence that the hotspot belogns to the lobe and is not a chance association of some completely different object shining through the lobe. Moreover, detection of synchrotron radiation over such a wide range of frequencies from radio to optical is evidence that highly energetic processes are accelerating electrons in the lobe.

Synchrotron radiation comes from accelerated electrons that are forced by an ambient magnetic field to follow helical paths. The corkscrewing motion of the electrons gives their emissions a strong polarization in a particular direction, and to an astronomer the finding of such polarized radiation indicates the presence of this synchrotron mechanism. According to Meisenheimer and Roser, the importance of finding the optical polarization is that the electrons that produce the optical frequencies must move in very short paths. Their radiation, therefore, maps the regions where the acceleration is happening more closely than does that of the radio waves, whose source electrons move much farther from the place of acceleration while they are radiating.

Meisenheimer and Roser did this work at the European Southern Observatory at Cerro La Silla, Chile, using the Max Planck Society's 2.2-meter telescope located on La Silla mountain. The very high sensitivity of changed coupled devices (CCDs), the photoelectronic sensors now used for the most delicae astronomical imaging, enabled them to dtermine the optical polarization of the hotspot. To determine polarization, these astronomers inserted into the telescope a rotatable plane-parallel double-calcite plate. Calcite separates a beam of light into two parts polarized at right angles to each other and sends them over slightly different paths. On the CCD this procedure makes two images of every object.

If the light from a given object is predominantly polarized in a particular direction, and if the calcite is rotated properly to align with it, the two imaes will show a marked difference in brightness; most of the light will take one path, the one corresponding to the dominant polarization. "After the first 30-minute exposure, a highly polarized optical object ner the hotspot of 3C33 south was immediately conspicuous," Meisenheimer and Roser write.

It is therefore possible for the first time to discuss the nature of the radiation-producing processes in such a hotspot on the basis of a range of frequencies emitted that runs from 1 billion to 100 trillion hertz, Meisenheimer and Roser say. On that basis, considering the probable physical characteristics of the neighborhood, they think it reasonable that the electrons are accelerated in shock waves.

This concurs with a belief based on the shapes of the lobes that they are material forcefully ejected by some high-powered "engine" in the center of the galaxy. (There is much other evidence for such an engine.) As such material moves outward through the tenuous gas in intergalactic space, it should produce shocks at its leading edges.