Very interesting galaxy

https://www.legacysurvey.org//viewer/?ra=345.9167&dec=1.0460&layer=hsc-dr2&zoom=14

Not only does this galaxy have some impressive tidal tails from a merger, it also has an AGN and there is a curious bright blue crescent within the outer shell of collision debris. Could this be starforming as a result of outward pressure from the AGN?

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You are correct that they are areas of star formation. This is a very common occurrence in the immediate aftermath of a collision.

My point was that this could be akin to the starburst region in Hanny’s voorwerp, not just a result of the collision. The shape and orientation of the streak of starburst does not seem to conform to any shock trail from the interaction (unlike the one on the other side of the nucleus, which does).

Oh, I see. There are all sorts of tidal forces at play here, with it being a mid-stage merger. Material is ending up all sorts of places. Doesnt look like a voorwerp to me, so my money’s still on star formation caused by the collision.

If you crank the contrast and brightness controls you will see the feature more clearly:

Screenshot 2021-09-17 at 10.57.20

This streak is clearly separate from - and distinctly differently oriented than - the shells of tidal debris formed by gravitational interaction during the merger. If it were gravitationally formed, I would not expect it to be truncated at either end so abruptly - there would probably be trails heading off a long way, rather like the topmost trail. These are the reasons I regard it as possibly indicative of some kind of interaction with the AGN, which may be precessing as a result of the recent merger and shooting a jet through the debris in a spiralling pattern. Judging by the small size and low intensity of the AGN signal in VLASS, and the absence of any jet plumes that would take many millions of years to form, this may be an example of an AGN that has only just switched on as a result of the recent merger.

And that is why I think it is noteworthy.

Its color is consistent with that of star formation, its part of a galaxy that is the process of merging, and the sdss spectrum indicates star formation. Perhaps you’re right, but i’m going with the significantly more likely option here.

Yes, it’s star formation. But what is causing it? There is star formation in Hanny’s Voorwerp that is the result of an outflow from the AGN striking the debris trail formed by the merger. Why would you not think it possible for a similar effect to be happening here as the AGN at the heart of this galaxy turns on following the merger?

The morphology here is not consistent with gravitational compression from the merger, as this streak is not aligned with any of the immediately adjacent shells of ejecta created by the merger. In fact, this streak crosses two of the shells as shown in the enhanced image, and it curves in the opposite sense, so I don’t think it fits the gravitational disturbance theory.

In my work I deal with people who go for “the most likely explanation” every day. They frustrate me, as they can’t be bothered to look at things critically enough. It is precisely because I DON’T go for “the most likely explanation” that I discovered a previously unknown phenomenon in semiconductor production. It was missed by every other expert who had been working on the project over several years, because they presumed it had the “most likely” explanation. It’s when you don’t dismiss things by choosing “the most likely explanation” without giving the subject sufficiently detailed scrutiny, that you make new discoveries. You won’t find something new every time, of course, but if you never bother to look at things critically enough you will never discover anything new.

Your reason for ruling out gravitational causes is that it isnt aligned with visible ejecta shells, but it could be in a foreground shell that is between us and the shells with visible boundaries, and have collided with another molecular cloud, triggering star formation. We see the images in 2d, but galaxies arent restricted to 2 dimensions. It all depends on the angle at which the galaxies collided.

Perhaps people want to go for the most likely explanation too often, but they will be correct in the majority of cases. And hey, maybe they will never discover anything new (though I dispute that, as anything that isnt explained by any current theories/understandings will inevitably result in a new theory). Looking for alternatives is never a bad thing, but perhaps if you do that every time, you may end up finding things that just arent there in many cases.

I don’t know which of our theories is correct. Perhaps neither of them are. This was a violent collision that has led to an extremely dynamic system. We can debate until the cows come home, but there’s no real way to prove either hypothesis as it stands.

Of course this cannot be resolved through simple discussion. It requires further study, but such study will not take place if people adopt the 'most likely cause" stance and don’t flag it for others to take a closer look.

I find your “logic” a little confusing. First you say that people who go for the most likely explanation will be correct in the majority of cases. Then you dispute that maybe they will never discover anything new, by saying anything that ISN’T explained by current theories will result in a new theory. So if they HAVE explained something by a current theory, by incorrectly choosing the “most likely” explanation, there won’t be any reason to come up with a new theory, will there?

It is my approach to science to challenge absolutely everything. Assuming that all your predecessors have got everything right gives you a poor foundation for discovery if they haven’t - that was precisely the case with my electrostatics work. Even after I had proven my contrary findings and they had been independently confirmed by others, there were “die-hards” in the electrostatic experts community who refused to accept that they had been wrong for thirty or more years and had built their careers on a falsehood.

And please, stop treating me like a five year old. Of course I understand that the universe is not two dimensional. I have pretty good spatial awareness and find it very easy to visualise things in 3D. I was even complimented on this during my PhD viva, after working out the crystal structures of metal surfaces from two-dimensional electron diffraction patterns. If you can explain how an isolated band of starburst like this could be formed at any point around this galaxy through gravitational effects, without any other similarly-oriented structures being visible nearby, I might accept your view, but at the moment you just seem to be dismissing any alternative in favour of the perhaps obvious and dare I say lazy interpretation.

You said they would “never” discover anything new. If something turns up (like hanny’s voorwerp) that is completely new, and isnt explained by any current theories, naturally we would have the ability to make the discovery and explain it with a new theory. Going for the most likely explanation may limit changes to current theory for phenomena we already know about, but it absolutely does not confine us to “never discover[ing] anything new”.

Don’t take this the wrong way, but I really don’t care about your electrostatic work. I have no doubt that you are excellent in your field, and have made some revolutionary breakthroughs, but there is a difference when you have a PhD and many decades of experience working in a field. You’re not exactly peddling a revolutionary new theory here. You just have a different interpretation of what is causing star formation in this galaxy. I’m not a die-hard astrophysicist who’s decades long career is based on gravitational reasons for starforming in galaxies; i’m a citizen scientist who is posting opinions on a discussion forum. I have no professional ego riding on what I post here, so I feel fairly comfortable going with the most likely explanation in this case.

Material is being thrown around left, right and center with such a dynamic system caused by this merger. It’s not inconceivable that a molecular cloud was ejected and collided with a thick medium of an ejecta shell and caused star formation. The momentum may even cause part of the resulting starforming region to move through a less dense region, hence the misalignment of part of the region with the visible shell boundaries.

Sorry for being a pedant about this, but if someone has incorrectly decided the existing theory explains something, they are NOT going to need to come up with a new theory, and hence they will not discover that the existing theory is wrong. Your logic is false.

My electrostatics work is merely an example of the thought process. The work itself is irrelevant to this problem, but the thought process isn’t, so you are wrong to ignore it as you clearly have. You are right, material can be thrown around in all directions by a merger. Weird shapes can be caused by interactions. But every once in a while, something different might be taking place, which would be rare (like Hanny’s Voorwerp). One has to look out for such things and be willing to explore them further to have any hope of making a new discovery. If you automatically write everything off as just a result of gravitational interaction, which seems to be your stance (just like the electrostatics experts in my example) you won’t go looking for anything else so you won’t find it, now will you?

Im pretty sure this is the one thing I agreed with you on?:

:man_shrugging:

I don’t automatically write everything off as gravitational interaction, but in this case, with such a huge amount of tidal activity, it would be miraculous if no material had collided to cause star formation. To me, it seems more likely to be the simpler explanation than an AGN jet. If you can attain more evidence that suggests it is in fact an AGN jet that has caused the star formation, i’d be happy to change my mind, but at the moment, i’m going with the preponderance of evidence.

I’m not a gravitational zealot. I just see more evidence for my hypothesis than yours at this time.

If you were agreeing with me, you have a very strange way of expressing it!

“If you can attain more evidence that suggests it is in fact an AGN jet that has caused the star formation, i’d be happy to change my mind, but at the moment, i’m going with the preponderance of evidence.”

…and how would you suggest that I might be able to come up with more evidence that this could be caused by a newly forming jet, other than by getting someone else to look at it with other equipment that could study it in other ways (as happened with Hanny’s Voorwerp)?

This situation is similar to the identification of potential gravitational lenses. Possible lenses can only be confirmed through spectroscopic analysis, which requires further study of the object. If you dismiss all possible lenses as probably being starburst and never bother to look at them more closely, you will never find out either way, will you?

You’re arguing that I shouldn’t just believe that it’s what the most likely explanation is. I’m asking what evidence I should base that change of opinion on. The only evidence you have presented for it being caused by a jet is that it isnt completely aligned with ejecta shells. I have provided an alternative explanation for this. I see no radio indication that there has been a jet. I see no x-ray indication that there has been a jet. I mean, i’m open to changing my mind if I see the right evidence, but at least give me something to work with…

I’ve already given the reasons for proposing that this may be the onset of jet formation from a newly formed AGN, but you have ignored them. The evidence that you seek will only be produced if someone looks at it with other tools than an optical telescope.

I don’t expect you to see it though - as Heywood put it “there are none so blind as those who will not see”.

I haven’t ignored them, I have rejected them as evidence of jets, as they all have simpler explanations.

If the evidence required to suggest that it could be jets needs to come from further observations, then I see no reason to believe its jets from the optical survey we currently have access to.

Good job with the quote though. You must be really confident in your hypothesis if you’re resorting to personal digs.

I’m not confident in my hypothesis beyond asking the question and putting forward some observations to justify asking the question.

No, the evidence required to suggest that it could be jets doesn’t need to come from further observation, because the optical evidence was sufficient for me to suggest it. Further evidence would show whether or not jets are involved. You wouldn’t bother to look for any other evidence, though, because your mind is already closed to the possibility of jets being involved. Hence the proverb is pertinent.

I’m open to the possibility, but so far you have not provided enough evidence to justify me believing that it is caused by jets. Your evidence that “suggests” that jets are the cause is not exclusive to that hypothesis. I believe that it is possible that jets are the culprit, but I believe that gravitational forces are significantly more likely based on the evidence I see.

You can’t propose a cause, provide ‘evidence’ that isnt conclusive, and then call me closed-minded when I don’t believe it. The proverb isn’t pertinent at all.

I feel like this thread is not going in a productive direction.

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