Exploring M87's Jet: Questions & Answers

[RJ Emery]

I have several questions concerning M87's jet:

1. From observations by the HST of the jet, its speed was determined to be several times faster than that of light. The superluminal effect has been attributed to visual effects, a conclusion I do not dispute. However, I do not understand how the measurement was made and what visual effects would give rise to a FTL result. Anyone have any insights?

2. Has there been an opposite jet detected?

3. If M87's jet were pointed directly towards us, what would we likely see (across the spectrum from gamma to radio)?

4. We have some measurements and knowledge of the galaxy with its jet pointed away from us. How might that data set be different with a head on view?

 

[Nereid]

I have several questions concerning M87's jet:

1. From observations by the HST of the jet, its speed was determined to be several times faster than that of light. The superluminal effect has been attributed to visual effects, a conclusion I do not dispute. However, I do not understand how the measurement was made and what visual effects would give rise to a FTL result. Anyone have any insights?

IIRC, the measurement was not made by the HST, but by one or more VLBI radio astronomy observations. 'Bright spots' (aka 'knots') in the jet were observed to have different positions on the sky, at different times. Geometry, plus a distance to M87, gives the apparent speed of these knots.

I don't recall the particular mechanism used to explain the supraluminal speed, for the M87 jet, but it's an illusion, much like the apparent speed of a lighthouse beam on a (very) distant screen, or the point where a giant pair of scissors closes - you have to separate what's actually moving from what seems to be moving.

2. Has there been an opposite jet detected?

IIRC, yes. However, a quick search failed to find any relevant papers, so I may be mis-remembering (or simply didn't search long enough).

3. If M87's jet were pointed directly towards us, what would we likely see (across the spectrum from gamma to radio)?

Pretty much the same as a http://en.wikipedia.org/wiki/Blazar" . I don't know how bright it would be - could even be unaided eye visible! - but it would certainly be spectacular. One thing that would be exciting would be the TeV gammas and UHECRs that we'd likely see coming from it.

4. We have some measurements and knowledge of the galaxy with its jet pointed away from us. How might that data set be different with a head on view?

As above.