It’s so unsettling, this dark matter stuff. When you finally get a little bit comfortable with the first actual “view” of dark matter ever obtained, through gravitational lensing of a collision of galaxy clusters in the Bullet Cluster, then a disturbing analysis of such a collision in the galaxy cluster Abell 520 shows something odd and takes all your security away! Never mind we don’t know what dark matter really is. Not knowing what it is and then not being able to understand when you see it, well frankly this is too much. Dark matter is adding insult to injury.
Maybe the picture is not that bleak, but maybe it is. We have analyzed cluster of galaxy collision ad infinitum on LMSP! Basically, this is how it goes:
There is a lot more gas than luminous/normal matter in clusters of galaxies. During collisions the gas is basically stripped off the colliding clusters of galaxies because the gas is slowed when colliding with other gas. But, the luminous matter sticks with the dark matter.
The dark matter is revealed by the extreme bending of light the dark matter facilitates from objects behind those involved in the collision. This intense signature of dark matter bends the light far more than the amount of luminous matter in the respective colliding clusters can. So, the dark matter which interacts so weakly with other matter and itself just passes on through the collision, and its presence is bellied by the gravitational lensing it causes.
Now, suddenly, however, in this Abell 520 collision, a dark matter core is just sitting there bending the light from objects beyond it. The problem is no luminous objects stayed behind with it.
Did this core of dark matter slow somehow in the collision, while the luminous matter kept on going? Don’t tell us now please that dark matter is “sticky” and that it could slow in a collision with other dark matter, because frankly, we don’t want to hear that. We don’t understand how that could be, beyond not understanding what dark matter is. Does that make any sense? Yes, it’s way too hard to admit just exactly how ignorant we are on this subject
Could it be that more than one galaxy cluster was involved in the collision than is evident now? I sure hope so, because that might be easier to explain. The dynamics of multiple cluster collisions beyond just two get very complicated and might yield such a result.
Or is it possible that very faint galaxies are in fact present where the dark matter is, just beneath the Hubble’s limiting magnitude? Now that would be best of all, because then we could go off just thinking that we are not even further confused beyond not knowing what dark matter is, let alone how it behaves.
Yes, I do have limits on how confused I want to be over this issue! Scientists have proposed other explanations but I am rooting for the latter two explanations of those NASA shares in its release of these findings.
The image I have posted today: http://hubblesite.org/newscenter/archive/releases/2012/10/image/a/format/xlarge_web/ elucidates the results that are not popular at this moment for the aforementioned reasons.
“This composite image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters.
“The natural-color image of the galaxies was taken with NASA's Hubble Space Telescope and with the Canada-France-Hawaii Telescope in Hawaii.
“Superimposed on the image are "false-colored" maps showing the concentration of starlight, hot gas, and dark matter in the cluster. Starlight from galaxies, derived from observations by the Canada-France-Hawaii Telescope, is colored orange. The green-tinted regions show hot gas, as detected by NASA's Chandra X-ray Observatory. The gas is evidence that a collision took place. The blue-colored areas pinpoint the location of most of the mass in the cluster, which is dominated by dark matter. Dark matter is an invisible substance that makes up most of the universe's mass. The dark-matter map was derived from the Hubble Wide Field Planetary Camera 2 observations by detecting how light from distant objects is distorted by the cluster of galaxies, an effect called gravitational lensing.
“The blend of blue and green in the center of the image reveals that a clump of dark matter resides near most of the hot gas, where very few galaxies are found. This finding confirms previous observations of a dark-matter core in the cluster. The result could present a challenge to basic theories of dark matter, which predict that galaxies should be anchored to dark matter, even during the shock of a collision.”
Credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University)