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A kiloparsec-scale internal shock collision in the jet of a nearby radio galaxy


Jets of highly energized plasma with relativistic velocities are associated with black holes ranging in mass from a few times that of the Sun to the billion-solar-mass black holes at the centres of galaxies1. A popular but unconfirmed hypothesis to explain how the plasma is energized is the ‘internal shock model’, in which the relativistic flow is unsteady2. Faster components in the jet catch up to and collide with slower ones, leading to internal shocks that accelerate particles and generate magnetic fields3. This mechanism can explain the variable, high-energy emission from a diverse set of objects4,5,6,7, with the best indirect evidence being the unseen fast relativistic flow inferred to energize slower components in X-ray binary jets8,9. Mapping of the kinematic profiles in resolved jets has revealed precessing and helical patterns in X-ray binaries10,11, apparent superluminal motions12,13, and the ejection of knots (bright components) from standing shocks in the jets of active galaxies14,15. Observations revealing the structure and evolution of an internal shock in action have, however, remained elusive, hindering measurement of the physical parameters and ultimate efficiency of the mechanism. Here we report observations of a collision between two knots in the jet of nearby radio galaxy 3C 264. A bright knot with an apparent speed of (7.0 ± 0.8)c, where c is the speed of light in a vacuum, is in the incipient stages of a collision with a slower-moving knot of speed (1.8 ± 0.5)c just downstream, resulting in brightening of both knots—as seen in the most recent epoch of imaging.

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Figure 1: A comparison of HST images of the jet in 3C 264 from 1994 to 2014.
Figure 2: Position versus time for knots B and C.
Figure 3: Change in optical flux at 6,000 Å in the colliding knots B and C over 20 years.


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E.T.M. acknowledges HST grant GO-13327.

Author information

Authors and Affiliations



E.T.M. performed the HST data analysis and wrote the paper. B.S., J.A., R.P.v.d.M. and S.T.S. were consulted on and contributed to the HST data analysis. M.G. contributed to the interpretation and performed the theoretical calculations in consultation with E.T.M. E.P. contributed radio data. J.B., C.N. and M.C. provided insight into the design of the observations and interpretation. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Eileen T. Meyer.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Position versus year for knot A.

Positions are taken from the contour analysis method. Errors are 1σ from contour-derived position measurement added to the systematic error on the mean position from the image registration in quadrature.

Source data

Extended Data Figure 2 Position versus year for knot D.

Positions are taken from the contour analysis method. Errors are 1σ from contour-derived position measurement added to the systematic error on the mean position from the image registration in quadrature.

Source data

Extended Data Figure 3 Mean flux along the midline of the jet.

Flux is averaged transversely over 0.1″ at each step along the jet. The epoch is denoted by arrows. Note the decay of knot A with time, while the knot B+C complex appears substantially brighter in the last epoch.

Extended Data Figure 4 Comparison of 1983 VLA midline flux to 1994 HST midline flux.

Flux is averaged transversely over 0.1″ at each step along the jet. We fit parabolic forms to the peaks seen in the radio contour to derive rough estimates of knot location, with values depicted with short dashed lines. Dotted lines connecting matched knots are shown to guide the eye.

Extended Data Figure 5 Comparison of 1983 radio positions of knots with HST data.

The locations of knots B and C in 1983 are denoted by triangles, and we include an estimated 15 mas error on the position based on a typical centroiding error of 10% of the beam size (0.15″). Optical data are identical to that described in Fig. 2. For knot C, a dashed line gives the fit to epochs 1983 and 1994. For knot B, the dashed line is a parabolic fit to all data. The location of knot A is noted with a dotted line.

Source data

Extended Data Figure 6 Depiction of background subtraction method.

a, The final 2014 image stack before subtraction. b, The mask used in modelling the galaxy and dust disk. (White areas were not used in fits.) c, The final 2014 galaxy and dust model. d, The same image as a after c has been subtracted. Physical scaling of all images and flux scaling of a, b and d are identical.

Extended Data Figure 7 Depiction of modelling results.

a, The resulting background-subtracted model image for the 2002 epoch. b, The real background-subtracted 2002 image. In both cases the image box is shown by the red outline. Only pixels within this area were used in the fit.

Extended Data Table 1 Maximum-likelihood modelling results

Supplementary information

Motion of the knots in 3C 264 from 1994 – 2014

These images correspond to the panels in Figure 1. Contours correspond to the 30% flux-overbackground level. Background galaxy light has been subtracted and the jet rotated into a horizontal position. Knot B, with an apparent speed of 7c, is highlighted in red. (MP4 124 kb)

Motion of the knots in 3C 264 from 1994 – 2014

No contours are plotted to allow an unobstructed view of the jet, which is shown in its native orientation with North up. (MP4 64 kb)

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Meyer, E., Georganopoulos, M., Sparks, W. et al. A kiloparsec-scale internal shock collision in the jet of a nearby radio galaxy. Nature 521, 495–497 (2015).

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