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tokens: This work required the use and integration of a number of Python software packages for science, including AstroPy (Astropy Collaboration et al. 2013 ), 5 5 http://www.astropy.org Matplotlib (Hunter 2007 ), 6 6 http://matplotlib.org NumPy, 7 7 http://www.numpy.org SciPy, 8 8 http://www.scipy.org/scipylib/ and yt (Turk et al. 2011 ). 9 9 http://yt-project.org We are thankful to the developers of these packages. We also thank Volker Springel for the use of the AREPO code. J.A.Z. thanks David Barnes, Rahul Kannan, Federico Marinacci, and Hui Li for useful discussions. He also thanks Grant Tremblay for assistance with a last-minute fix to a figure. J.A.Z. acknowledges support through Chandra Award Number G04-15088X issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. J.Z. acknowledges support by a Grant of Excellence from the Icelandic Research Fund (grant No. 173929−051). The numerical simulations were performed using the computational resources of the Advanced Supercomputing Division at NASA/Ames Research Center.