Asymptotic behaviour for small mass in the two-dimensional   parabolic-elliptic Keller-Segel model
  The Keller-Segel system describes the collective motion of cells that are attracted by a chemical substance and are able to emit it. In its simplest form, it is a conservative drift-diffusion equation for the cell density coupled to an elliptic equation for the chemo-attractant concentration. This paper deals with the rate of convergence towards a unique stationary state in self-similar variables, which describes the intermediate asymptotics of the solutions in the original variables. Although it is known that solutions globally exist for any mass less $8\pi $, a smaller mass condition is needed in our approach for proving an exponential rate of convergence in self-similar variables. 
Adrien Blanchet (GREMAQ), Jean Dolbeault (CEREMADE), Miguel Escobedo,   Javier Fern\'andez