ESO: Spectacular Galaxy Cluster –“Observed Harboring a Voracious Central Cannibal”




Galaxy clusters do not come in neatly defined shapes so it is difficult to determine exactly where they begin and end. However, astronomers have estimated that the centre of the Fornax Cluster is in the region of 65 million light-years from Earth. What is more accurately known is that it contains nearly sixty large galaxies, and a similar number of smaller dwarf galaxies. Galaxy clusters like this one are commonplace in the Universe and illustrate the powerful influence of gravity over large distances as it draws together the enormous masses of individual galaxies into one region.

At the center of this particular cluster, in the middle of the three bright fuzzy blobs on the left side of the image shown below, is what is known as a cD galaxy — a galactic cannibal. cD galaxies like this one, called NGC 1399, look similar to elliptical galaxies but are bigger and have extended, faint envelopes. This is because they have grown by swallowing smaller galaxies drawn by gravity towards the center of the cluster.


The central galaxy is often the brightest galaxy in a cluster, but in this case the brightest galaxy, NGC 1316 , is situated at the edge of the cluster, just outside the area covered by the image below. Also known as Fornax A, it is one of the most powerful sources of [radio waves] — in the sky. The radio waves, which can be seen by specialised telescopes sensitive to this kind of radiation, emanate from two enormous lobes extending far into space either side of the visible galaxy. The energy that powers the radio emission comes from a supermassive black hole lurking at the center of the galaxy which is emitting two opposing jets of [high-energy particles]. These jets produce the radio waves when they plough into the [rarefied gas] which occupies the space between galaxies in the cluster. The spectacular image was taken by the VLT Survey Telescope at ESO's Paranal Observatory in Chile.  This image shows the central part of the cluster in great detail. At the lower-right is the elegant barred-spiral galaxy NGC 1365 and to the left the big elliptical NGC 1399. (ESO, Aniello Grado and Luca Limatola).




The Chandra x-ray image at the top of the page actually explores the center of a much more extended cosmic family — the Fornax cluster of galaxies some 65 million light-years away. Spanning nearly 900,000 light-years, the Chandra Observatory composite image reveals high-energy emission from several giant galaxies near the Fornax cluster center and an immense, diffuse cloud of x-ray emitting hot gas. On the whole, the hot cluster gas seems to be trailing toward the upper left in this view. As a result, astronomers surmise that the Fornax cluster core is moving toward the lower right, encountering an intergalactic headwind as it sweeps through a larger, less dense cloud of material. In fact, along with another visible galaxy grouping at the outskirts of the cluster, the Fornax cluster core galaxies seem to be moving toward a common point, attracted by the dominating gravity of unseen structures of dark matter in the region.

Indeed, there is evidence that this process is happening before our eyes — if you look closely enough. Recent work by a team of astronomers led by Enrichetta Iodice (INAF – Osservatorio di Capodimonte, Naples, Italy), using data from ESO's VST, has revealed a very faint bridge of light between NGC 1399 and the smaller galaxy NGC 1387 to its right. This bridge, which has not been seen before (and is too faint to show up in this picture), is somewhat bluer than either galaxy, indicating that it consists of stars created in gas that was drawn away from NGC 1387 by the gravitational pull of NGC 1399. Despite there being little evidence for ongoing interactions in the Fornax Cluster overall, it seems that NGC 1399 at least is still feeding on its neighbours.

Towards the bottom right of this image is the large barred spiral galaxy NGC 1365. This is a striking example of its type, the prominent bar passing through the central core of the galaxy, and the spiral arms emerging from the ends of the bar. In keeping with the nature of cluster galaxies, there is more to NGC 1365 than meets the eye. It is classified as a Seyfert Galaxy, with a bright active galactic nucleus also containing a supermassive black hole at its center.



A 2004 survey made with the Anglo-Australian Telescope (AAT) shown above revealed dozens of previously unsuspected miniature galaxies in the Fornax cluster.They belong to a class of galaxies dubbed "ultra-compact dwarfs" (UCDs), which was unknown before the same team of astronomers discovered 6 of them in the Fornax cluster in 2000. Now they say the UCDs outnumber the "conventional" elliptical and spiral galaxies in the central region of the Fornax cluster and they have found some in the Virgo galaxy cluster too. It is possible that at least some are left-over examples of the primordial `building blocks' that formed large galaxies by merging together. It is likely that they are very common but have been overlooked because they resemble nearby stars at first sight.

The Daily Galaxy via ESO

Image credit: Central region, C. A. Scharf, D. R. Zurek, M. Bureau (Columbia Univ.), CXC, NASA

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