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<div>I got this email message below from Prof.<span style="color: rgb(0, 0, 0); font-size: 12px;"> </span><span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">Nicholas Suntzeff (College Station). really exciting news
about dark matter.</span></div>
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<div>Carlos </div>
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<span style="font-weight:bold">From: </span>nsuntzeff <<a href="mailto:nsuntzeff@tamu.edu">nsuntzeff@tamu.edu</a>><br>
<span style="font-weight:bold">Date: </span>Wed, 7 Oct 2015 00:24:15 -0500<br>
<span style="font-weight:bold">To: </span><<a href="mailto:faculty-talk@physics.tamu.edu">faculty-talk@physics.tamu.edu</a>><br>
<span style="font-weight:bold">Cc: </span>"Astrophysics@TAMU" <<a href="mailto:astro@listserv.tamu.edu">astro@listserv.tamu.edu</a>><br>
<span style="font-weight:bold">Subject: </span>[Faculty-talk] Interacting dark matter<br>
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Dear All,
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<div class="">The European Southern Observatory has released some photos and analysis that are quite puzzling. See:</div>
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<div class=""><a href="http://www.eso.org/public/images/eso1514a/" class="">http://www.eso.org/public/images/eso1514a/</a></div>
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<div class="">What they have found is that through gravitational lensing, they have mapped out the dark matter content around four interacting galaxies buried deep in the core of a cluster of galaxies. What is strange is that there is one galaxy where the dark
matter appears to have lagged behind the galaxy. This seems to imply that dark matter is interacting in a non-gravitational way, and perhaps is self-interacting. </div>
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<div class="">This is not like the Bullet Cluster where two galaxies passed through each other, and the interaction separated the gas from the stars/dark matter. Here there is no gas and as the galaxies collide, both the stars and the dark matter will pass
through each other. Gravity will slow the components as they pass through each other due to gravitational friction, but the separation of the dark matter and the stars was not expected as seen in the image.</div>
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<div class="">The interpretation is not iron clad at this point, but it is very intriguing.</div>
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<div class="">The picture of the cluster in the article above is also intriguing. I attach the HST image of the cluster. You can see the four elliptical galaxies enveloped in a much larger luminous halo, which is not uncommon in very rich clusters. But if you
look closely into the haze, you can see four images of a single galaxy that looks like a blue loop with a dot in the center. It is this multiply imaged galaxy, along with some other arcs that allow such a reconstruction of the dark matter distribution.</div>
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<div class="">In this image you can see the derived contours of the dark matter (in dark blue lines). The galaxy to the 10 o’clock position of the quartet has the dark matter contours clearly not centered on the galaxy, which they are interpreting as perhaps
due to self interaction of the dark matter. The other three galaxies do not have enough contrast from the lensing to detect such an offset.</div>
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<div class="">cheers nick</div>
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