Formation and Behavior of Dirac Singularities in the Parabolic-Elliptic Keller-Segel System in Dimensions $n\geq 3$
We consider nonnegative radially symmetric solutions of the parabolic-elliptic Keller-Segel system \begin{align*} \left\lbrace \begin{array}{r@{}l@{\quad}l} &u_t=Δu-\nabla \cdot \big(u\nabla v\big),\\ &0=Δv -μ+ u , \\ \end{array}\right. \end{align*} where $μ$ is the spatial average of $u$, under homogeneous Neumann boundary conditions in a ball in $\mathbb R^n$ for $n\geq 3$. In two dimensions, it is well established that solutions blowing up in finite time converge to a Dirac profile in the vague topology. In contrast, for $n\geq 3$, blow-up solutions with finite existence time do not appear to exhibit such concentration behavior. By generalizing to measure-valued solutions corresponding to accumulated densities of $u$, we extend the analysis beyond the blow-up time. Within this framework, we establish the existence of a minimal solution \[ u(t)=θ(t)δ_0 + ρ(\cdot,t) dx, \qquad t \geq 0, \] where $ρ$ is integrable and $θ$ is increasing and right-continuous. We further construct a class of initial data for which $θ(t_0)>0$ for some $t_0>0$, thereby establishing the formation of a Dirac mass at the origin. Unlike in the case $n=2$, the singular mass does not jump to a positive level instantaneously; instead, $θ$ becomes positive continuously. Moreover, $θ$ is strictly increasing on $[t_0,\infty)$, and the entire mass is asymptotically absorbed at the origin.