The Jet of M87 at Tenth-Arcsecond Resolution: Optical, Ultraviolet, and Radio Observations^*

The European Space Agency's Faint Object Camera (FOC) on the Hubble Space Telescope (HST) was used to obtain optical and UV imaging of the jet of M87 prior to the HST refurbishment. We present a detailed comparison to new VLA radio observations at similar spatial resolution and flux information for the radio, optical, and UV bands.

While the radio and optical images present a remarkable degree of similarity, there are nevertheless significant differences. The optical/UV images show intrinsically higher contrast than the radio, with compact regions of emission localized within the knots. The jet is narrower in the optical/UV; the emission is more concentrated to the jet's center in the optical/UV than in the radio band. The radio-to-optical spectral index of the interknot regions is steeper than that of the knots themselves. There are also differences in the detailed knot structure of the optical emission compared to the radio, and there is a weak overall spectral steepening with distance from the nucleus beyond knot A.

The jet does not show pronounced limb brightening in the optical/UV. This indicates that the emission occurs within the jet and not in a thin boundary around the jet, as in some jet models.

We explore an idealized synchrotron model of jet emission and derive volume-deprojected physical parameters for the model. While the knots themselves are overpressured with respect to the surroundings, the pressure of the interknot regions in this observationally based model drops to the ambient external pressure, as in theories that invoke jet recollimation at shocks. Alternatively, internal shocks may be triggered by boundary instabilities or time-dependent power output from the nucleus. Downstream from knot A, the situation is less ordered, although in situ acceleration near the jet's center line also seems to be required, as the optical jet remains narrower than the radio jet there.