Spiral Galaxies Rotate. Elliptical Galaxies do not rotate. Why?

I believe the conventional assumption concerning angular moment in the universe on very large scales is that it is net zero. (i.e. The universe does not rotate.) As far as I know there is no observational evidence on a very large scale or on a large scale to question that base assumption.

For example a cluster of galaxies does not rotate. If clusters of galaxies did rotate as a group that would be evidence that the net angular moment in the universe was not zero.

I have not seen a good explanation as to why the stars in a spiral galaxy rotate as a group which means that a spiral galaxy has net angular moment.

Just as puzzling for the opposite reason, is why the stars in an elliptical galaxy do not rotate as a group. The stars in an elliptical galaxy orbit the elliptical galaxy's centre such that the net angular moment of the entire elliptical galaxy is zero.

Why do the stars as a group in a spiral galaxy rotate? Can angular moment be created?

Also curiously, elliptical galaxies have almost no gas and dust, whereas, spiral galaxies, I believe, have 20% of their mass in gas and dust.


The diagram in the attached link shows how the net angular moment of spiral galaxy is positive while it is net zero for an elliptical galaxy.


http://cass.ucsd.edu/public/tutorial/Galaxies.html

Spiral galaxies form from a single collapsing cloud of gas. Ellipticals form from the merger of multiple other galaxies. Thus spirals will poses whatever angular momentum the gas has as it collapses into a disk, while stars in elliptical galaxies have "random" orbits due to the various angles of the mergers that created them. And this also explains why ellipticals have little to no gas or dust: merging stirs it up and kicks it out.

This is covered in pretty much any textbook on galaxy formation.

And, once again, since you are still ignoring me:

I'm going to keep quoting myself until you cease ignoring me.

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In reply to parejkoj’s answer:

Elliptical Galaxies
The problem with the elliptical dry merger hypothesis is that mergers should continue to occur throughout the life of the universe, which will produce a range of ages of stars in elliptical galaxies, which is not observed. i.e. If two spiral galaxies merge, there would be a range of stellar ages in the resultant as there is a range of stellar ages in the spiral galaxies. Elliptical galaxies have been observed with uniformly old evolved stars.

Is it correct that the merger hypothesis is?: Elliptical galaxies are the resultant of mergers, the first galaxies produced would be spiral. Some of the spiral galaxies would merge. Some of the merges would retain the spiral characteristic, others would not and would produce a elliptical galaxy.

A possible clue to the mechanism difference between an elliptical and a spiral is the largest elliptical galaxies "cD", which are the largest galaxies (Radius up to around 1Mpc. Some mechanism limits spiral galaxy size.) is the cD elliptical galaxies are only found near the centres of dense galactic clusters.


Spiral Galaxy – Conservation of Angular Moment Problem
In the early universe, with closed spaced clusters of gas clouds and population III stars, net angular moment should be conserved. It is expected that early galaxy formation should therefore be similar to the very large elliptical galaxies where the net angular moment of the galaxy is zero.


In reply to parejkoj's other comment:
Your question concerning what are O/B stars is reasonable. I have the same questions.

The two disks of stars noted in the paradox of youth thread, are not randomly orientated. They are as you noted roughly perpendicular to each other. The stars in both disks were formed roughly at the same time.

I will not mention the MECO charge and fragmentation hypothesis, again until I can provide additional observational support for that specific hypothesis or can site a paper.

Factually incorrect statement. Elliptical galaxies most certainly do rotate, and this is not a "recent" discovery; i.e., van der Marel & van Dokkum, 2007; Thomas, et al., 2006; van Zee, Skillman & Haynes, 2004; Pedraz, et al., 2002; Hanlan & Bregman, 2000; Jedrzejewski & Schechter, 1989; Bertola & Capaccioli, 1979 & etc.

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In reply to Tim Thompson's comment:

Your comment is correct. Elliptical galaxies do have some residual rotation. Stellar motion in elliptical galaxies is however, predominately radial, which explains why elliptical galaxies are elliptical in shape as opposed to disk like. The residual rotation occurs from galactic mergers. The 2007 paper you provided a link to showed a reduction in elliptical galaxy residual rotation with redshift. The hypothesis to explain the reduction in residual rotation, with time, has there was time for more mergers which is what one would expect.

So is the conclusion, that only elliptical galaxies where involved in mergers?

Why wouldn't early merging proto spiral galaxies have become like the elliptical galaxies. If there is no mechanism to make a galaxy rotate, then random frequent merges in the early universe would have created galaxies with a trend to zero net rotation, which what is observed with ellipticals. i.e. The more mergers there are, if the galaxies have random rotation, galaxy rotation reduce as one moves toward current time.


From the 2007 paper:

From Wikipedia:
General characteristics of Elliptical
Elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. The motion of stars in elliptical galaxies is predominantly radial, unlike the disks of spiral galaxies, which are dominated by rotation. Furthermore, there is very little interstellar matter (neither gas nor dust), which results in low rates of star formation, few open star clusters, and few young stars; rather, elliptical galaxies are dominated by old stellar populations, giving them red colours. Large elliptical galaxies typically have an extensive system of globular clusters.

As I understand this; Galaxies form in a rotating halo of dark matter. As the halo/galaxy interacts with other similar systems either accreting or passing, the rotation and movement of the DM halo and bayonic components start to differentiate. If the halo separates from the stellar component then the stars are no longer bound in rotation and are free to move tranjentially away from the centre of the galaxy; this results in a gravitic relaxation of the galaxy and elliptical morphology.

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Or another way to say it, the ellipticals have found some way to transfer angular momentum from the stars to the dark matter. The interaction is generally assumed to be purely gravitational, but can occur in models.

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Here are some observational data and analysis, related to galactic merging frequency and resultant.

Interesting is galactic mergers do not increase with redshift, out to z<1.2. Slightly more than half the time the merger does not change the galactic morphology. (i.e. Spiral and Spiral produces a Spiral. Elliptical and Elliptical produces an Elliptical.)

'"The Evolution of Galaxy Mergers and Morphology at z<1.2 in the Extended Groth Strip", by Lotz, Davis, Faber, Guhathakurta, Gwyn, Huang, Koo, Le et al.

http://arxiv.org/abs/astro-ph/0602088

The follow are excerpts from the paper:

Since the universe was smaller and denser in the distant past, one would think that mergers would not simply "occur throughout the life of the universe," but would occur much more frequently in the early universe (as has been observed), and then less and less frequently as the universe ages.

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Wow! Now that's something to think about.

Thanks, John M.

In reply to Cougar's comment:
The problem is that as mergers are predicted to increase roughly as (1+z)^3, how does one explain why the predominate galaxy type in the current universe, is not elliptical. (i.e. a spiral galaxy merged with a spiral galacy produces an elliptical galaxy, an elliptical galaxy merged with an elliptical galaxy always produces an elliptical galaxy.)

There galaxies at redshift z=6.5, where the merger rate should be roughly 400 times greater that at redshift z=1.2. At redshift z=1.2 almost half of the spiral galaxy major mergers become elliptical galaxies and roughly 10% of the galaxies undergo major mergers. Is it reasonable to assume from z=6.5 to z=1.2 that all spiral galaxies would have been involved in major merges? If it is, then the percentage of galaxies that are elliptical should be roughly 50% or greater, rather than 18%.

Link to a list of high redshift galaxies.

http://www.astro.wisc.edu/~stanway/r.../highzobj.html

Excerpt from this paper which discusses the analysis of data, to determine the formation history of elliptical galaxies.

http://arxiv.org/abs/astro-ph/0509725v2

Another complication which any theory must explain is: Why do the spiral galaxy halos, have net zero angular moment (i.e. Stars and gas in the halo, orbit the halo in random directions such that their net angular moment is zero.) The following is the conventional hypothesis to explain this observation from An Introduction to Modern Astrophysics 2nd Edition, page 1024.

I do not support the above hypothesis. I believe the observation that spiral galaxies’ halos have net zero angular moment and a set of metallicity anomalies within spiral galaxies provides support for the hypothesis that there is a mechanism that causes the spiral arms of galaxies to rotate.

I very much doubt that elliptical galaxies do not rotate. I expect that it would be very unusual to find a mass, regardless of size, that does NOT rotate. I would be curious to know whether actual measurements have failed to detect motion, including Doppler measurements on edges of elliptical galaxies. If only attempts have been made to detect proper motion in small portions of elliptical galaxies, I wouldn't find it surprising that none were found.

If separation of the DM halo though external interaction is the mechanism for for the formation of elliptical morphology, then it is possible that the continuing existance of a halo acts as a buffer in collisions, or at least simply not all collisions strip away the halo, so maintaining the spiral shape after post interaction.

also, there are a range of sprial morphologies than could be included as a result of partial separation of the DM component.

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Posts six and seven. Wow! Just wow! Good thing the Universe doesn't run by what I expect it can do.

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I've noticed quite a bit of ignoring others going on, lately. Evidently some people's manners are either slack or non-existant.

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If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

The Monolithic Scenario. But I also heard the theory that long-term in-fall of intergalactic gas into an elliptical galaxy can (if the gas has enough angular momentum) produce a disc, turning the elliptical into a lenticular or spiral galaxy. Has this been debunked, or does it still hold water?

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I honestly don't know the answer to that, though it is a good question. I suspect it might be necessary for the "continuous" formation of dwarf galaxies in voids and underdensities, but I don't know about the current state of the art in that regard. I'm not even sure the answer is yet known, though I'll see if I can find anything.

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report