D. Evidence for the Presence of a Counter-Jet
The twin-relativistic jet model of Blandford and Rees (1974) supposes that the central engine is rotationally symmetric and should produce two diametrically opposed jets. The fact that M87 has two lobes and two jet-like structures on large scales (> 1.5 kpc) supports such a picture, but efforts to detect a counter-jet on scales 1 kpc have been entirely unsuccessful. Limits on the ratio of the jet to counter-jet brightness have been derived from radio data, and a conservative limit is > 150:1 (BOC89). A somewhat stronger limit (> 380:1) can be obtained if we assume the counter-jet would appear identical to the jet, but this is perhaps a poor assumption, since light travel time effects (among others) could cause the jet and counter-jet to look very different. The limiting factor here is not the quality of the radio images, but rather the radio emission from the lobes, whose complex morphology could easily camouflage a weak counter-jet. Stronger limits of > 450:1 are obtained in the optical (Stiavelli, Möller, Zeilinger 1992), where the galaxy light presents a simple background which is easily subtracted.
An important advance in this area is the recent discovery of an optical "hot spot" which is placed precisely where we might expect the end of the counter-jet to be located (Stiavelli et al. 1992; Sparks, Macchetto, and Owen 1992; Figure 19). This hot spot can also be seen in older images in the literature (e.g., Keel 1988). This new feature is located opposite the jet to a precision well within the jet's width. And its distance from the nucleus of 25" places it at a distance corresponding to knot H at the end of the visible jet. It is spatially coincident with a bright arc of emission seen in the radio. It is virtually certain that the emission from this feature is synchrotron emission, and not, for example, line emission. It is seen in at least three optical bands (V, R, and I), and both its radio-to-optical spectral index (0.86) and optical spectral index (1.9 ± 1.0) are similar to those of knot H. Furthermore, its optical polarization properties are similar to those of the spatially coincident radio emission. (We note this new feature appears unassociated with Arp's counter-jet (1967) which is slightly more distant from the nucleus, and consists primarily of line emission.)
Figure 19. Comparison of optical and radio morphology of hot spot in M87 east radio lobe (inside boxes on left). The optical image (top) is the sum of I, R, and V band observations; the radio image (bottom) is from 15 GHz VLA observations. Nucleus is near center of each panel with the jet extending towards the right. North is up.
This optical hot spot provides evidence for an unseen counter-jet, because the lifetime of the electrons emitting at these frequencies is very short. At radio frequencies the electron lifetime is about 106 years, so that the radio emission can be ascribed to events in the distant past. However, the lifetime for the optically emitting electrons in the hot spot is only about 1.6 x 103 years, indicating that something is supplying high-energy electrons at nearly the present epoch. The most obvious candidate for such a supply is some sort of unseen counter-jet.