How spiral galaxy works?

[OceanBreeze]

Here is a link to an arXiv open access paper by Jerry Sellwood and Karen Masters on “Spirals in Galaxies”

I haven’t read the entire article yet but the introduction looks interesting:

“Spirals in galaxies have long been thought to be caused by gravitational instability in the stellar component of the disk, but discerning the precise mechanism had proved elusive. Tidal interactions, and perhaps bars, may provoke some spiral responses, but spirals in many galaxies must be self-excited. We survey the relevant observational data and aspects of disk dynamical theory. The origin of the recurring spiral patterns in simulations of isolated disk galaxies has recently become clear and it is likely that the mechanism is the same in real galaxies, although evidence to confirm this supposition is hard to obtain. As transient spiral activity increases random motion, the patterns must fade over time unless the disk also contains a dissipative gas component. Continuing spiral activity alters the structure of the disks in other ways: reducing metallicity gradients and flattening rotation curves are two of the most significant. The overwhelming majority of spirals in galaxies have two- or three-fold rotational symmetry, indicating that the cool, thin disk component is massive. Spirals in simulations of halo-dominated disks instead manifest many arms, and consequently do not capture the expected full spiral-driven evolution. We conclude by identifying areas where further work is needed.”

 

I have read elsewhere:

The wave pattern is thought to be caused by some sort of disturbance which results in the wave. Think of dropping a pebble into a bucket of water. The disturbance can be caused by the self-gravity of the stars acting on gas clouds (the spirals are only observed in galaxies that contain gas) or the disturbance can be caused by some interaction between two galaxies.

[Dandav]

23 hours ago, OceanBreeze said:

Here is a link to an arXiv open access paper by Jerry Sellwood and Karen Masters on “Spirals in Galaxies”

I haven’t read the entire article yet but the introduction looks interesting:

Thanks for your support and for the above interesting article.

It is statute:

"3.2. Tidally-driven  Spirals:  A spiral pattern may also be triggered by the tidal field of a passing companion galaxy: M51 and M81 are particularly clear examples."

"Simulations have also shown that tidal encounters can trigger bar formation".

"We have only a few specific models of spirals being tidally driven, and we need to know the mass range and orbit parameters of encounters with companion galaxies that can excite a spiral response without triggering a bar (§3)."

Hence, once we accept the idea that tidal could have an impact on the Oval shape of the galaxy, we can go on and try to verify how this tidal force really works.

Please look at the following image:

 

 

We see that Tidal has two main forces.

1. Vertical Tidal force - This vertical force squeezes the objects (from pole to pole).

2. Horizontal tidal force - The horizontal force push the matter outwards almost symmetrically from the center to both sides.

Due to those forces, the object is converted from a ball shape to Oval shape.

Therefore, if we start the simulation with a ball shape central bulge (with billions of stars), and set around it in an orbital motion a smaller star cluster (with millions of stars), the central bulge should get the oval shape.

In other words, (based on correct mass proportional between the objects) due to the vertical tidal force, the massive ball shape central bulge with a starting radius of 2R, could potentially be squeezed by its poles to a radius of only 1R, while due to the horizontal tidal force that push the stars outwards almost Symmetrically to both sides the horizontal radius would be increased from 2R to 3R.

This can explain why the major horizontal radius at the milky way central bulge is 3KPC while its minor vertical radius is only 1KPC.

The vertical tidal force, would prevent from a star at the bar to move backwards. Therefore, as the central bulge rotates, it is expected that a star that is located at the major axis at 2KPC would orbit twice the velocity of a star that is located at only 1KPC at the same axis.

Let's verify the MILKY WAY Galaxy Rotation Curves at the central bulge (up to 3KPC)

 

 

https://www.astronomy.ohio-state.edu/thompson.1847/1101/lecture_darkmatter_darkenergy.html

 

We clearly see that the velocity is increasing linearly up to about 3KPC. This fully meets the expectation from the central bulge oval/bar shape due to the impact of the tidal forces.

Any idea why they call it solid body rotation?

Edited January 17 by Dandav

[OceanBreeze]

4 hours ago, Dandav said:

Any idea why they call it solid body rotation?

Because the ratio of V/R = Ω (angular velocity in rad/s) is constant up to a R of 3 KPC.

The linear (tangential) velocity is increasing linearly with radial distance up to a radius of ~ 3KPC.

That is true for how a solid body, such as a dinner plate, rotates. It is solid body rotation.

The story is different for the spiral arms. In their case, it is V the linear velocity is kept constant, or nearly so.

With V constant it is obvious that Ω must vary with distance R.

Therefore, it would seem that the outer part of the spiral arms would rotate slower than the inner sections causing the outer section of the spiral to lag behind the inner section.

That seems to be very intuitive and simple to understand until some astronomer had to go and spoil everything and discover galaxies that have leading spiral arms, exemplified by galaxy NGC 4622.  Leading spiral arms means the tips of the spirals point in the same direction as the rotation!

The best answer the astronomers and astrophysicists have for now is the spirals are actually a wave pattern superimposed on the star disk.

 

[Dandav]

4 hours ago, OceanBreeze said:

Because the ratio of V/R = Ω (angular velocity in rad/s) is constant up to a R of 3 KPC.

The linear (tangential) velocity is increasing linearly with radial distance up to a radius of ~ 3KPC.

That is true for how a solid body, such as a dinner plate, rotates. It is solid body rotation.

Thanks for your important answer.

Now it is clear that base on the linear (tangential) velocity of the central bulge / Bar the bar acts as a solid dinner plate or solid body rotation.

 

4 hours ago, OceanBreeze said:

The story is different for the spiral arms. In their case, it is V the linear velocity is kept constant, or nearly so.

With V constant it is obvious that Ω must vary with distance R.

I fully agree with you that the story with the spiral arm is different from the bar as it is a different object.

Therefore, there shouldn't be any contradiction between the activity at the spiral arms with reference to the activity at the bar or the ring.

When we try to understand the activity at the bar , Ring or spiral arms, we must focus ONLY on what we observe at each object.

As we observe that the central bulge / bar acts as a solid dinner plate, then we should try to explain the activity while the central bulge / Bar is a solid object regardless of the activity at the spiral arms or the ring.

So, what does it mean - a solid bar?

Do you confirm that the idea is that the stars are bonded to each other by gravity force (as a solid arm)?

In other words, each star in the bar can move locally at a relatively low velocity, while all the stars move with the bar at the rotation velocity of the bar?

Please advise if you agree with the following explanation of how the solid bar is created:

The vertical tidal force squeezes the stars in the central bulge, while the horizontal gravity force pushes them outwards.

Those two orthogonal forces, push the stars together, bond them by gravity and create the solid gravitational arm that is called - Bar. 

Edited January 17 by Dandav

[Dandav]

I have just found a breakthrough article dated from NOVEMBER 30, 2010:

https://phys.org/news/2010-11-milky-stars-mysterious-ways.html

It is stated that "stars move towards the exterior of the Galaxy at an average speed of around 10 kilometers per second, which is considerably faster than previously thought."

"To reach this conclusion, the team systematically analyzed the velocities of over two hundred thousand stars located within a radius of a little over six thousand light years around the Sun. Using data from the major star survey RAVE (RAdial Velocity Experiment) collected since 2003 by the Australian Astronomical Observatory's Schmidt telescope, they were able to measure for the first time the radial velocities of hundreds of thousands of stars and determine whether they were moving towards or away from us."

"The researchers were thus able to ascertain that the average speed of stars towards the exterior of the Galaxy increases with their distance from the Sun in the direction of the Galactic center, reaching 10 kilometers per second at a distance of 6,000 light years from us (in other words, 19,000 light years from the Galactic center). This result was completely unexpected and all the more surprising as it appeared to mainly affect old stars, several billion years old."

Hence, for more than 13 years we know that stars in the galaxy "move towards the exterior of the Galaxy at an average speed of around 10 kilometers per second".

Our Sun is just one star in the spiral arms galaxy.

Don't you agree that as stars move towards the exterior of the Galaxy, then by definition our Sun MUST also move towards the exterior of the Galaxy?

So, how can we claim that the sun stays at the same radius for several billion years?

Why do we refuse to accept this clear observation?

This article is fully correlated with the Tidal impact on the Bar.

The vertical tidal force pushes the stars outwards from the central bulge to the Bar while stars in the spiral arms are also move outwards towards the exterior of the Galaxy.

Once we accept this observation - we can easily understand how spiral galaxy really works.

So, are we willing to accept the observation that stars in the spiral arms move towards the exterior of the galaxy (at an average speed of around 10 kilometers per second)?

 

Edited January 18 by Dandav

[OceanBreeze]

12 hours ago, Dandav said:

I have just found a breakthrough article dated from NOVEMBER 30, 2010:

https://phys.org/news/2010-11-milky-stars-mysterious-ways.html

It is stated that "stars move towards the exterior of the Galaxy at an average speed of around 10 kilometers per second, which is considerably faster than previously thought."

"To reach this conclusion, the team systematically analyzed the velocities of over two hundred thousand stars located within a radius of a little over six thousand light years around the Sun. Using data from the major star survey RAVE (RAdial Velocity Experiment) collected since 2003 by the Australian Astronomical Observatory's Schmidt telescope, they were able to measure for the first time the radial velocities of hundreds of thousands of stars and determine whether they were moving towards or away from us."

"The researchers were thus able to ascertain that the average speed of stars towards the exterior of the Galaxy increases with their distance from the Sun in the direction of the Galactic center, reaching 10 kilometers per second at a distance of 6,000 light years from us (in other words, 19,000 light years from the Galactic center). This result was completely unexpected and all the more surprising as it appeared to mainly affect old stars, several billion years old."

Hence, for more than 13 years we know that stars in the galaxy "move towards the exterior of the Galaxy at an average speed of around 10 kilometers per second".

Our Sun is just one star in the spiral arms galaxy.

Don't you agree that as stars move towards the exterior of the Galaxy, then by definition our Sun MUST also move towards the exterior of the Galaxy?

So, how can we claim that the sun stays at the same radius for several billion years?

 

Not that it matters (since I am not an astrophysicist and obviously neither are you) I cannot agree with what you have written. It is increasingly apparent that you have not researched this subject enough to have a solid fundamental understanding.

To answer your question, nobody is claiming “that the sun stays at the same radius for several billion years”. I freely admit that I have a (very) limited understanding of this subject, gained from reading scientific journals and papers published by reliable sources. From what I understand, the position and motion of our sun is well understood:

“The Sun circles the Galaxy in a wobbly path that continually changes in response to the large, irregular mass concentrations that pervade a galactic disc. The Sun is currently about 8.5 kpc from the Galactic centre and about 20 pc above (north) of the Galactic plane. It is moving radially inwards toward the core at 10 km s-1, tangentially forward at ~5 km s-1 faster than the average star at this radius from the Galactic centre, and about 7 km s-1 vertically from the the Galactic plane. In a side view it would look like an eccentric ellipse with a very large precession.”

 

Quote

Why do we refuse to accept this clear observation?

Who is the "we" you you are referring to?

 

Quote

 

This article is fully correlated with the Tidal impact on the Bar.

The vertical tidal force pushes the stars outwards from the central bulge to the Bar while stars in the spiral arms are also move outwards towards the exterior of the Galaxy.

Once we accept this observation - we can easily understand how spiral galaxy really works.

 

So far you have only mentioned the tidal effect of gravity. Until you start using terms such as angular momentum, thermodynamics, hydrodynamic Mach speed (the speed of a shock wave through its medium), magneto dynamics, central black hole, Lindblad radii, fluid dynamics and density perturbations, just to mention a few areas of interest related to galaxy morphism, there really is no reason to read anything more that you write.

I don’t mean to be overly critical: I give you credit for bringing up an interesting topic, but compared to the scientific literature that is readily available on the net, your coverage of this subject is not even at entry level. Also, the way you try and spoon feed your ideas, by asking leading questions, is frankly annoying.

I suggest that you take some time to read and understand papers like this, for example: https://arxiv.org/abs/1311.2609v1

When you can converse on this topic with something close to the same level of understanding exemplified by that paper by John Kormendy, it will be worthwhile to read what you have to say.

Because this subject is interesting, this thread should remain open, in my opinion, but moved to  Alternative Theories/Strange claims.

 

 

[Dandav]

On 1/18/2024 at 5:21 PM, OceanBreeze said:

From what I understand, the position and motion of our sun is well understood:

“The Sun circles the Galaxy in a wobbly path that continually changes in response to the large, irregular mass concentrations that pervade a galactic disc. The Sun is currently about 8.5 kpc from the Galactic centre and about 20 pc above (north) of the Galactic plane. It is moving radially inwards toward the core at 10 km s-1, tangentially forward at ~5 km s-1 faster than the average star at this radius from the Galactic centre, and about 7 km s-1 vertically from the the Galactic plane. In a side view it would look like an eccentric ellipse with a very large precession.”

The sun wobbling motion is very clear. Actually, it is expected that every star in the galaxy would have a wobbling motion.

However, the key question is: Could it be that over time stars drift towards the exterior of the Galaxy?

Unfortunately, it is very difficult for us to get accurate long time drifting measurements with regards to the sun due to its wobbling motion.

In the following article it is stated:

https://phys.org/news/2010-11-milky-stars-mysterious-ways.html

Scientists "were able to measure for the first time the radial velocities of hundreds of thousands of stars and determine whether they were moving towards or away from us."

So, although each one of those hundreds of thousands of stars has a wobbling motion, I assume that scientists set an accurate statistical measurement to understand how that group of stars behave.

Their conclusion is very clear:

"Stars move towards the exterior of the Galaxy at an average speed of around 10 kilometers per second, which is considerably faster than previously thought."

Please be aware that those hundreds of thousands of stars are located relatively close to our location (6,000 LY away from us).

On 1/18/2024 at 4:42 AM, Dandav said:

at a distance of 6,000 light years from us (in other words, 19,000 light years from the Galactic center).

So, out of all of those relatively near by hundreds of thousands of stars the scientists have concluded that the "Stars move towards the exterior of the Galaxy at an average speed of around 10 kilometers per second".

In other words, those hundreds of thousands of stars are drifting towards the exterior of the Galaxy at an average speed of around 10 kilometers per second (with reference to our location). 

If this measurement is correct, then how can we believe that those hundreds of thousands of stars are the ONLY stars in the galaxy that are drifting towards the exterior of the Galaxy, while all the other billions stars (including our sun) stay at the same radius (more or less, assuming that we ignore the wobbling motion) ?

Why can't we understand that this observation indicates that every star in the galaxy is drifting towards the exterior of the Galaxy?

Actually, if we could eliminate the impact of the sun wobbling motion, we should find its drifting velocity towards the exterior of the Galaxy.

This observation is very critical for our understanding about the activity of the spiral arm galaxy.

Therefore, I wonder why from NOVEMBER 30, 2010, no new measurements have been set on this critical understanding.

[Dandav]

On 1/18/2024 at 5:21 PM, OceanBreeze said:

suggest that you take some time to read and understand papers like this, for example: https://arxiv.org/abs/1311.2609v1

 

When you can converse on this topic with something close to the same level of understanding exemplified by that paper by John Kormendy, it will be worthwhile to read what you have to say.

Thanks for this important article

I read some of it very carefully.

They offer the following image:

It is stated:

"1.4.1 Orbital resonances in a galactic disk Orbital resonances are the key to understanding bars and spirals. The main resonances – inner Lindblad resonance, corotation, and outer Lindblad resonance – will recur in many of the lectures at this school. I therefore need to introduce them in some detail. The general orbit of a star in a galactic disk is an unclosed rosette, because the potential is not Keplerian (that is, the galaxy mass is distributed in radius and not all in one central point as in, for example, the Solar System)."

"For bars, corotation happens near the end of the bar or slightly beyond it (see Sellwood & Wilkinson 1993 for a review)"

https://ned.ipac.caltech.edu/level5/Sept14/Kormendy/Kormendy4.html

Figure 10 shows disk orbits as seen in a frame of reference that rotates clockwise (orange arrow) at the pattern speed Ωp of some coherent structure. In this figure, I show a spiral arm (red), because spirals almost always trail in the rotation direction (that is, the arm is convex in the direction of rotation).

Corotation is the most important resonance in the galaxy, because the mean position of the star with respect to the global pattern never changes as long as Ωp is fixed."

 

So what is corotation: 

 

https://www.merriam-webster.com/dictionary/corotate

 "To rotate in conjunction with or at the same rate as another rotating body. corotation. ˌkō-rō-ˈtā-shən. noun."

The real meaning is that the stars in the arms are locked by gravity to other nearby stars.

In other words, this "corotation" means that near the end of the bar and at the spiral arms stars are locked by gravitational bonding to other nearby stars. This is the main explanation why the sun is wobbling. In reality, the sun is locked by gravity to the nearby stars and therefore, it is corotated. Hence, the wobbling motion is just a direct outcome from the corotation.

Due to tidal gravity force, we have already understood the symmetrical shape of the two bar arms. The tidal force sets the gravitational bonding between the stars near the end of the bar.

Therefore, at the end of the bar there is no "free" stars. The stars are bonding together (as a group of stars).

However, from time to time while the vertical gravity pushes the stars more and more outwards, some group of stars might be disconnected from the bar.

Please be aware that due to tidal force, the two arms MUST be fully symmetrical.

Therefore, if one group / section of stars in the edge of one bar arm would be disconnected, then immediately on the edge of the other arm (even if it is 6KPC away - 3KPC for each bar) a similar section of stars would also be disconnected.

Therefore, in my understanding, we should see the following:

Over time the two bar arms should be longer and longer - fully symmetrically. However, from time to time, once one of the bar arms lose a section from its far end, then at the same moment the other arm should lose a similar section. Therefore, they would stay symmetrically under any conditions.

Once the far section of stars is disconnected from the edge of the bar, it would drift outwards as a solid object and be connected to the Ring (3KPC arm). 

From the ring it would move to the base of the spiral arm.

Therefore, the ring acts as a buffer between the motion/delivery of stars sections from the edge of the bar to the base of the spiral arms.

This new section of stars would increase the length of the spiral arms from inside.

As new section is added to the spiral arm from inside, stars in the spiral arms would drift towards the exterior of the Galaxy.

Therefore, the spiral arm could be a rigid object without breaking its spiral structure over time.

New sections of stars are coming from the bar while stars in the spiral arm are drifting towards the exterior of the Galaxy.

In this case, the two symmetrical spiral arms can keep their rigid structure for billions of years while new segments of stars are coming constantly from the edge of the bar arm.

Hence, as long as the density of stars is high enough, the spiral arms can keep/hold the stars by gravity bonding.

The density of G star at our location is exactly 64 stars per 50LY. (or 512 stars per 100LY) we have clear measurement for that.

Please be aware that the diameter of the spiral arms at the base is 3,000 LY, at our location is 1000 LY and at the edge of the spiral arm it is only 400LY.

When the diameter of the spiral arm drops below 400, the density of stars can't hold the stars there anymore, and the far end section of the spiral arm is disconnected from the arm.

Therefore, while new stars are coming symmetrically from the bar to the base of the spiral arms, all the current stars in the spiral arms are drifting towards the exterior of the Galaxy and at some point, the stars at the edge of the spiral arms would be disconnected from arm.

On 1/18/2024 at 5:21 PM, OceanBreeze said:

Until you start using terms such as angular momentum, thermodynamics, hydrodynamic Mach speed (the speed of a shock wave through its medium), magneto dynamics, central black hole, Lindblad radii, fluid dynamics and density perturbations, just to mention a few areas of interest related to galaxy morphism, there really is no reason to read anything more that you write.

 

There is no need for big words.

No need for dark matter and no need for density wave.

Tidal gravity force by itself can explain the whole activity at the spiral galaxy!

This is the only explanation why the edge of the bar arm, the ring (3KPC arm) and the base of spiral arm could meet at 3KPC, why the galaxy has two symmetrical bar arms, two symmetrical 3KPC arms (ring) & two symmetrical spiral arms, why the bar acts as a solid object, why the spiral arm looks rigid while it keeps the star velocity at almost any radius and why 70% of the observed galaxies are spirals.

Just one single phenomenon for the entire spiral galaxy structure and activity

 

 

Edited January 20 by Dandav

[Dandav]

 

Please look at the following image:

 

https://grade8science.com/1-3-2-how-is-our-solar-system-moving-in-the-milky-way-galaxy/

 

"if you look in any small region of the galaxy, such as the box drawn in Figure 1.30 to represent our “local solar neighborhood,” you’ll find that stars move essentially at random relative to one another. Scientists can measure the speeds at which stars are moving, and they are quite impressive; on average, nearby stars are moving relative to us at about 70,000 kilometers per hour."

70,000 kilometers per hour means – 20 Km/s.

So, in our local solar neighborhood stars move at a random direction relative to one another on average velocity of 20Km/s.

We might think that over time they all would move away from each other, but this is a severe mistake.

They would stay together as they are all bonded together by local gravity force in the local neighborhood  – ("corotation" - rotate in conjunction with or at the same rate as other rotating bodies).

The density of G stars at the spiral arm in our location: 

http://members.fcac.org/~sol/solcom/stars3/100-gs.htm

As many as 512 or more stars of spectral type "G" (not including white dwarf stellar remnants) are currently believed to be located within 100 light-years or (or 30.7 parsecs) of Sol -- including Sol itself. Only around 64 are located within 50 light-years (ly), while some 448 are estimated to lie between 50 and 100 light-years -- a volume of space that is seven times as large as the inner sphere within 50 ly of Sol. 

We observe 512 stars per 100LY. That means about 64 Stars per 50LY. That is exactly what we observe in 50LY around us. Therefore, it is an indication that average density of G stars per 50LY in the Orion local neighborhood is 64. 

If one of those stars would dare to more away from the arm and be disconnected from its local neighborhood, it would be ejected from the galactic disc at ultra-high velocity as this hypervelocity star:

https://www.space.com/25843-hypervelocity-star-dark-matter-black-hole.html

The so-called "hypervelocity" star is traveling at 1.4 million mph (2.3 million km/h) relative to our solar system, about three times faster than a typical star, a new study reports. The star lies 42,400 light-years from Earth, above the disk of the Milky Way, and is cruising toward the halo of dark matter that surrounds the galaxy.

Therefore, each star holds itself by gravity force in the local neighborhood spiral arm and go with the arm wherever it goes.

The spiral arm is quite flexible. Stars are bonded together by local gravity force in the local neighborhood. All the stars in the arm are constantly drifting towards the exterior of the Galaxy while new stars are delivered from the edge of the bar (by the ring buffer) to the base of the spiral arm. Therefore, the spiral arm looks rigid, while each star at any radius can maintain its 220 Km/s velocity.

Tidal is the only force in the universe that can set the symmetrical shape of the two bar arms. Those arms are responsible for the two symmetrical 3KPC arms (ring) and the two main symmetrical spiral arms.

Edited January 21 by Dandav

[OceanBreeze]

14 hours ago, Dandav said:

Thanks for this important article

I read some of it very carefully.

They offer the following image:

 

I am glad you read it.

 

 

 

 

Quote

 

There is no need for big words.

No need for dark matter and no need for density wave.

Tidal gravity force by itself can explain the whole activity at the spiral galaxy!

This is the only explanation why the edge of the bar arm, the ring (3KPC arm) and the base of spiral arm could meet at 3KPC, why the galaxy has two symmetrical bar arms, two symmetrical 3KPC arms (ring) & two symmetrical spiral arms, why the bar acts as a solid object, why the spiral arm looks rigid while it keeps the star velocity at almost any radius and why 70% of the observed galaxies are spirals.

Just one single phenomenon for the entire spiral galaxy structure and activity

 

Too bad you didn’t learn anything from it.

Dandav; in response to your erroneous statement “Tidal gravity force by itself can explain the whole activity at the spiral galaxy!” I will just say:

The general principle that drives the evolution of self-gravitating systems is this: it is energetically favorable to spread.

The second law of thermodynamics states that the total entropy of a system either increases or remains constant in any spontaneous process; it never decreases. An important implication of this law is that heat transfers energy spontaneously from higher- to lower-temperature objects, but never spontaneously in the reverse direction.

The total energy will be divided up in the way that maximizes the number of ways (the number of degrees of freedom) the system can be arranged, and hence maximizes the entropy. That is, the system has a "tendency to go to a lower energy state," which means that transferring energy from the system to the environment results in an increase in entropy.

How this applies to the secular evolution of disk galaxies depends on whether the system is dominated by rotation or by random motions (heat).

If the system is supported by heat, evolution is by heat transport. If the center of the system gets hotter than the periphery, then heat tends to flow outward. The inner parts shrink and get still hotter. This promotes further heat flow. The outer parts receive heat; they expand and cool.

If the disk system is supported by rotation, evolution is by angular momentum transport which acts to minimize the total energy at fixed total angular momentum. Disks spread when a unit mass at radius r2 moves outward by gaining angular momentum L from a unit mass at radius r1 < r2. This is energetically favorable and in accordance with the second law of thermodynamics; disk spreading leads to a lower energy state.

In general, disk spreading, outward angular momentum flow, and energy dissipation accompany one another in disk and spiral galaxy evolution.

The above explanation, based on the general principle of entropy, is by no means a complete explanation of spiral galaxies. However incomplete it may be, this explanation destroys your naive claim that: “Tidal gravity force by itself can explain the whole activity at the spiral galaxy!”

Dandav, you evidently have an aversion to “big words” and prefer to try to explain something as complex as spiral galaxies in terms of only tidal forces. it is abundantly clear that you have no real understanding at all of this subject and furthermore, because of your adversity to learning about those “big words”, you may never understand it.

Moderator Note: Unfortunately, it is in the best interest of both Dandav, as well as this forum, to close this thread and suspend his posting privilege. Hopefully, he will use the suspension to thoroughly read the paper by John Kormendy and learn the meaning of those big words. I don’t expect Dandav to gain a complete understanding of galaxy evolution, which includes very complicated nonlinear dynamics. Even the paper by John Kormendy, with all those “big words” is a simplified explanation, but still far beyond the childlike “theory” he is spouting here.