Any system of randomly moving particles will likely have a small residual angular momentum. As the system collapses (e.g., under its own gravity, such as when a galaxy or a solar system forms), conservation of angular momentum causes it to rotate faster. Associated with this rotation is the plane in which the rotation takes place. Due to mutual interactions of the particles in the system (especially when gas and dust are present so the interactions are not purely gravitational), motion that is perpendicular to the plane of rotation tends to be dampened; the overall rotation, however, remains constant because once again, angular momentum is conserved. As a result, over a long period of time the system tends to flatten out.

For a more detailed explanation, see, for instance, http://physics.stackexchange.com/questions/25950/why-are-some-galaxies-flat

Any system of randomly moving particles will likely have a small residual angular momentum. As the system collapses (e.g., under its own gravity, such as when a galaxy or a solar system forms), conservation of angular momentum causes it to rotate faster. Associated with this rotation is the plane in which the rotation takes place. Due to mutual interactions of the particles in the system (especially when gas and dust are present so the interactions are not purely gravitational), motion that is perpendicular to the plane of rotation tends to be dampened; the overall rotation, however, remains constant because once again, angular momentum is conserved. As a result, over a long period of time the system tends to flatten out.

For a more detailed explanation, see, for instance, http://physics.stackexchange.com/questions/25950/why-are-some-galaxies-flat

Charles, I don't think Sanjit's question was about the large-scale alignment. Perhaps I misunderstood but I think he was simply wondering about why planets in the solar system all orbit in roughly the same plane, or why stars in the Milky Way all orbit in roughly the same plane, not how the rotation planes of distant galaxies may statistically align.

Still, gravitational collapse within an amorphous gas cloud commonly produces protoplanetary disks - It seems that inertial rotation must be fundamental to the process of collapsing particles...

Hi, Sanjib!

The "statistical" argument is that if you have a large loose collection of co-orbiting matter with a net angular momentum around its common centre of gravity, then over a large period of time that angular momentum will tend to be shared and smeared and averaged out amongst the orbiting material by collisions and gravitational flybys. It's a "road traffic" problem - if any matter is moving through the cloud too fast, or too slow, or in the wrong rotation plane (or in the opposite direction!), then it'll have an increased rate of collisions and gravitational interactions that will keep changing its orbit. The lowest rate of orbit-changing collisions and flybys happens when everything is rotating in step with its neighbours (or as close as can be achieved bearing in mind orbital mechanics), so that tends (statistically) to be the end result. Ish.

If the disc is a thick disc then it'll tend to collapse to form a thinner disc, because there's no obvious forces to keep it "fat" to counteract gravitational effects perpendicular to the plane. Anything that orbits slightly out of the plane, but sharing a common centre of gravity with the rest of the material will have to cross the plane twice per orbit, again with a greater chance of having it's orbit changed by a collision or gravitational flyby each time that it crosses.

However, much further away from the centre of the solar system, we reckon that there's something called the Oort Cloud, at a distance of maybe 2000 to 5000 times the distance of the Earth from the Sun. This is reckoned to be pretty much spherical, with its bodies orbiting pretty much any which way - however, the cloud must be sufficiently rarefied, with the rate of collisions and gravitational interactions so small, that it's never had the need to establish a more orderly "one way system". Come back in a zillion zillion zillion years, and perhaps you might find that the Oort cloud has settled down into a co-rotating disk, too.

We assume that the Oort Cloud exists because of the occasional icy comets that come into the solar system at crazy angles that take them reasonably close to the Sun. We know that at least some of these are orbiting (like Halleys), but we also know that they seem to only be able to make a limited number of passes before evaporating, so somewhere out there there should be a reserve of icy objects orbiting far from the Sun that occasionally throws something our way after an interaction, so that the supply of fresh orbiting comets is kept "topped up" (otherwise there probably wouldn't be enough of them left by now for us to notice).

Is there any preference for direction of rotation?

James Williams,

There is a report of a study of rotation in >15,000 spiral galaxies in the Sloan Digital Sky Survey (SDSS) data (mostly in the Northern hemisphere) that found a 7% excess of 'counter-clockwise' rotation (I think this also implies some preference in orientation). See http://physicsworld.com/cws/article/news/2011/jul/25/was-the-universe-born-spinning.

However, a subsequent study of the SDSS data evaluating the rotation of >125,000 spiral galaxies found that their rotational preference depends on their position within the sky - potentially indicating a universal spin orientation! See http://astrobites.org/2012/07/24/is-god-right-handed-spiral-galaxies-rotation-and-isotropy/ - it provides links to several other studies with somewhat similar findings. I cringe at the title, however...

James, the Shamir study is very interesting. Thanks for the link. In my own meager observations, I also saw a higher degree of right-handed rotation. The question is why this is so.

James,

Yes - No one can definitively answer that question yet, but the simplest conjecture would be that the initial universe was spinning. I also wonder if a spinning universe might very directly explain some initial preference for the condensation of matter rather than antimatter...

Galaxies rotate, so they are in a battle between the center seeking forces

of gravity, and the center fleeing forces of rotation.

I suspect that galaxies began life as approximately spherical accumulations

of gas and dust, but contraction set the galaxies to spinning, so they

became oblate spheroids which gradually became more and more

oblate until they formed a disc as part of the material migrated toward the

center of the galaxy, and part of the material migrated toward the perimeter of the galaxy.

We like to think of galaxies as static, because we see them in static pictures,

but they are dynamic, which means they are constantly changing, which means

they are constantly moving materials either toward the center of the galaxy, or they

are moving materials toward the perimeter of the galaxy, and/or both are

occurring simultaneously. In addition, larger Galaxies consume smaller

galaxies. The process of the motion of materials toward or away from the

center of the galaxy also changes the rotational speed of the galaxy in

localized areas, so the inside speeds up, and the outside slows down, so

the galaxy forms spiral arms.

In our galaxy, the position of our Solar system is in a fortunate location about

3/5 ths of the way from the center to the outside. This is the quiet zone where

material is neither moving toward the center, nor moving toward the perimeter,

so the number of collisions between objects of differing sizes is greatly

reduced, because the difference between rotational velocities of objects

about the galactic center is diminished.

There should actually be a thin band where the materials neither moves

toward the galactic center, nor migrates away from the galactic center,

but may oscillate in waves back and forth through the galactic plane.

Michael,

As I understand, the motions of all rotating objects is determined by the net influence of gravitational and centrifugal effects. Even objects at the periphery of galactic disks are affected not only by centrifugal effects resulting from their rotational velocities, but their gravitational bindings with billions of comparably massive neighboring objects.

Unfortunately, most evaluations consider only their gravitational attraction to a collective center of mass, which seems surprisingly insufficient to prevent their expulsion from the disk. This leads to the misconception that some undetected mass must be contributing to the gravitational potential of the 'virtual' central point mass - to prevent expulsion resulting from high velocity centrifugal effects...

As for the idea that:

"In our galaxy, the position of our Solar system is in a fortunate location about 3/5 ths of the way from the center to the outside. This is the quiet zone where material is neither moving toward the center, nor moving toward the perimeter, so the number of collisions between objects of differing sizes is greatly reduced, because the difference between rotational velocities of objects about the galactic center is diminished."

The rotational velocity of objects at the same radius from a central mass would vary _only_ if the masses were significantly different _and_ were significant in relation to the mass of the idealized central object (m1+m2 for each planetary orbital was significantly different from the other). In all cases of disk objects, it's considered that each orbital's mass can be disregarded in evaluating gravitational effects since it is so much smaller than the idealized collective mass (improperly) represented by m1.

However, galaxy disks are not independently orbiting planetary systems - actually, locally bound components of the disk interact, rotating collectively, at nearly identical speeds.

JAMES

You are one of the few people that I know that understands that gravity

between 60 Billion objects in a Galaxy means that each object in the

Galaxy has 60 Billion gravitational vectors working on it, and there are then

an incredible number of total gravitational vectors acting as a gravitational

network that attempts to contract the Galaxy into a smaller space, but in reality,

it only accomplishes a change in the rotation of the Galaxy.

For want of a better concept, it is a gravitational network of vectors that tends

to align itself in a spinning plane of balanced forces between gravitational

contraction, and rotational dispersion.

In short the simplified version of the two party Newtonian equation only addresses

1 / ( e+1) of the total gravitational forces = 26.8941421 % which is

usually rounded to 27 %. They then create the rest = 73% as Dark Matter.

A gravitational network thus has ( e+1 ) / 1 times the simplified version

of Gravity = M1 x m2 / r^2, and no dark matter is needed. The actual

summation of all of the gravitational vectors is thus 3.718281828......times the

calculated amount.

Objects such as planets are gravitation-ally locked to their parent star simply

because the distances are vastly shorter, so the forces are vastly greater

between the planet and its star, compared to the planet, and its galaxy.

The same is also true of planets and Moons. The gravitational force between

a Planet and its moon is generally vastly greater than the force between a

Moon and its star. This is also a distance thing, but you notice that neither

Mercury, nor Venus have Moons, but, all of the rest have Moons,

including tiny Pluto.

In the smaller scale, M1 x m2 / r^2 works because of the relatively small number

of the r^2. In reality gravitational forces a vectorially additive, but the Newtonian

M1 x m2 works because M1 is generally vastly bigger than m2, so m2's

additive vector forces is extremely small in the two party system.

In reality objects orbiting the Earth even up to the size of the Moon, can

be expressed as m2 multiplied by a constant G M (Earth) / r^2.

Because the product GM (Earth) is so enormous, you can just multiply

it by m2 because each kilogram of m2 has the same forces on it, so

you can either multiply the 1 kilogram forces of each kilogram, and add them

up, or, you can add them up, and then multiply by all of the 1 kilogram forces at

once. This only gives the summation of the individual forces of the

Earth acting on the individual kilograms of m2. What is not accounted for

is the summation of the extremely small individual forces of

one kilogram acting on the Earth. In short G m2 / r^2 is very tiny so

it is not considered as part of the summation of the gravitational forces

of the system.

To clarify: I summed up all of the individual one kilogram units and multiplied them

by a very big gravitational force of G M ( Earth ) at a distance r, but we also

need to add in all of the 1 kilograms of the Earth and multiply it by a tiny

force of Gm2 at a highly variable distance r which can vary from near zero

to greater than 12, 742 km. The second force is incredibly small.

For each Kilogram of m2 the forces is G which is about 6.67 x 10^-11 divided

by r^2.

All of these tiny forces do not come into play until you add up all of the vector forces

inside a Galaxy from all of the Star systems acting upon all of the other Star

Systems, so the total sums to ( e+1 ) , which is the original value calculated

plus e times the original value calculated.

Michael,

Yes - at least to the extent that I can follow. I guess it takes engineering or information analysis experience to recognize that galactic scale (and greater) objects are compound structures that can't be properly represented as simplistic two-body mass distributions!

I really don't do math, but as I understand, the fundamental issue in classical treatments of galaxy gravitation is that, in a network of interacting bodies, the gravitational force imparted by each body to a subject body is determined not only by its distinct mass, but also by (the inverse square of) its distinct separation distance from the subject!

As I understand, galactic evaluations commonly represent a vast distribution of bodies by a simple aggregation of its mass and a single separation distance (r). There is no attempt to approximate the non-linear effect of varying separation distances. As a result, the estimated mass required to produce observed rotational velocities - at a separation distance equal to the subject's radial distance - is far greater than if a significant portion of the distributed mass was evaluated at its actual separation distance from the subject - requiring a summation of at least many individual vectors - as you imply.

I hope this explanation & my perhaps inappropriate terminology can be properly understood. You might also find it (more) interesting to consider a complex method of radially partitioning the disk evaluation in an attempt to more closely approximate actual separation distances: see http://dx.doi.org/10.1088/1674-4527/11/12/005 or http://arxiv.org/abs/1104.3236 - referenced in my informal profile essay. It is good to hear from someone with a similar perspective!

I've read that galaxies are self-organizing systems. What are your thoughts?