The ALMA REBELS Survey: the dust content of $z \sim 7$ Lyman Break
  Galaxies
  We include a fully coupled treatment of metal and dust enrichment into the
Delphi semi-analytic model of galaxy formation to explain the dust content of
13 Lyman Break Galaxies (LBGs) detected by the Atacama Large millimetre Array
(ALMA) REBELS Large Program at $z\simeq 7$. We find that the galaxy dust mass,
$M_d$, is regulated by the combination of SNII dust production, astration,
shock destruction, and ejection in outflows; grain growth (with a standard
timescale $\tau_0= 30$ Myr) plays a negligible role. The model predicts a
dust-to-stellar mass ratio of $\sim 0.07-0.1\%$ and a UV-to-total star
formation rate relation such that $log (\psi_{\rm UV}) = -0.05 ~[log
(\psi)]^{2} + 0.86 ~log(\psi) -0.05$ (implying that 55-80\% of the star
formation is obscured) for REBELS galaxies with stellar mass $M_* = 10^{9-10}
M_\odot$. This relation reconciles the intrinsic UV luminosity of LBGs with
their observed luminosity function at $z=7$. However, 2 out of the 13 systems
show dust-to-stellar mass ratios ($\sim 0.94-1.1\%$) that are up to $18\times$
larger than expected from the fiducial relation. Due to the physical coupling
between dust and metal enrichment, even decreasing $\tau_0$ to very low values
(0.3 Myr) only increases the dust-to-stellar mass ratio by a factor $ \sim 2$.
Given that grain growth is not a viable explanation for such high observed
ratios of the dust-to-stellar mass, we propose alternative solutions.
