Is it likely, unlikely, or impossible for an elliptical structure to form when a gas cloud collapses?

Due to the conservation of angular momentum, one would expect that disk structures are much more likely to form than elliptical structures. Elliptical galaxies then formed by merging of disk structures (or of earlier elliptical structures, which themselves must have originated from disk structures). The presence of mostly very massive elliptical galaxies in the centers of galaxy clusters (with high mass densities) would support such a merging history.

Were the first galaxies all disk-like and are old large ellipticals the result of continuous merging events?

On the other hand, the majority of stars in ellipticals seem to have formed in a relatively short rapid burst of star formation. If elliptical structures were the results of mergers, wouldn't it be unlikely for the stars in these merging galaxies to have all formed at the same time? Shouldn't we, therefore, find various stellar populations in ellipticals that are old but do not have the same age?

Or perhaps the currently observed old populations in ellipticals were formed during the merging process, and the remaining gas of the merging partners was used up then. Still, then we should be able to detect some older stars from before the merging.

An interesting question then is what proportion do these "pre-merger stars" represent in ellipticals?

  • 2
    $\begingroup$ I feel like there is more than one question here; one about the dynamics of gas clouds and one about identifying stellar populations in large ellipticals? $\endgroup$
    – antlersoft
    May 29, 2023 at 16:15
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    $\begingroup$ Yes, that is probably, right feel free to ask any of them. If I have time this week I will make two seperate posts. $\endgroup$
    – trynerror
    May 30, 2023 at 6:13

2 Answers 2


The shape of a collapsing cloud is influenced by various factors, including the initial shape, angular momentum, ability to dissipate heat, and sub-clumping processes. When a gas cloud collapses, it can only contract by a factor of 2 in each direction before the rising heat content requires energy dissipation for it to continue its collapse. Generally, gas can dissipate and so the collapse proceeds but quite slowly. If the cloud forms sub-clumps at only a few collapse factors, that is, it rapidly forms stars, globular clusters, or molecular clouds, it maintains its shape at this point. Those gas clouds that were not rotating rapidly before the collapse would be primarily supported through an anisotropic velocity dispersion (pressure from random velocities), not rotational support, and will likely end up in an elliptical shape.

From the distribution of specific angular momentum of primordial gas clumps in cosmology simulations, one expects around 20% of first collapse galaxies would be ellipticals. This is about the fraction of galaxies that are elliptical and indicates that ellipticals formed by mergers are a minority of all elliptical.

For a gas clump that possesses significant angular momentum and sub-clumps slowly enough, it will collapse along the rotation axis into a flat rotating disk. This is the Population II stars in spirals. The stars that formed earlier, before the clump formed a flattened disk, compose the elliptical shaped stellar halo of Population I stars.

The halo plays an essential role in this process. With a dissipating gas cloud embedded in a non-dissipating halo, the dynamics become more complex. Because the universe began as a homogeneous expanding gas, there is very limited angular momentum. All angular momentum is picked up by tidal interactions between neighboring density enhancements. The dark matter halo only collapses by a factor of two, enabling the baryonic matter (ordinary matter) to collapse by a few extra factors which leads to higher rotational velocities that we observe in galaxies. Without the influence of this dark matter, more galaxies would be ellipticals.

The prevalence of ellipticals in clusters does not support the picture that all ellipticals are formed by mergers. Mergers between galaxies require velocities to be closely matched. This is not the case in clusters because they have high velocity dispersions. Thus, the merger rate is low in clusters once it forms. This suggests that the elliptical galaxies in clusters are primordial rather than formed by the late merger process.

There are a couple of potential explanations for the preferential formation of ellipticals in clusters. Firstly, galaxies forming in clusters may have collapsed into stars at a faster rate due to the higher densities present. The dense environment could lead to more efficient star formation processes, resulting, as described above, in the formation of elliptical galaxies.

Secondly, the process of tidal torque spin-up, which contributes to the formation of spirals, might be less effective in cluster environments. Tidal torque spin-up typically requires pure expansion to avoid tidal locking.

  • $\begingroup$ Two caveats for the "merger rate is low in clusters" argument: 1) the buildup of cluster velocity dispersions may be gradual, allowing some merging to continue for a while; 2) ellipticals at the centers of clusters are clearly the result of multiple mergers. $\endgroup$ May 30, 2023 at 20:36
  • $\begingroup$ Yes. The cD galaxies at the center of clusters grow by mergers as galaxies repeatedly fall through the center. I forgot to mention that. The build up of cluster velocity may be slow as the density rises. But, that means that one can't point to the high density in clusters as the reason for the predominance of ellipticals. Also, the cores of rich clusters formed quite early. $\endgroup$
    – eshaya
    Jun 1, 2023 at 0:56

You said "Due to the conservation of angular momentum disk structures are more likely to form" . Well, there may not be any angular momentum in the first place, so disc structures can not form at all. Angular momentum can for instance be transferred from the central region of a rotating gas cloud via 'angular momentum transfer' to the outer regions via magnetic fields created by the 'dynamo effect' in partially ionized gases (see http://th.nao.ac.jp/MEMBER/tomisaka/Lecture_Notes/StarFormation/5/node94.html for more). So there will always be a tendency for the central region of gaseous structures to have small angular momentum (just look at the angular momentum distribution in the present solar system). A galaxy cluster will therefore tend to have smaller systematic velocities (i.e. be closer to hydrostatic equilibrium) in the central region as compared to the outer region.

  • $\begingroup$ It's probably impossible for there to be zero angular momentum. $\endgroup$ May 30, 2023 at 20:31

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