  We study the Neumann initial-boundary problem for the chemotaxis system $$
\left\{\begin{array}{ll} u_t= \Delta u - \nabla \cdot (u\nabla v), & x\in
\Omega, \, t>0, 0=\Delta v - \mu(t)+w, & x\in \Omega, \, t>0, \tau w_t + \delta
w = u, & x\in \Omega, \, t>0, \end{array} \right. \qquad \qquad (\star) $$ in
the unit disk $\Omega:=B_1(0)\subset \R^2$, where $\delta\ge 0$ and $\tau>0$
are given parameters and $\mu(t):=\mint_\Omega w(x,t)dx$, $t>0$. It is shown
that this problem exhibits a novel type of critical mass phenomenon with regard
to the formation of singularities, which drastically differs from the
well-known threshold property of the classical Keller-Segel system, as obtained
upon formally taking $\tau\to 0$, in that it refers to blow-up in infinite time
rather than in finite time: Specifically, it is first proved that for any
sufficiently regular nonnegative initial data $u_0$ and $w_0$, ($\star$)
possesses a unique global classical solution. In particular, this shows that in
sharp contrast to classical Keller-Segel-type systems reflecting immediate
signal secretion by the cells themselves, the indirect mechanism of signal
production in ($\star$) entirely rules out any occurrence of blow-up in finite
time. However, within the framework of radially symmetric solutions it is next
proved that whenever $\delta>0$ and $\io u_0<8\pi\delta$, the solution remains
uniformly bounded, whereas for any choice of $\delta\ge 0$ and $m>8\pi\delta$,
one can find initial data such that $\io u_0=m$, and such that for the
corresponding solution we have \bas \|u(\cdot,t)\|_{L^\infty(\Omega)} \to
\infty \qquad \mbox{as} t\to\infty.
