The apparent anti-correlation between the mass opacity of interstellar
  dust and the surface-density of interstellar gas
  Recent analyses of ${\it Herschel}$ observations suggest that in nearby disc
galaxies the dust mass opacity at $500 \, {\rm \mu m}$, $\kappa_{500}$,
decreases with increasing gas surface density, $\Sigma_{\rm ISM}$ (Clark et al.
2019). This apparent anti-correlation between $\kappa_{500}$ and $\Sigma_{\rm
ISM}$ is opposite to the behaviour expected from theoretical dust evolution
models; in such models, dust in denser, cooler regions (i.e. regions of
increased $\Sigma_{\rm ISM}$) tends to grow and therefore to have increased
$\kappa_{500}$. We show, using a toy model, that the presence of a range of
dust temperatures along the line of sight can lead to spuriously low estimated
values of $\kappa_{500}$. If in regions of higher $\Sigma_{\rm ISM}$ the range
of dust temperatures extends to lower values (as seems likely), the magnitude
of this effect may be sufficient to explain the apparent anti-correlation
between $\kappa_{500}$ and $\Sigma_{\rm ISM}$. Therefore there may not be any
need for spatial variation in the intrinsic dust properties that run counter to
theoretical expectations.
