Dust Grain Growth at High Redshift: Starburst-driven CMB-Dark
  Supershells
  We present a novel scenario for the growth of dust grains in galaxies at
high-redshift ($z\sim 6$). In our model, the mechanical feedback from massive
star clusters evolving within high-density pre-enriched media allows to pile-up
a large amount of matter into massive supershells. If the gas metallicity
($\geq$ Z$_{\odot}$), number density ($\geq 10^6$ cm$^{-3}$) and dust-to-gas
mass ratio ($\sim 1/150 \times Z$) within the supershell are sufficiently
large, such supershells may become optically thick to the starlight emerging
from their host star clusters and even to radiation from the Cosmic Microwave
Background (CMB). Based on semi-analytic models, we argue that this mechanism,
occurring in the case of massive ($\geq 10^7$ M$_{\odot}$) molecular clouds
hosting $\geq 10^6$ M$_{\odot}$ star clusters, allows a large mass of gas and
dust to acquire a temperature below that of the CMB, whereupon dust grain
growth may occur with ease. In galaxies with total stellar mass $M_{*}$, grain
growth within supershells may increase the dust mass by $\sim 10^6$ M$_{\odot}$
$(M_{*}/10^{8}$ M$_{\odot}$).
