John Phillips edited Intro.tex  over 9 years ago

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The standard picture of the evolution of substructure in the Universe involves the collapse of dark matter into halos, which may host luminous galaxy. Such halos may exist within the bounds of larger halos; in these cases the galaxies they may host are typically called satellite galaxies, and their evolution differs substantially from galaxies that are not satellites in ways not fully understood. Analysis of the spatial and kinematic distributions of such galaxies can inform our ideas of how satellites and the systems in which they are found evolve. Substantial evidence exists that satellite galaxies are not isotropically distributed around their hosts. \cite{West2000,Bailin_2008}, nor are their velocities isotropic \textbf{cite}. This is also seen in simulations; subhaloes of hosts typically are typically distributed anisotropiclly in both position and velocity space \cite{VDB99,Knebe,Zentner_2005,Faltenbacher_2010}.  Local group satellites are highly anisotropically distributed both around the Milky Way and M31. The disk-like arrangement of MW satellites was first pointed out by \citet{Lynden-Bell74}. Later studies argued further for the existence of a disk-like structure of Milky Way satellites, \cite{Kroupa05}  and noted that the arrangement of the satellites strongly favors the Northern Hemisphere. Around M31, dramatic evidence has been found for a disk of satellites, many of which exhibit coherent rotation along the line of sight \cite{Ibata_2013}. The M31 structure seems particularly difficult to square with our picture of galaxy evolution; \cite{Ibata_2014} argues that \cite{Ibata_2014} that alignments similar to the one found around M31 are essentially non-existant in numerical simulations. Much recent work has gone into investigating the possibility of similar satellite distributions around galaxies outside of the local group. Recently, work by \citet{Ibata_2014} (hereafter I14) pointed to the possibility of corotation seen in diametrically opposed satellite pairs in Sloan Digital Sky Survey (SDSS), finding 20 out of 22 oppositely aligned satellite pairs corotating along the line of sight. This result was contested by \citet{Cautun14}, arguing that the results of \citet{Ibata_2014} are strongly dependant on selection criteria and are not robust. The original authors then claimed that less-massive satellites than originally considered exhibit a spacial over-density consistent with the claimed existence of co-rotating sturctures frequently seen in SDSS. Still, the consensus is far from clear as to the prevelence of co-rotating satellite disks in the non-local Universe is unclear.