Seventy-five years ago H. D. Curtis noted a "curious straight ray" in M87 which was "apparently connected with the nucleus by a thin line of matter" (Curtis 1918). This observation marked the first discovery of an extragalactic jet, though its physical nature, as well as that of its host nebula, would remain obscure for several more decades. Later, Virgo A would be one of the first discrete sources discovered by the new "radio" astronomy, and its association with M87 (Bolton, Stanley, and Slee 1949) and the "curious" feature (Baade and Minkowski 1954) were quickly recognized. Eventually radio interferometer images revealed the bright radio lobes and the optical jet itself (Turland 1975). Similarly, the jet would be detected at X-ray frequencies, once the resolution was adequate to separate it from the bright thermal emission (Schreier, Gorenstein, and Feigelson 1982; Figure 1). While several hundred jets have now been discovered (Bridle and Perley 1984; Keel 1988; Fraix-Burnet et al. 1990; Liu and Xie 1992), M87 remains one of the nearest examples, making it an ideal target for study of the extragalactic jet phenomenon. Only one other extragalactic jet is substantially closer, and that is Centaurus A (NGC5128) which is at a very low declination and therefore is difficult to study with northern radio interferometers.
Figure 1. M87 at radio, optical, and X-ray bands. All images shown at similar scale, where 1 kpc = 12.8" assuming a distance of 16 Mpc. North is up. (a.) Radio image (5 x 109 Hz) at 0.4" resolution from VLA. Nucleus, jet, and radio lobes are prominent. (b.) Optical image (4 x 1014 Hz) obtained with Palomar 1.5 m telescope. Resolution 1.2". The nucleus, knots A, B, and C in the jet, and stellar light are prominent. (c.) X-ray image (2 x 1017 Hz) obtained with Einstein satellite. Resolution is ~ 3". Nucleus and knot A are evident. Original data from BSH91.
M87 (NGC4486) is now recognized as a giant elliptical (E0/1) galaxy near the center of the Virgo Cluster. It is one of two dominant galaxies in the cluster, the other being NGC4472. The associated radio source (Virgo A, 3C274, 1228+127) is classified as an FR-I source based on its low luminosity (P178MHz ~ 1 x 1025 W H-1) and edge-darkened morphology (Fanaroff and Riley 1974). In spite of this classification, its one-sided jet is more typical of FR-II sources, and perhaps this is related to its situation near the FR-I/II division at P178MHz ~ 3 x 1025 W H-1. Bridle (1984) has also noted that it has an unusually weak core for a one-sided jet. Its distance has been measured to be 15.9 Mpc using a redshift independent technique (Tonry 1991; z = 0.0043). We will assume a distance of 16 Mpc throughout this paper; this gives a linear scale of 78 pc per arcsecond, and a proper motion of 1 milliarcsecond (mas) per year corresponds to 0.254c, where c is the velocity of light.
In the following section I review the present state of the observational material, and briefly describe some straightforward conclusions drawn from the observations. In Section 3 some problems posed by the observations, and a possible model for the kinematics, are discussed.