4.3.2 Velocity field

Figure 18: Contours of the velocity field for the fitted models. Top: model with Gaussian profile; bottom: model with spine and shear layer. Contours are shown at intervals of 0.05$c$ and fiducial contours are labelled in the outer jets. The panels correspond to the same area projected on the plane of the sky and differ very slightly in size because the values of $\theta$ are not identical in the two models. Both cover very nearly 12 kpc in the plane of the jet.

Figure 19: Profiles of the velocity along streamlines for the SSL model. Full line: spine (on-axis) and shear layer $s = 0$; short dash: shear layer $s = 0.5$; long dash: shear layer $s = 1$ (jet edge).

The inferred velocity field is shown as contour plots in Fig. 18 and as longitudinal profiles in Fig. 19. The on-axis velocity of the inner jet is poorly constrained, although extremely low values ($\beta < 0.4$) are ruled out. The best fits show an abrupt decrease in velocity across the inner boundary (Fig 19; see Section 5.1 for a more detailed discussion), but continuity cannot be entirely ruled out. Further out, the velocity field is much better constrained (±0.1c and ±0.05c, for the flaring and outer regions) and both models agree almost exactly. The on-axis velocity remains roughly constant ( $\beta \approx 0.77$) between 1 and 2.5kpc and then drops abruptly to $\beta \approx 0.55$ at the outer boundary. Quantitatively, the exponent H in the velocity law for the flaring region (Table 4) is required to be > 3. Thereafter, $\beta$ declines smoothly to $\approx$0.22 at 12 kpc.

The transverse velocity profile is hardly constrained at all in the inner region. The best fits in the flaring and outer regions require an edge velocity close to 0.7 of the central value (Figs 18 and 19), independent of distance from the nucleus. The error analysis shows, however, that we cannot exclude a flat-topped profile at the inner boundary, so some evolution of the profile along the jet could occur. Very low velocities at the edge of the jet are not consistent with the observed sidedness ratios in these regions.

2002-06-13