Spreading speeds of a parabolic-parabolic chemotaxis model with logistic source on $\mathbb{R}^{N}$

The current paper is concerned with the spreading speeds of the following parabolic-parabolic chemotaxis model with logistic source on $\mathbb{R}^{N}$, \begin{equation} \begin{cases} u_t=\Delta u-\chi\nabla\cdot ( u\nabla v) + u(a-bu),\quad x\in\mathbb{R}^{N},\cr {v_t}=\Delta v -\lambda v+\mu u,\quad x\in \mathbb{R}^{N}. \end{cases}(1) \end{equation} where $\chi, \ a,\ b,\ \lambda,\ \mu$ are positive constants. Assume $b>\frac{N\mu\chi}{4}$. Among others, it is proved that $2\sqrt{a}$ is the spreading speed of the global classical solutions of (1) with nonempty compactly supported initial functions, that is, $$\lim_{t\to\infty}\sup_{|x|\geq ct}u(x,t;u_0,v_0)=0\quad \forall\,\, c>2\sqrt{a}$$ and $$\liminf_{t\to\infty}\inf_{|x|\leq ct}u(x,t;u_0,v_0)>0 \quad \forall\,\, 0<c<2\sqrt{a}.$$ where $(u(x,t;u_0,v_0), v(x,t;u_0,v_0))$ is the unique global classical solution of (1) with $u(x,0;u_0,v_0)=u_0$, $v(x,0;u_0,v_0)=v_0$, and ${\rm supp}(u_0)$, ${\rm supp}(v_0)$ are nonempty and compact. It is well known that $2\sqrt{a}$ is the spreading speed of the following Fisher-KPP equation, $$u_t=\Delta u+u(a-bu),\quad \forall\,\ x\in\mathbb{R}^{N}.$$ Hence, if $b>\frac{N\mu\chi}{4}$, the chemotaxis neither speeds up nor slows down the spatial spreading in the Fisher-KPP equation.