We present the velocity dispersion of red giant branch (RGB) stars in M31'shalo, derived by modeling the line of sight velocity distribution of over 5000stars in 50 fields spread throughout M31's stellar halo. The dataset wasobtained as part of the SPLASH (Spectroscopic and Photometric Landscape ofAndromeda's Stellar Halo) Survey, and covers projected radii of 9 to 175 kpcfrom M31's center. All major structural components along the line of sight inboth the Milky Way (MW) and M31 are incorporated in a Gaussian Mixture Model,including all previously identified M31 tidal debris features in the observedfields. The probability an individual star is a constituent of M31 or the MW,based on a set of empirical photometric and spectroscopic diagnostics, isincluded as a prior probability in the mixture model. The velocity dispersionof stars in M31's halo is found to decrease only mildly with projected radius,from 108 km/s in the innermost radial bin (8.2 to 14.1 kpc) to $\sim 80$ to 90km/s at projected radii of $\sim 40$ to 130 kpc, and can be parameterized witha power-law of slope $-0.12\pm 0.05$. The quoted uncertainty on the power-lawslope reflects only the precision of the method, although other sources ofuncertainty we consider contribute negligibly to the overall error budget.
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