On the dust properties of high redshift molecular clouds and the connection to the 2175 {\AA} extinction bump

We present a study of the extinctionand depletion-derived dust properties of gamma-ray burst(GRB) absorbers at 1 extinctionlaws, we discover a broad range of dust models characterizing the GRB CI absorption systems. In addition to the already well-established correlation between the amount of CI and visual extinction, A_V, we also observe a correlation with the total-to-selective reddening, R_V. All three quantities are also found to be connected to the presence and strength of the 2175 {\AA} dust extinctionfeature. Contrary to these SED-derived dust properties, the amount of CI is not found to be correlated with the depletion-derived dust content, derived from [Zn/Fe] and N(Fe)_dust. To reconcile this, we discuss a scenario where the observed extinctionis dominated by the composition of dust particles confined in the molecular gas-phase of the ISM. We argue that since the depletion level trace non-carbonaceous dust in the ISM, the observed extinctionin GRB CI absorbers is primarily produced by carbon-rich dust in the molecular cloudand is therefore only observable in the extinctioncurves and not in the depletion patterns. This also indicates that the 2175 {\AA} dust extinctionfeature is caused by dust and molecules in the cold and molecular gas-phase. This scenario provides a possible resolution to the discrepancy between the depletion- and SED-derived amounts of dust in high-z absorbers.