Thousands of galaxies crowding an area on the sky roughlythe size of the full moon have been captured in a new photo released today.
The new cosmic photo, a wide-field view from the EuropeanSouthern Observatory, reveals many thousands of distant galaxies, including alarge group belonging to the massive galaxy cluster known as Abell 315.
Yet, as crowded as it may appear, this assembly of galaxies? like most galaxy clusters ? is dominated by darkmatter that remains unseen. And while the actual existence of dark matterremains largely unexplained, this mysterious stuff has helped scientists piecetogether other parts of the cosmic puzzle. For instance, dark matter'sgravitational pull on galaxy clusters helped researchers calculate the mass ofAbell 315.
When stargazers scan the night sky with the unaided eye, theymostly see only stars within our own Milky Way galaxy and some of its closestneighbors. More distantgalaxies tend to be too faint to be perceived by the human eye, but if theycould be seen, they would literally cover the entire sky.
These galaxies span a vast range of distances. For thosethat are relatively close, it is possible to distinguish their spiral arms orelliptical halos, particularly in the upper part of the image.
The more distant galaxies appear much fainter. The lightfrom these galaxies have travelled through the universe for 8 billion years ormore before reaching Earth. The universe is about 13.7 billion years old.
The thing about Abell (315)
The galaxy cluster Abell 315 can be seen in the imagebeginning in the center and extending below and to the left. Abell 315 ?so-called because it was designated with the number 315 in the cataloguecompiled by the American astronomer George Abell in 1958 ? is a concentrationof about a hundred yellowish galaxies.
The galaxy cluster is located about 2 billion light-yearsaway from Earth in the constellation of Cetus (the Whale).
Galaxy clusters are some of the largest structures in the universeheld together by gravity, and while these types of images can reveal a rathercrowded field of view, there is actually more in these structures than what wesee. ?
In these clusters, the galaxies themselves contribute toonly 10 percent of the mass, with hot gas in between the galaxies accountingfor another ten percent. The remaining 80 percent is made up of an invisibleand unknown ingredient called darkmatter that can be found in between the galaxy structures.
The presence of dark matter is revealed through itsgravitational effect: the enormous mass of a galaxy cluster acts on the lightfrom galaxies behind it (much like a cosmic magnifying glass), bending thetrajectory of the light and thus making the galaxies in the background appearslightly distorted.
Dark matter secrets
By observing and analyzing the twisted shapes of thesebackground galaxies, astronomers can infer the total mass of the clusterresponsible for the distortion, even when this mass is mostly invisible.
Still, this effect is tiny, and to be accurate, it is necessaryto measure it over a huge number of galaxies to obtain significant results.With Abell 315, the shapes of almost 10,000 fainter galaxies in this image werestudied in order to estimate the total mass of the cluster, which amounts toover a hundred thousand billion times the mass of our sun.
A handful of other objects that are closer to Earth and muchsmaller than galaxies and galaxy clusters are also scattered throughout theimage, including stars belonging to our own galaxy, as well as many asteroidsthat are visible as blue, green or red trails. These objects belong to the mainasteroid belt, located between the orbits of Mars and Jupiter.
The image was taken by the Wide Field Imager on the MPG/ESO2.2-meter telescope at ESO's La Silla Observatory in Chile.
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