Abell 520

Abell 520 is a cluster of galaxies in the constellation of Orion, whose name derives from its entry in the Abell catalog. The exact coordinates are right ascension 04h 54m 03.80 s and declination 02 ° 53 ' 33.00 ". At a redshift of z = 0.201 it is about 2.4 billion light -years away from Earth and the sky has an optical extent of about 10 arc minutes.

Specificity and significance

The total mass of the system is about 1015 solar masses, spread upon closer inspection, several currently merging clusters of galaxies. Abell 520 provides for the Bullet Cluster is the second system in which the dark matter could be observed separately from the baryonic ( ordinary ) matter; this could be helpful to better understand the nature of dark matter. Special significance at the pile in that it (still) is not in an equilibrium state.

The situation in the case of Abell 520 is significantly more complex than the Bullet Cluster. A good understanding ( both cases) requires further observations and theoretical, numerical considerations. Abell 520 is a challenge for current models of the development of clusters of galaxies, but this does not always exist a contradiction.

Details

As part of a multi -wavelength campaign, the mass distribution in 50 massive clusters of galaxies was analyzed with the help of the gravitational lens effect. The following are the results for Abell 520 are summarized.

The reconstruction of the mass distribution of Abell 520 shows four mass concentrations ( entries 1-4), which are strung like pearls on a chain in NE-SW direction ( see Figure as well as references for further illustrations ). Due to this arrangement, call Mahdavi et al. 2007 Abell 520 as a " cosmic train wreck " ( cosmic train wreck ). A fifth mass concentration located to the east of it. On the basis of the brightest galaxies in the mass concentrations, their radial velocities can be determined. It turns out that they all differ markedly from each other, so that it is moving relative to each other mass gatherings, in fact. This situation is unusual, but hardly of particular importance. The only becomes apparent when one examines the "composition" of the individual mass gatherings in more detail. So here the respective mass fractions are meant that in galaxies ( stars ) are in diffuse intergalactic gas and dark matter. For a typical galaxy cluster, the proportion of stars with less than ten per cent and for intergalactic gas at about 15-20 percent. The radical and thus the major 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 produced as much light and is typically given in units of solar masses per solar luminosity (also below). For most galaxies, it is of the order of 200: This means, for example, for ourselves, that still about 200 times the same mass depends on the total mass of our solar system in our cosmic environment, which is in the non-luminous form. The ratio is so high because most of the mass is not bound in shining stars, but either in the form of dark matter or gas, which emits relatively little light ( compared to the Sun ).

In the analysis of the MLV for each mass concentrations of the Abell 520 system, two stand out particularly. These are among the mass number 3 and 5 The first is approximately in the center of the Zugwracks and No. 5, the mass concentration is east of the wreck. For # 3, see Mahdavi et al. 2007 MLV of about 720, which is significantly more than the normal case, while for the fifth mass concentration only a value of about 60 is found. 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 well within expectations, as dark and baryonic matter (which, mainly in the form of intergalactic gas rather than stars ) should behave differently at a pile of merger ( see also " Bullet cluster" for a discussion of the dynamics ). In other words, a separation of baryonic and dark matter is not surprising. Unusually, however, the distribution of galaxies. Of these, it is expected that they behave collision ( ie how the dark matter ). It turns out, however, that the center of Zugwracks in which the bulk of the dark matter is concentrated, is as good as free from galaxies. In particular, none of the three brightest cluster galaxies located near the center. This is a crucial point, as well heaps are known which have an unusually high MLV, but there fall the positions of the brightest galaxies and dark matter together. With Abell 520, a case was first investigated here, where this is different. In contrast, the easternmost baryonic mass concentration to an overabundance of galaxies.

Assuming the mass concentrations of 3 and 5 together, the result is assigned to no abnormalities; it looks like an average galaxy clusters. This suggests that both have a common " ancestor ". It seems as if galaxies and dark matter were separated by a not yet understood effect.

In this effect, it could be, for example, to complex three -body interactions, the " spin " the galaxies from the center. It would also be conceivable that the dark matter is a non- gravitational interaction shows that can explain their spatial separation of the galaxies. The question of the reason for the separation of galaxies and dark matter is yet unanswered.