The Pan Andromeda Archeological Survey ( PAndAS ) CFHT Megaprime survey of the M31-M33 system has found a star stream which extends about 120 kpc NW from the center of M31 .
The great length of the stream , and the likelihood that it does not significantly intersect the disk of M31 , means that it is unusually well suited for a measurement of stream gaps and clumps along its length as a test for the predicted thousands of dark matter sub-halos .
The main result of this paper is that the density of the stream varies between zero and about three times the mean along its length on scales of 2 to 20 kpc .
The probability that the variations are random fluctuations in the star density is less than 10 ^ { -5 } .
As a control sample we search for density variations at precisely the same location in stars with metallicity higher than the stream , [ Fe/H ] = [ 0 , -0.5 ] and find no variations above the expected shot noise .
The lumpiness of the stream is not compatible with a low mass star stream in a smooth galactic potential , nor is it readily compatible with the disturbance caused by the visible M31 satellite galaxies .
The stream ’ s density variations appear to be consistent with the effects of a large population of steep mass function dark matter sub-halos , such as found in LCDM simulations , acting on an approximately 10 Gyr old star stream .
The effects of a single set of halo substructure realizations are shown for illustration , reserving a statistical comparison for another study .