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

  1. #1
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    Spiral Galaxies Rotate. Elliptical Galaxies do not rotate. Why?

    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

  2. #2
    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:

    Quote Originally Posted by me
    Now, can you provide the math that shows that a MECO can actually fragment, and show that the physical conditions that you assume in that model are physically viable? Please start a thread in the ATM section where you work this through, or cite a paper that does so.
    ...
    I'm waiting to see the quantitative predictions of your model of MECO fragmentation, including how the fragmentation products would look exactly like O and B stars and why they would have randomly oriented orbits.
    I'm going to keep quoting myself until you cease ignoring me.

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    Conservation of Angular Moment Problem

    In reply to parejkoj’s answer:

    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.
    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:
    Originally Posted by me
    Now, can you provide the math that shows that a MECO can actually fragment, and show that the physical conditions that you assume in that model are physically viable? Please start a thread in the ATM section where you work this through, or cite a paper that does so.
    ...
    I'm waiting to see the quantitative predictions of your model of MECO fragmentation, including how the fragmentation products would look exactly like O and B stars and why they would have randomly oriented orbits.
    I'm going to keep quoting myself until you cease ignoring me.
    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.

  4. #4

    Lightbulb Ellipticals Do Rotate

    Quote Originally Posted by William View Post
    Spiral Galaxies Rotate. Elliptical Galaxies do not rotate. Why?
    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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    Elliptical Galaxies Predominantly Radial

    In reply to Tim Thompson's comment:

    Factually incorrect statement. Elliptical galaxies most certainly do rotate, and this is not a "recent" discovery.
    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:
    An alternative interpretation for the difference between the histograms in Figure 4 is to assume that there is a true redshift evolution in the internal dynamics of elliptical galaxies. Since elliptical galaxies are collisionless systems, mergers are required to significantly change their rotation rate. There are different types of mergers and these can affect the evolution of the observed rotation rate in different ways. We will discuss the relevant types in turn. Independent of the type of merger, we are only concerned here with mergers that happen between z ≈ 0.5 and z = 0, i.e., in the last 5 Gyr.

    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.

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    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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    Galactic Merging Frequency & Resultant

    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:

    Some mergers may not follow the standard Toomre (1977) picture of two gas-rich disks transforming into a gas-poor elliptical. Our results suggest that at least half of the time the merging process does not dramatically transform the galaxies’ morphologies. This conclusion is supported by recent work which finds that the end product of some gas-rich mergers can be disk dominated (Springel & Hernquist 2005; Hammer et al. 2005; Robertson et al. 2006) while spheroidal-spheroidal mergers will produce spheroidals (Bell et al. 2006a).
    The theoretical predictions for the galaxy merger rate over this epoch have changed in recent years. Ahough n-body simulations of galaxy halos predict a halo merger fraction ∝ (1+z)^3 (Gottl¨ober, Klypin, & Kravtsov 2001), the galaxy merger rate may evolve less dramatically than the halo merger rate because multiple galaxies occupy the same halo at late times (Berrier et al. 2006). Interestingly, many semi-analytical models do not predict a dramatic increase in the fraction of merging galaxies righter than MB = −20 from z ∼ 0 to z ∼ 1 (Bell et al. 2006a; Benson et al. 2002)
    We find that the fraction of merger candidates is ∼ 10±2% and does not evolve strongly from z ∼ 1.2 to z ∼ 0.2. The fraction of E/S0/Sa has increased by a factor of ∼ 2 from 21±3% at z ∼ 1.1 to 44±9% at z ∼ 0.3. The fraction of Sb-Ir has declined from 64±6% to 47±9% over the same redshift range.

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    Quote Originally Posted by William View Post
    The problem with the elliptical dry merger hypothesis is that mergers should continue to occur throughout the life of the universe...
    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.
    Everyone is entitled to his own opinion, but not his own facts.

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    More Dark (Groan) Matter

    Quote Originally Posted by Ken G View Post
    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.
    Wow! Now that's something to think about.

    Thanks, John M.

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    High Redshift Merger Rate & Elliptical Resultant

    In reply to Cougar's comment:
    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.
    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

    We identify 312 merger candidates brighter than 0.4L∗B and find that the fraction of major merger candidates is roughly constant at ∼ 10% for 0.2 < z < 1.2 for both samples.

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    Spiral Galaxy Halo's Have Net Zero Angular Moment

    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.

    …according to the hierarchical model, collisions and tidal interactions between merging fragments… these proto-Galaxy fragments that were initially moving in a retrograde direction relative to the eventual orbital motion of the Galaxy disk and inner halo produce the net zero rotation of the outer halo that is observed today.
    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.

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    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.
    Last edited by dcl; 2008-May-07 at 04:18 PM.

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    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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    Quote Originally Posted by parejkoj View Post
    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.
    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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    Quote Originally Posted by parejkoj View Post
    Spiral galaxies form from a single collapsing cloud of gas.
    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?

  18. #18
    Quote Originally Posted by Eroica View Post
    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?
    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.

  19. #19
    Quote Originally Posted by William View Post
    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
    This is an old thread, but the question was not answered. And for good reason, astronomers do not know the answer to why ellipticals which tend to be common in clusters have little support from rotation, while spirals, which tend to be common in the field between clusters have most support from rotation. In the case of ellipticals the majority of the support (from collapse) is from random motions. But, there is an established theory on the origin of the angular moment in galaxies. It was first described by Jim Peebles in 1969 (ApJ and A&A). It is the idea of tidal torque. If a protogalaxy is not spherical then there is some major axis with a direction (called the quadrupole moment). Either a neighboring galaxy or group or cluster will be exerting a tidal field which acts to torque the major axis toward the neighbor. This spins up the galaxy. Now, since galaxies tend to expand away from each other by the time the galaxy turns enough to point toward the neighbor, the tidal field has subdued because of distance. Also the collapse of the protogalaxy reduces its moment arm and that reduces the torque. So, the galaxy breaks free to continue spinning in whatever direction it started to turn.

    Originally, this effect was too low by about a factor of 5 or so, but when dark matter was introduced into the calculation it worked out just right. Another piece of evidence in favor of dark matter.

    So, this explains spirals but whence ellipticals? Well, one might suppose that some fraction of galaxies will by coincidence either already be lined up with a neighbor or just happen to be quite spherical. Or, perhaps there just is no net tidal field because neighbors are small or they cancel each other. So some small fraction of galaxies just never get spun up. This is not bad to explain the low fraction of ellipticals in the field and perhaps the central galaxies of clusters which just do not have important neighbors.

    But, it did not explain the preponderance of ellipticals in clusters. In 1984, I wrote a little paper in ApJ that received no attention and has never been tested that tried to explain this. The idea is cute even if not correct and it goes like this: The tidal torque mechanism requires expansion, but what of galaxies that are forming in a collapsing environment. Then, instead of breaking away before reaching alignment, the galaxy passes through 0 degrees and starts to get torqued in the opposite direction. In other words, there is a tidal locking instead of a tidal spinup. This would be consistent with the first generation of galaxies which form as they fall into the cluster and make the cluster very elliptical rich. Later on some spirals would fall in as well.

    Well, the merging hypothesis came along and that provided a new mechanism to form elliptical galaxies. Here, two similar size galaxies fall together and the baryons, because they dissipate energy fall to smaller radii earlier. At smaller radii, they go around the center of mass faster and get ahead of the dark matter. The dark matter pulls back on the baryons and removes more angular moment, so the baryons crash to the center having exchanged most of its angular moment with the dark matter. This forms an elliptical. This is all sound and it must also be happening some fraction of the time. But, we don't know how often. It can't be too often because spirals are also formed by the merging smaller systems as all galaxies are. It requires fairly similar massed systems and usually that is not the case. Also, it is not clear how this explains the difference in populations in the cluster and the field. Perhaps it is the high rate of collisions in a cluster that makes the difference.

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