2021MNRAS.507.1175M

We present a novel scenario for the growth of dust grains in galaxies at high redshift (z ∼ 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 (>=Z_☉), number density (>=10⁶ cm^–3), and dust-to-gas mass ratio (∼1/150 x 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 (≥10⁷ M_☉) molecular clouds hosting ≥10⁶ M_☉ 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 ∼10⁶ M_☉ (M_*/10⁸ M_☉).