  Let $G$ be a finitely generated torsion-free nilpotent group. The
representation zeta function $\zeta_G(s)$ of $G$ enumerates twist isoclasses of
finite-dimensional irreducible complex representations of $G$. We prove that
$\zeta_G(s)$ has rational abscissa of convergence $a(G)$ and may be
meromorphically continued to the left of $a(G)$ and that, on the line
$\{s\in\mathbb{C} \mid \textrm{Re}(s) = a(G)\}$, the continued function is
holomorphic except for a pole at $s=a(G)$. A Tauberian theorem yields a precise
asymptotic result on the representation growth of $G$ in terms of the position
and order of this pole.
  We obtain these results as a consequence of a more general result
establishing uniform analytic properties of representation zeta functions of
finitely generated nilpotent groups of the form $\mathbf{G}(\mathcal{O})$,
where $\mathbf{G}$ is a unipotent group scheme defined in terms of a nilpotent
Lie lattice over the ring $\mathcal{O}$ of integers of a number field. This
allows us to show, in particular, that the abscissae of convergence of the
representation zeta functions of such groups and their pole orders are
invariants of $\mathbf{G}$, independent of $\mathcal{O}$.
