The cold dark matter model predicts cuspy dark matter halos .
However , it has been found that , in some low-mass galaxies , cored dark halos provide a better description of their internal dynamics .
Here we give constraints on the dark halo profile in the Sextans dwarf spheroidal galaxy by studying the longevity of two cold kinematic substructures detected in this galaxy .
We perform N -body simulations of a stellar clump in the Sextans dwarf galaxy , including a live dark matter halo and the main stellar component .
We find that , if the dark halo is cuspy , stellar clumps orbiting with semi-major axis \approx 400 pc are disrupted in \sim 5 Gyr , even if the clump is initially as compact stellar cluster with a radius of r _ { c } = 5 pc .
Stellar clusters in an initial orbit with semi-major axis \leq 250 pc may survive to dissolution but their orbits decay towards the center by dynamical friction .
In contrast , the stellar clumps can persist for a Hubble time within a cored dark matter halo , even if the initial clump ’ s radius is as extended as r _ { c } = 80 pc .
We also study the evolution of the clump in the MONDian context .
In this scenario , we find that even an extended stellar clump with radius r _ { c } = 80 pc survives for a Hubble time , but an unrealistic value for the stellar mass-to-light ratio of 9.2 is needed .