<p>I think the most common explanation for this kind of structure is ram pressure from the galaxy's motion through the surrounding <a href="https://en.wikipedia.org/wiki/Intracluster_medium" rel="noreferrer">intracluster medium (ICM)</a>. In the rest frame of the galaxy and jet, the ICM is blowing past with a velocity of several hundred km/sec, and this is enough to divert the flow of the jet. In this picture, the jet isn't moving back <em>toward</em> the galaxy; this is a projection effect (the inner jet is thought to be pointed toward us, only 30 degrees or so away from our line-of-sight to the galaxy).</p>

<p>A second factor may be that the point where the jet seems to bend is where its smooth, relativistic flow breaks up due to the <a href="https://en.wikipedia.org/wiki/Kelvin%E2%80%93Helmholtz_instability" rel="noreferrer">Kelvin-Helmholtz instability</a>. The initial small kink visible at the right-hand edge of the HST-WFPC2 image (upper right) is probably a sign of the instability starting to disrupt the jet. Once the smooth flow of the jet breaks up, it is likely to slow down, making it more easily diverted by the ram pressure of the ICM.</p>

CC BY-SA 4.0
2018-09-03T17:58:19.013Z
2018-09-03T17:58:19.013Z
Peter Erwin
<p>The end of the relativistic jet that emerges from M87 looks curved like a hook. See here:</p>

<p><a href="https://i.sstatic.net/WQvir.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/WQvir.png" alt="enter image description here"></a></p>

<p>Data from the VLA array show more of the jet: at the far end, the jet makes a complete 180 degree turn backwards toward the galaxy it emerges from.</p>

<p>What forces the jet to make such a drastic turn around? </p>

CC BY-SA 4.0
2018-09-03T15:37:19.987Z
Interesting question.  I would guess that it is due to precession of the spin access of the accretion disk that forms the jet having the jet point in slightly different directions over time, but I couldn't begin to do the math.
antlersoft
CC BY-SA 4.0
2018-09-06T14:25:05.490Z
I assume you meant "spin axis". That would't make the jet go backwards. I guess a combination of precession, huge proper motion, and our viewing angle could give complex results. But please see Peter Erwin's answer.
PM 2Ring
<galaxy>
Why is M87's relativistic jet bent?
h22
# Why is M87's relativistic jet bent?

(Asked by: username_0 on 2018-09-01)

The end of the relativistic jet that emerges from M87 looks curved like a hook. See here:

[![enter image description here](https://i.sstatic.net/WQvir.png)](https://i.sstatic.net/WQvir.png)

Data from the VLA array show more of the jet: at the far end, the jet makes a complete 180 degree turn backwards toward the galaxy it emerges from.

What forces the jet to make such a drastic turn around?

**Tags:** galaxy

> **Comments:**
> **username_1**: Interesting question.  I would guess that it is due to precession of the spin access of the accretion disk that forms the jet having the jet point in slightly different directions over time, but I couldn't begin to do the math.  
> **username_2**: I assume you meant "spin axis". That would't make the jet go backwards. I guess a combination of precession, huge proper motion, and our viewing angle could give complex results. But please see username_3's answer.  

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## 1 answers

### Answer by username_3 on 2018-09-03. Score: 7 | Accepted answer

I think the most common explanation for this kind of structure is ram pressure from the galaxy's motion through the surrounding [intracluster medium (ICM)](https://en.wikipedia.org/wiki/Intracluster_medium). In the rest frame of the galaxy and jet, the ICM is blowing past with a velocity of several hundred km/sec, and this is enough to divert the flow of the jet. In this picture, the jet isn't moving back *toward* the galaxy; this is a projection effect (the inner jet is thought to be pointed toward us, only 30 degrees or so away from our line-of-sight to the galaxy).

A second factor may be that the point where the jet seems to bend is where its smooth, relativistic flow breaks up due to the [Kelvin-Helmholtz instability](https://en.wikipedia.org/wiki/Kelvin%E2%80%93Helmholtz_instability). The initial small kink visible at the right-hand edge of the HST-WFPC2 image (upper right) is probably a sign of the instability starting to disrupt the jet. Once the smooth flow of the jet breaks up, it is likely to slow down, making it more easily diverted by the ram pressure of the ICM.


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