tokens: − 2.4 per cent and a dispersion of ∼ 10 per cent (8.7 per cent when adjusted for random uncertainty), in the outer r / r max > 0.2 region of the haloes. Similar level of dispersion ( ∼ 7 per cent or ∼ 6 per cent when adjusted for random uncertainty) is also observed for the case of the DM haloes of BJ05 but with positive bias of ∼ 3 per cent. In perfect conditions, a spherical system in equilibrium, we can correctly reconstruct the mass profile and there is no bias and dispersion that can be accounted by random uncertainty on the estimated mass. When applied to data from simulations, we do see some bias and dispersion, meaning the bias and dispersion can be either due to system being aspherical and out-of-equilibrium or due to an unknown effect in simulations. We find little correlation of mass offsets with asphericity, so out-of-equilibrium effects seem to be the main cause behind the observed mass offsets. The out-of-equilibrium effects will shift the mass estimates at any given radius and this will lead to a non-zero dispersion. So the dispersion, in principle, sets a limit on the accuracy with which we can expect to measure mass at a given radius for any halo using the spherical Jeans equation. This limit is set due to the inherent nature of the ΛCDM haloes and is independent of the quality and the quantity of the observational data. Using galaxia (Sharma et al. 2011b ), a stellar population synthesis software utilizing Padova isochrones (Marigo et al. 2008 ), we estimate that, till the magnitude limit of 17 in V band, Gaia will have more than 5 times the number of tracers that we have investigated here. So, the error on the mass estimates of the Galaxy using stellar tracers provided by the Gaia using the Jeans analysis will be limited by the non-equilibrium effects as well as uncertainties in observed distances and tangential velocities rather than the sample size. The fact that the two different simulations (with DM particles as tracers) give similar level of dispersion (10 per cent for surfs and 7 per cent for BJ05 haloes) in estimated masses is reassuring for our estimate of the dispersion. When applied to BJ05 stellar haloes (stellar particles as tracers), we get a higher dispersion (14 per cent). This is the case that is of practical use, as we observe stars rather than dark matter particles. The higher dispersion here is also as expected, since the stellar haloes have more sub-structures than dark matter haloes. Dispersion could also be due to random uncertainty associated with our mass estimator (effect of Poisson noise). We have shown that the random uncertainty is a factor of 2 smaller than the measured dispersion for all analysed cases. The total variance being the sum of squares of random and intrinsic scatter, the random uncertainty should make very little contribution to the dispersion. Taking the random uncertainty into account, we estimate the intrinsic scatter due to non-equilibrium effects to be 12 per cent for the case
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