Abell 520
| Galaxy clusters | ||
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| Abell 520 | ||
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| False color image of the galaxy cluster. (Orientation: north up, east left.) The distribution of dark matter is shown in blue, while the red areas indicate the regions of increased baryonic density. | ||
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| Abell 520 is just north of the circled star π 5 Ori. | ||
| Constellation | Orion | |
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Position equinox : J2000.0 |
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| Right ascension | 04h 54.3m | |
| declination | + 02 ° 57 ′ | |
| Further data | ||
| Brightness (visual) |
17.5 likes |
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| Angular expansion |
10.0 |
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| history | ||
| Catalog names | ||
| ACO 520 • RXC J0454.1 + 0255 | ||
| Aladin previewer | ||
Abell 520 is a galaxy cluster in the constellation Orion , the name of which is derived from its entry in the Abell catalog . The coordinates are right ascension 04h 54.3m and declination + 02 ° 57 '. With a redshift of z = 0,201 it is about 2.4 billion light years away from the earth and has an optical expansion of about 10 arc minutes in the sky .
Peculiarity and meaning
The total mass of the system is around 10 15 solar masses , which, on closer inspection, are distributed over several currently merging galaxy clusters. After the bullet cluster, Abell 520 represents the second system in which dark matter could be observed separately from baryonic (ordinary) matter ; this could be helpful in understanding the properties of dark matter better. The heap is of particular importance because it is not (yet) in a state of equilibrium .
The situation in the case of Abell 520 is much more complex than that of the bullet cluster. A good understanding (of both cases) requires further observations and theoretical, numerical considerations. Abell 520 poses a challenge to previous models of the evolution of galaxy clusters, but this does not necessarily have to be a contradiction.
Details
As part of a multi- wavelength - campaign was determined using the gravitational lens effect , the mass distribution in 50-mass galaxies analyzed. The results for Abell 520 are summarized below.
The reconstruction of the mass distribution of Abell 520 shows four mass concentrations (No. 1–4), which are lined up like a pearl necklace in NE-SW direction (see figure and references for further illustrations). Because of this arrangement, Mahdavi et al. 2007 Abell 520 also known as "cosmic train wreck ". A fifth mass concentration is to the east of it. Based on the brightest galaxies in the respective mass concentrations, their radial velocities can be determined. It turns out that they all differ noticeably from one another, so that they are in fact a matter of mass accumulations moving relative to one another. This constellation is unusual, but hardly of particular importance. This only becomes apparent when the “composition” of the individual masses is examined more closely. This means the respective mass fractions that are in galaxies (stars), in diffuse intergalactic gas and dark matter. For a typical galaxy cluster, the proportion for stars is less than ten percent and for intergalactic gas around 15–20 percent. The rest, and thus the largest part, is in the form of dark matter.
A measure of the mass ratio of baryonic matter (stars and gas) to dark matter is the so-called "mass-luminosity ratio (MLV)" . This indicates how much mass how much light is generated, and is typically given in units of solar masses per solar luminosity (see below). For most galaxy clusters it is in the order of magnitude of 200. For ourselves, for example, this means that for the entire mass of our solar system there is about 200 times the same mass in our cosmic environment that is present in non-luminous form. The ratio is so high that most of the mass is not bound in shining stars, but is either in the form of dark matter or gas, which emits relatively little light (compared to the sun).
When analyzing the MLV for the individual mass concentrations of the Abell 520 system, two particularly stand out. These belong to masses number 3 and 5. The first is roughly in the center of the collision, number 5 is the mass concentration east of it. For No. 3, Mahdavi et al. In 2007 an MLV of around 720, which is significantly more than in the normal case, while only a value of around 60 is found for the fifth mass concentration. This result can be interpreted as follows: mass concentration No. 3 contains almost only dark matter, while No. 5 contains almost only baryonic (luminous) matter. This corresponds to the expectations, since dark and baryonic matter (which is mainly in the form of intergalactic gas and not stars) should behave differently in a cluster merger (see also “ Bullet cluster ” for a discussion of the dynamics). In other words: a separation of baryonic and dark matter is not surprising. However, the distribution of the galaxies is unusual. These are expected to behave collision-free (like dark matter). It turns out, however, that the center of the collision, in which most of the dark matter is concentrated, is as good as free of galaxies. In particular, none of the three brightest galaxies in the cluster are near the center. This is a crucial point, since clusters are known to have an unusually high MLV, but the positions of the brightest galaxies and dark matter coincide there. With Abell 520, a case was examined for the first time where this is different. In contrast, the baryonic mass concentration to the east shows an abundance of galaxies.
If the mass concentrations No. 3 and 5 are taken together, the result does not show any abnormalities; it then looks like an average cluster of galaxies. This suggests that both have a common "ancestor". It seems as if galaxies and dark matter have been separated by a previously unknown effect.
This effect could be, for example, complex three-body interactions that “throw” the galaxies out of the center. But it is also conceivable that dark matter shows a non-gravitational interaction, which can explain its spatial separation from the galaxies. The question of the reason for the separation of galaxies and dark matter has not yet been answered.
Web links
- Chandra X-ray observatory. Illustrations and explanations (English) for Abell 520
- An article on the topic (German) on astronews.com