  If a graph submanifold $(x,f(x))$ of a Riemannian warped product space
$(M^m\times_{e^{\psi}}N^n,\tilde{g}=g+e^{2\psi}h)$ is immersed with parallel
mean curvature $H$, then we obtain a Heinz type estimation of the mean
curvature. Namely, on each compact domain $D$ of $M$, $m\|H\|\leq
\frac{A_{\psi}(\partial D)}{V_{\psi}(D)}$ holds, where $A_{\psi}(\partial D)$
and $V_{\psi}(D)$ are the ${\psi}$-weighted area and volume, respectively. In
particular, $H=0$ if $(M,g)$ has zero weighted Cheeger constant, a concept
recently introduced by D.\ Impera et al.\ (\cite{[Im]}). This generalizes the
known cases $n=1$ or $\psi=0$. We also conclude minimality using a closed
calibration, assuming $(M,g_*)$ is complete where $g_*=g+e^{2\psi}f^*h$, and
for some constants $\alpha\geq \delta\geq 0$, $C_1>0$ and $\beta\in [0,1)$,
$\|\nabla^*\psi\|^2_{g_*}\leq \delta$, $\mathrm{Ricci}_{\psi,g_*}\geq \alpha$,
and $det_g(g_*)\leq C_1 r^{2\beta}$ holds when $r\to +\infty$, where $r(x)$ is
the distance function on $(M,g_*)$ from some fixed point. Both results rely on
expressing the squared norm of the mean curvature as a weighted divergence of a
suitable vector field.
