Steady state solutions for the Gierer-Meinhardt system in the whole space

We are concerned with the study of positive solutions to the Gierer-Meinhardt system $$\begin{cases} \displaystyle -\Delta u+\lambda u=\frac{u^p}{v^q}+\rho(x) &\quad\mbox{ in }\mathbb{R}^N\, , N\geq 3,\\[0.1in] \displaystyle -\Delta v+\mu v=\frac{u^m}{v^s} &\quad\mbox{ in }\mathbb{R}^N,\\[0.1in] \end{cases}$$ which satisfy $u(x), v(x)\to 0$ as $|x|\to \infty$. In the above system $p,q,m,s>0$, $\lambda, \mu\geq 0$ and $\rho\in C(\mathbb{R}^N)$, $\rho\geq 0$. It is a known fact that posed in a smooth and bounded domain of $\mathbb{R}^N$, the above system subject to homogeneous Neumann boundary conditions has positive solutions if $p>1$ and $\sigma=\frac{mq}{(p-1)(s+1)}>1$. In the present work we emphasize a different phenomenon: we see that for $\lambda, \mu>0$ large, positive solutions with exponential decay exist if $0< \sigma\leq 1$. Further, for $\lambda=\mu=0$ we derive various existence and nonexistence results and underline the role of the critical exponents $p=\frac{N}{N-2}$ and $p=\frac{N+2}{N-2}$.