I have the following naive (and inexpert) question about the
reduction of Shimura curves at primes dividing the discriminant
of the underlying quaternion algebra. It requires some background
to state. That is, let $F$ be a totally real field of degree $d$.
Fix a real place $\tau_1$ in the set of real
places $\lbrace \tau_1, \ldots, \tau_d \rbrace$ of $F$. Let
$B$ be a quaternion algebra over $F$ that is split at $\tau_1$
and ramified at $\tau_2, \ldots, \tau_d$. Let $H \subset
\widehat{B}~(= B \otimes \widehat{Z})$ be a compact open
subgroup. Let $M_H$ denote the Shimura curve over $F$ of
level $H$, with complex points given by \begin{align*}
M_H({\bf{C}}) &= B^{\times}\backslash \widehat{B}^{\times}
\times \left({\bf{C}}-{\bf{R}} \right)/H.\end{align*}
Fix a prime $v \subset \mathcal{O}_{F}$. Assume that
$H$ can be factored as $H^v \times H_v$, with $H_v \subset
B_v^{\times}~(= B^{\times} \otimes F_v)$ maximal. If $v$ does
not divide the discriminant of $B$, then it is known by
work of Morita and Carayol that $M_H$ has good reduction
over $v$, hence that there exists a smooth model ${\bf{M}}_H$
of $M_H$ over $\mathcal{O}_{(v)}$. If $v$ divides the discriminant
of the quaternion algebra $B$, then it is known by work of Varshavsky
for instance that there exists an integral model
${\bf{M}}_{H}^V$ of $M_H$ over $\mathcal{O}_{(v)}$. (N.B. there
is apparently also a model due to Drinfeld, described extensively
in the literature for the case of $F={\bf{Q}}$, though it
is not clear to me why Drinfeld's work, which seems
to require a moduli theoretic description of $M_H$, extends
to the general totally real fields setting). Anyhow, let
$F_v$ denote the completion of $F$ at $v$, with $\kappa_v$ the
residue field and $\pi_v$ a uniformizer. By
Cerednik's theorem, the completion of ${\bf{M}}_H^V$
along its closed fibre is canonically isomorphic to the product
\begin{align*} GL(F_v)\backslash \widehat{\Omega}^{unr} \times
D^{\times}\backslash \widehat{D}^{\times}/\overline{H}^v.
\end{align*} Here, $\widehat{\Omega}^{unr}$ denotes the product
$\widehat{\Omega} \times_{\operatorname{Spf}\mathcal{O}_{F_v}} \operatorname{Spf}
\mathcal{O}_{F_v}^{unr}$, where $\widehat{\Omega}$ denotes the $v$-adic
upper half plane (viewed as a formal scheme), and
$\mathcal{O}_{F_v}^{unr}$ the ring of Witt vectors with coefficients
in $\overline{\kappa}_v$. The action of $\gamma \in GL(F_v)$ on
$\widehat{\Omega}^{unr}$ is via the image of $\gamma$ in $PGL(F_v)$
on the component $\widehat{\Omega}$, and via multiplication by
$\operatorname{Frob}_v^{n(\gamma)}$ on
$\widehat{\mathcal{O}}_{F_v}^{unr}$, where $n(\gamma) = -
ord_v \left( \det(\gamma) \right)$. As well,
$D$ denotes the totally definite quaternion algebra over $F$ obtained
from $B$ by switching invariants at $v$ and $\tau_1$, with
$\overline{H}^v$ the compact open subgroup of $\widehat{D}^{\times v}$
corresponding to $H^v$ under a fixed isomorphism $B^{\times v} \cong
D^{\times v}$. The theory of Mumford-Kurihara unifomization then gives
the following information about this curve ${\bf{M}}_{H}^V$:
- The curve
${\bf{M}}_{H}^V$is an admissible curve over$\mathcal{O}_{F_v}$in the sense of Jordan-Livne, i.e.
(i) ${\bf{M}}_H^V$ is a flat, proper curve over $\mathcal{O}_{F_v}$
with a smooth generic fibre.
(ii) The special fibre of ${\bf{M}}_H^V$ is reduced; the normalization
of each of its irreducible components is isomorphic to ${\bf{P}}^1_{\kappa_v}$,
and its only singular points are $\kappa_v$-rational, ordinary double points.
(iii) The local ring ${\bf{M}}_{H, x}$ at any singular point $x$ of
the special fibre is isomorphic as an $\mathcal{O}_{F_v}$-algebra to
$\mathcal{O}_{F_v}[[X,Y]]/(XY - \pi_v^{m(x)})$, for $m(x) \geq 1$ a uniquely
determined integer.
- The dual graph
$\mathcal{G}({\bf{M}}_H^V) = (\mathcal{V}({\bf{M}}_H^V),\mathcal{E}({\bf{M}}_H^V))$of the special fibre of${\bf{M}}_H^V$is
isomorphic to$GL(F_v)^{+} \backslash \left( \Delta \times D^{\times}\backslash \widehat{D}^{\times}/\overline{H}^v \right)$, minus any loops. Here,$GL(F_v)^+ \subset GL(F_v)$denotes the collection of matrices with determinant having even $v$-adic valuation, and $\Delta = (\mathcal{V}(\Delta), \mathcal{E}(\Delta))$ the Bruhat-Tits tree of $SL(F_v)$.
My question is the following: why is the edgeset
$\mathcal{E}({\bf{M}}_H^V)$ nonempty? The dual graph
$\mathcal{G}({\bf{M}}_H^V)$ is clearly disconnected, and
seen easily to be given by the disjoint union of connected
graphs \begin{align*} \coprod_i \mathcal{G}_i &=
\coprod_i \overline{\Gamma}_i \backslash \Delta.
\end{align*} Here, each $\overline{\Gamma}_i$ denotes
the image in $PGL(F_v)$ of a suitable arithmetic subgroup
$\Gamma_i \subset D^{\times} \cong D_v^{\times} \cong GL(F_v)$.
Each component graph $\mathcal{G}_i = (\mathcal{V}_i,
\mathcal{E}_i)$ is connected. Now, since $\Delta$ is a tree,
each component graph $\mathcal{G}_i = \overline{\Gamma}_i
\backslash \Delta$ is a tree. It is then well known that
each (first) Betti number $\beta(\mathcal{G}_i) :=
\vert \mathcal{E}_i \vert - \vert \mathcal{V}_i \vert + 1$ must
vanish, i.e. $\vert \mathcal{E}_i \vert =
\vert \mathcal{V}_i \vert -1 = 0$. If so, then the cardinality
of the edgeset $\mathcal{E}({\bf{M}}_H^V)$ must also equal
zero. i.e. the special fibre of ${\bf{M}}_H^V$ would have no
singular points ... what have I missed here?
