Imaging the Coma Cluster

At a distance of 100 Mpc, Coma is the nearest example of a rich cluster of galaxies. Once thought to be in virial equilibrium, Coma is now understood to be a cluster in the making, with several subunits in the throes of merging, making it a very dynamic and interesting place to study.

We obtained wide field images of Coma using the 0.6 meter (24 inch!) Burrell Schmidt telescope at Kitt Peak National Observatory. Despite the small size of the telescope, the wide field, large pixel format is excellent for detecting extended low surface brightness features.

Here is a pretty picture of the core of the cluster. This is a true-color image made from the R, V, and B band Schmidt images, approximating R-G-B.

Click on the image to see a full resolution version or here to see a GIF image of an even wider field of view or, if you prefer, a TIF image .

We noticed several unusual low surface brightness features in our deepest R-band image.

The most spectacular is a plume of stars about 130 kpc (500,000 light years) long in the heart of the cluster. That's 4 times the diameter of the Milky Way! The total luminosity is not as great as our Galaxy; the plume is the equivalent of about 3 Large Magellanic Clouds.

To get a better view of the "plume", the foreground stars and galaxies can be digitally removed (lower panel). Click on the image to see a better resolution version.

We then obtained much better images of the plume using the Canada-France-Hawaii Telescope wide field CCD imager. The new improved version reveals many details in the body of the plume, perhaps dwarf galaxies and intergalactic star clusters in the making.

Two other large, low surface brightness objects are also easily seen in these cutouts from the Schmidt images of the Coma cluster. The lower panel to the right shows a particularly interesting patch of stars following in the wake of the spiral galaxy NGC4911 as it plunges into the core of the cluster at more than 1000 km/s.

The plume and the other objects, including a fourth smaller arc noticed by Trentham & Mobasher (1998) are most easily understood as tidal debris that has been liberated through galaxy-galaxy or galaxy-cluster interactions in the relatively recent past (within the last several hundred million years). Over time, it will fade into the background sea of stars that permeates the Coma cluster.

To try to understand the content and origin of the Coma tidal debris, we have obtained WFPC2 images of NGC4911 and its wake of tidal debris. We are pursuing spectroscopy of the objects in and around the debris using the Keck Observatory; the redshifts of the objects can place them in the cluster or the background.

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UCD Physics Dept.
Cosmology Research Group
Institute of Geophysics and Planetary Physics at the Lawrence Livermore National Laboratory

Modified February, 2003
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