Galaxy clusters, like the universe as a whole, are composed of both baryonic (“ordinary”) matter and dark matter, with about 1 part in 6 of the former, and five parts in 6 of the latter. A curious thing happens – usually – when two clusters collide. Both the visible baryonic matter, consisting mainly of luminous galaxies, and the dark matter, continue on their way after the collision. They show almost no signs of the encounter, although they may be fated, ultimately, to merge.
However, there appears to be substantial interaction of the intergalactic gas that existed before the encounter in both clusters. As a result, the gas loses most of its momentum and is left behind, between the two departing clusters, after the collision. This gas is very hot, and is easily detected at X-ray wavelengths. The best known example of this phenomenon is the Bullet Cluster, about which research was reported in 2006. (News articles: here, here, here, here; Research: here).
Yet now there is confirmation of a galaxy cluster collision – Abell 520 (A520) – in which a large quantity of dark matter appears to have been left behind. This is surprising, since the main reason given to explain why this should not happen is that dark matter is expected to interact very little with itself. The magnitude of this interaction is expressed as the ratio of the interaction “cross section” to mass. In the Bullet Cluster, the upper limit for this was found to be at most 1 cm2g-1. But in A520 this same measure is estimated at ~3.8 cm2g-1.
A520 seems to be the exceptional case, rather than the the Bullet Cluster, since results similar to the latter have been found in other cluster collisions, such as MACS J0025.4-1222 (News article: here; Research: here), Abell 2744 (News article: here; Research: here), and DLSCL J0916.2+2951.
All of these examples have been studied by the technique of weak gravitational lensing. In standard gravitational lensing a massive foreground object creates one or more magnified images of a more distant object in the background. But in weak lensing, the massive foreground object (or group of objects, such as colliding galaxy clusters) merely distorts the shape of galaxies in the background. The amount of mass (both ordinary and dark) in a small region is inferred by estimating, using statistical techniques, the degree to which shapes are distorted.
Differences in mass distribution across the entire field show up as differences in background galaxy distortion (red in the image above). Usually, though not always, large bright galaxies show up in regions with higher densities of ordinary and dark matter. The location of extremely hot gas, on the other hand, is shown by direct imaging of X-rays, using the Chandra X-ray Observatory (blue in the image).
So, what exactly have the researchers found in A520? This unusual conglomeration of galaxy clusters was first reported by mostly the same team of astrophysicists in 2007. (News articles: here, here, here, here; Research: here) A520, which is at redshift z=0.201 (comoving distance 2.65 billion light-years), was reported to contain at least 5 distinct concentrations of mass, based on the evidence of weak gravitational lensing. The updated research has identified one additional mass concentration. All of them appear to contain dark matter, and all but one (#3) also are associated with at least a few bright galaxies.
No bright galaxies seem to be associated with the third component, which is referred to as P3, but it is in the same location as the main concentration of X-ray luminous hot gas. It is in the center of A520 and evidently consists mostly of dark matter. (The hot gas makes up about 13% of its mass.) Because of its location and composition, it’s also referred to as the “dark core”.
The newly reported research largely confirms what was reported over 4 years ago. The main difference is that use of the Hubble Space Telescope has allowed three times as many lensed galaxies to be included in the survey, so that the statistical significance of resulting measurements is substantially higher. The mass inferred for P3 is about 18% less than previously, but is still quite comparable to the other components, all 6 of which are in the range 4-5×1013 M⊙. The net result remains the same: A520 is rather unlike other cases of galaxy cluster mergers that have been studied, in that a large concentration of dark matter seems to have become separated from the ordinary matter making up luminous galaxies.
Although the researchers do not emphasize it, A520 is also different from other cases in having 6 separate massive components, whereas the Bullet Cluster, for instance, has only 2. So the history of A520 could well be atypical of cluster collisions – perhaps 3 or more separate clusters participated in this “train wreck”. One of the authors of the research has said that perhaps there are “some sort of freak initial conditions that would create this amount of dark matter.” Computer simulations are being done to examine this possibility.
The researchers do present several hypotheses that might conceivably account for the existence of the “dark core”, although some are mostly ruled out by the data.
One possibility is that P3 actually does contain a cluster of massive but low luminosity galaxies. If so, the mass/luminosity ratio is unusually large. However, since the range of possible M/L ratios for large galaxy groups is not well established, this hypothesis can’t be ruled out yet. It could be that P3 does contain many massive but atypically dim galaxies, which more powerful telescopes might detect.
A second possibility is that a very massive cluster is on the line of sight to P3, but behind it at a high redshift (z>1). This would confound the weak lensing measurements (by distorting even more distant galaxies). This is considered doubtful, since galaxies behind P3 but at lower redshift (z<1) are still distorted.
A third possibility is that bright galaxies were scattered away from P3 during the collision. This is ruled out, since simulations indicate that only smaller galaxies, and not larger ones, might have been ejected from P3.
A fourth possibility is that the interaction cross-section of dark matter is substantially higher than inferred in the Bullet Cluster and other collision examples. In this case, there really would be a concentration of mostly dark matter where P3 is, as a result of dark matter interacting with itself. As noted above, the discrepancy between the cross-section/mass ratio of A520 and the Bullet Cluster is at least a factor of 4. This discrepancy – if it exists – is the most puzzling aspect of the results. It is somewhat smaller between A520 and other examples, but still large. The researchers suggest that further investigation of the Bullet Cluster might be in order.
The final possibility is that the structure of dark matter filaments passing through A520 includes one filament directed along our line of sight, passing through P3. This would obviously affect the lensing of background galaxies. Such a filament would need to be sparse enough to not contain many substantial galaxies along its length, or else be thin enough that it passes only through P3. Various observations are considered to make such a filament unlikely, though still possible.
What’s the bottom line? Just this: P3 is most likely a real concentration of dark matter with little ordinary matter, not an artifact. So we still have a lot to learn about the dynamics of galaxy cluster collisions, and how dark matter behaves in such events.
|Jee, M., Mahdavi, A., Hoekstra, H., Babul, A., Dalcanton, J., Carroll, P., & Capak, P. (2012). A STUDY OF THE DARK CORE IN A520 WITH THE HUBBLE SPACE TELESCOPE: THE MYSTERY DEEPENS|
The Astrophysical Journal, 747 (2) DOI: 10.1088/0004-637X/747/2/96