[Kryptid]

Do you have an idea which kind of theories they have used to form this simulation?

The theory of gravity, most importantly.

Is it all realistic?

Take a look at the video I posted. It compares a simulated galactic collision simulation with observations of real galactic collision. So yes, I'd say it is realistic.

Actually at any other point of time in the simulation, we didn't get that "direct line" which is the most important feature in the real photo.

It's a matter of degree. Not all galaxy collisions are going to look exactly the same. If you want one that looks more like that, take a look at this simulation:

The time between 0:31 to 0:34 has the galaxies connected by a straight line of gas and stars.

Could it be that they didn't take in their simulation the real impact of this tidal force?

Simulating gravity would automatically simulate tidal forces.

Hence, do you agree that Tidal force is a key ellement in the formation of spiral galaxy?

I thought we were talking about a galactic collision, not the formation of a galaxy (unless you are talking about the formation of a new, unified galaxy after the collision of two different galaxies has settled down).

Hence, it is not about a gravity impact between the real matter (stars) in each galaxy, but between each star to the dark matter in the galaxy.

It's both.

How do they know the real impact on the dark matter during this collision?

We know the distribution of dark matter in our galaxy because we've measured it via star velocities.

Why they assume that while stars are ejected outwards from the galaxies, the combined dark matter would move to the center of the galaxies?

Some of the dark matter would likely be ejected as well.

[Origin]

I mean the impact due to dark matter

Then you need to ask that question I'm not the telepathy guy...

[evan_au]

in our current understanding each star is connected by gravity to the center of its galaxy by the dark matter?

Our star is connected to the supermassive black hole in the center of our galaxy by the mutual gravitation of the Sun and  the supermassive black hole.
- The same goes for every other star in our galaxy
- The same goes for every star and every other star in our galaxy
- The same goes for every star and all the Dark Matter in our galaxy (whatever it is)

Unfortunately, when you get 3 or more objects interacting gravitationally, it is too hard to write down a general equation for their motion ("in closed form" is the jargon)
- So astronomers have to use simulations of the gravitational interactions between stars and dark matter when simulating one galaxy, or the interactions between colliding galaxies.

[Origin]

Do you confirm that based on our current understanding each star is connected by gravity to the center of its galaxy by the dark matter?

Why do you do that?  You start with "do you confirm" and then state some convoluted story that is partially correct and partially your made up stuff with just plain wrong concepts sprinkled in.
So the simple answer to your question is no.  The longer answer is; part yes, part kind of and part no.

[Dave Lev (OP)]

in our current understanding each star is connected by gravity to the center of its galaxy by the dark matter?

Our star is connected to the supermassive black hole in the center of our galaxy by the mutual gravitation of the Sun and the supermassive black hole

No!
You have a severe mistake.
In the following article it is stated:
https://www.e-education.psu.edu/astro801/book/export/html/1863

"If you take the semi-major axis of the Sun's orbit to be 8 kiloparsecs and the orbital period to be 250 million years, you can determine that the Milky Way's mass interior to the Sun's orbit is approximately 10^11 solar masses, or 100 billion times the mass of the Sun."
The SMBH mass is only 4*10^6 solar masses.
Therefore, it is clear that Our SUN is NOT connected to the supermassive black hole in the center of our galaxy by the mutual gravitation of the Sun and the supermassive black hole as the SMBH has not enough mass to hold the Sun by gravity.
Therefore, our scientists claim that there is a need for dark matter.

Please let me know if I understand correctly the current main stream explanation:
The Sun motion is affected by two kinds of gravity forces at the same time.
1. The gravity due to dark matter that holds it in its orbital motion around the galactic center.
However, the dark matter can't hold the Sun in the galactic disc by itself. There is a need for one more gravity force.
2. The Gravity due to the galactic disc that holds it in the disc. Therefore, based on the main stream the Sun is bobbling around the disc while it orbits around the galactic center

Please look again at the following video at 0:12
https://svs.gsfc.nasa.gov/10687
We clearly see that the galactic disc is lifted due to the collision.
Once you lift the disc you actually destroy the second gravity force that keeps the stars on the disc.
Hence, there is no way to keep the disc structure of the galaxy.
At the best scenario, you would get a spherical galaxy.
Do you agree with that?

 

If you want one that looks more like that, take a look at this simulation:
The time between 0:31 to 0:34 has the galaxies connected by a straight line of gas and stars.

Yes, this one looks better, however, there are still few issues.
Let me just focus on the following two issue:
1.In the simulation we see that those two massive galaxies collide which each other while their galactic disc is fully aligned with each other. What is the chance for that?
2. Please focus on the arm that is connected between the galaxies.
We see that at 0:31 there is a significant mass cloud around it.
At 0:35 that mass cloud is already absorbed in the arm and makes it thicker.
The arm keeps itself to almost 0:51. However, it gets more concentrated over time
Therefore, don't you agree that this arm is all about stars that are connected/bonded to each other by gravity.
https://www.nasa.gov/image-feature/goddard/2022/hubble-inspects-a-pair-of-space-oddities
Hence, why do you refuse to accept the simple idea that the bright line that we see in Nasa Photo is all about stars (or cluster of stars) that are connected to each other by gravity?

[Kryptid]

1.In the simulation we see that those two massive galaxies collide which each other while their galactic disc is fully aligned with each other. What is the chance for that?

There are an awful lot of galaxies in the Universe. You'd expect collisions like that (or close to that) to happen once in a while. Perhaps the galaxy pair you have been talking about this whole time is an example of exactly that.

Therefore, don't you agree that this arm is all about stars that are connected/bonded to each other by gravity.

Gravity most certainly is an important factor in the shape of the arm.

Hence, why do you refuse to accept the simple idea that the bright line that we see in Nasa Photo is all about stars (or cluster of stars) that are connected to each other by gravity?

I'm not refusing it. I acknowledge that gravity plays an important role here. To be honest, I still don't really get what your new theory is and how it differs from what we already know about physics.

[Dave Lev (OP)]

Gravity most certainly is an important factor in the shape of the arm.

Therefore, don't you agree that this arm is all about stars that are connected/bonded to each other by gravity.

Gravity most certainly is an important factor in the shape of the arm.

Hence, why do you refuse to accept the simple idea that the bright line that we see in Nasa Photo is all about stars (or cluster of stars) that are connected to each other by gravity?

I'm not refusing it. I acknowledge that gravity plays an important role here.

Many Thanks!!!
I really appreciate that you are willing to accept the idea that the arm which is connected between the two galaxies is all about Gravitational arm where stars and gas clouds are connected to each other by gravity.

[Kryptid]

I really appreciate that you are willing to accept the idea that the arm which is connected between the two galaxies is all about Gravitational arm where stars and gas clouds are connected to each other by gravity.

I guess that depends on what you mean when you say "all about".

[Dave Lev (OP)]

I really appreciate that you are willing to accept the idea that the arm which is connected between the two galaxies is all about Gravitational arm where stars and gas clouds are connected to each other by gravity.

I guess that depends on what you mean when you say "all about".

I mean that each star in that gravitational arm bridge holds itself by gravity force to the other stars in the arm

[Kryptid]

I mean that each star in that gravitational arm bridge holds itself by gravity force to the other stars in the arm

In a sense... but it's not like some sturdy structure like the steel beams that hold a building up. The distances between those stars are constantly changing. This arm is dynamic, unstable, and will eventually lose its current form.

[Dave Lev (OP)]

I mean that each star in that gravitational arm bridge holds itself by gravity force to the other stars in the arm

In a sense... but it's not like some sturdy structure like the steel beams that hold a building up. The distances between those stars are constantly changing. This arm is dynamic, unstable, and will eventually lose its current form.

Well, this arm is dynamic and stable or unstable as any orbital system that holds itself by gravity.
I fully agree that eventually it should lose its current form as expected from any orbital system.
The life time of a star in that arm is not relevant.
Sooner or later all the stars would be ejected from the arm (as a single star or as a cluster of stars)

[Kryptid]

So what is it about contemporary science that you are disputing here? I assume you put this into "New Theories" for a reason.

[Origin]

So what is it about contemporary science that you are disputing here? I assume you put this into "New Theories" for a reason.

I'm slightly curious also.  His idea that gravity holds galaxies together doesn't seem like a new theory.

[Halc]

Do you confirm that based on our current understanding each star is connected by gravity to the center of its galaxy by the dark matter?

The word 'connected' implies a physical EM connection like bars and strings and such, stuff of which objects of varying degree of rigidity are made.  Stars are not connected in this way. They instead interact via gravity.  All matter attracts all other matter as described by F=GMm/r². That's it.

The visible structure of a galaxy is due to more than just gravity since without the other forces there'd be nothing to see at all, an there'd be perhaps a gathering of mass but no stars, arms, black holes, or anything. All those things require more than just gravity to form. No, they're not 'connected'. There's effectively empty space between the parts.

As for the 2nd part of your statement, there's nothing in F=GMm/r² that distinguishes dark matter from baryonic matter. There's not a separate formula for dark matter. It's the same either way.

Why they assume that dark matter would move in the expected direction and density?

Who is 'they'?  Nobody is assuming this. Nobody sees the dark matter. It isn't in your picture. The dark matter does the same as the regular matter, except it doesn't interact via the other forces. The evidence of EM interaction is very obvious in some of the simulations, especially the super-symmetric one.

Please let me know if I understand correctly the current main stream explanation:

Dave, if you actually did understand something, I don't think you could bring yourself to display that understanding. It's not what you do.

The Sun motion is affected by two kinds of gravity forces at the same time.

Wrong. There's only one kind of gravity. Newtonian mechanics works just fine for a galaxy collision, and F=Mm/r² describes it exactly for every kind of matter.

The gravity due to dark matter that holds it in its orbital motion around the galactic center.

Orbital motion is a term usually reserved for simple 2-body systems in a Keplerian relationship. The path of no object around the galaxy is really an orbit. It isn't elliptical. It happens to sort of look like a circle drawn by somebody with Parkinson's disease, but it's pushing it to call it an orbit since there's no object being orbited.

There's nothing special about the galactic center. There's no special 'connection' to it. There's nothing particularly big there. We're attracted to all matter everywhere (not just in our galaxy), and F=Mm/r² perfectly governs our path, plus whatever other forces are involved such as meteors hitting the Earth which definitely divert it from the path gravity would otherwise have taken it.

[Dave Lev (OP)]

Do you confirm that based on our current understanding each star is connected by gravity to the center of its galaxy by the dark matter?

The word 'connected' implies a physical EM connection like bars and strings and such, stuff of which objects of varying degree of rigidity are made.  Stars are not connected in this way. They instead interact via gravity.  All matter attracts all other matter as described by F=GMm/r². That's it.

Dear Halc
Thanks for your excellent explanation.
Yes, I fully agree that: "All matter attracts all other matter as described by F=GMm/r²" and from now on I should call it: "interact" (or Interaction) via gravity instead of connect (or connection) via gravity.

You discuss about the "Bar":

The word 'connected' implies a physical EM connection like bars and strings and such

There is a bar in the MY galaxy.
We can clearly see this BAR at the center of our galaxy:
https://phys.org/news/2016-07-earth-milky.html
Do you confirm that this bar is there due to gravity interaction?
However, interaction with what???
If it was just a gravity interaction between the stars in the bar it was expected to get a spherical shape as a nice cluster of stars.
Evan _au offered an excellent explanation for tidal force that could form a Bar:

would you agree that an object on Earth should fall upward to the moon due to gravity?

Yes, it does - it is called a "tide".
- But the gravitational force of the Moon (1/80 the mass of the Earth, at a distance of almost 400,000km)
- Is much less than the gravitational force of the Earth (at a distance of 6,300km)
- So the ocean tide is pretty small (typically 1 or 2 meters around here)
- And there is an equivalent tidal force on the far side of the Earth

Please look again at the following image:
https://phys.org/news/2016-07-earth-milky.html
We clearly see that the bar has also gravity interaction with the two main spiral arms:
Preseus & Scutum-Centaurus
Therefore,
Do you agree that tidal gravity interaction between the stars/matter in the bar to the stars/matter in the two main spiral arms have set the bar shape (similar to the tidal impact between the earth/moon)?

Now let's go back to the simulation:
https://svs.gsfc.nasa.gov/10687
Please watch it at 0:16

Watch the video Origin posted. At 0:16,  it looks a lot like that galaxy pair you posted. Then just watch the video from there to see what happens.

We clearly see the gravitational bridge arm and we also see the free arm that is located at the far side of each galaxy.
Based on my calculation, the "free arm" keeps its structure for almost 720 M year.

3. If we focus on the free arm of each galaxy, in this simulation it took it almost from 014 till 0:44 (30 sec = 720M years) to complete only one orbital cycle.

Without gravity interaction between all the stars in this free arm, how could it keep its structure for so long time?
Do you trust this simulation?
"The scientific theoretical model and the computational data output were developed by Drs. Brant Robertson and Lars Hernquist. AVL rendered more than 80 gigabytes of this data using in-house rendering software and Virtual Director for camera choreography."
Do you trust this work by Drs. Brant Robertson and Lars Hernquist?
If so, don't you agree that this "free" arm (exactly on the other side of the "gravitational arm bridge") is also there due to Gravity/tidal interaction?
Hence, can we also call it "gravitational arm"?
If you agree with the above, why can't we consider the two main spiral arms in the MY galaxy as gravitational arms?

Please look at the following image:
https://cdn.britannica.com/40/167240-050-FEAB05E2.jpg
We see that those two main arms go all around the galaxy (more or less).
Please remember - 70% of the galaxies in the universe are spirals.
What kind of force can keep spiral arms in their spiral structure for so long time except of gravity force?

Please also see the image of the galaxy from the side.
Do you confirm that the spiral arms get narrower as they are located further away from the center?
At the base of the arm (3KPC) - it is 3000LY, while at the edge of the arm (12- 15KPC) - it is 400LY.
What kind of force can do it except of gravity force?

[Kryptid]

it was expected to get a spherical shape

By whom?

What kind of force can keep spiral arms in their spiral structure for so long time except of gravity force?

I'm not aware of any astrophysicists who claimed that it wasn't gravity that held the arms together.

[Dave Lev (OP)]

I'm not aware of any astrophysicists who claimed that it wasn't gravity that held the arms together.

Wow!
So do you confirm that each star in the gravitational spiral arm is interacted with the other stars in the arm by gravity force and goes together wherever the arm goes?

[Kryptid]

So do you confirm that each star in the spiral arm is interacted with the other stars in the arm by gravity force and goes together wherever the arm goes?

Only to an extent. If you watch the simulation, those stars don't form an arm indefinitely, so those stars definitely aren't staying together long-term.

[Dave Lev (OP)]

So do you confirm that each star in the spiral arm is interacted with the other stars in the arm by gravity force and goes together wherever the arm goes?

Only to an extent. If you watch the simulation, those stars don't form an arm indefinitely, so those stars definitely aren't staying together long-term.

Yes, I fully agree.
Gravity interaction is limited in time duration.
It could be one million years or one billion years.
However, eventually, all the stars in the arm would be ejected outwards.
That is very clear.
Hence, as long as the star is interacted by gravity to the arm, it would go with the arm wherever it goes.

[Kryptid]

Hence, as long as the star is interacted by gravity to the arm, it would go with the arm wherever it goes.

Only to an extent.