tokens: of dust producers (Bolmer et al. 2018 ; Zafar et al. 2018b ) could also change the effect of total Fe on extinction. It is now simple to compute the average total column density of Fe along a sightline back to a given redshift, and then use the fit in Fig. 4 to convert this to an average ‘cosmic dust dimming’ CDD(). In Fig. 6 , we provide this quantity, CDD(), back to a redshift of = 5. Figure 6. The average ‘cosmic dust dimming’, CDD(), is computed as the integrated A V from redshift 0 to . The A V is computed from the average amount of dust along the line of sight based on the distribution of Fe (blue points) shown in Fig. 5 . 4.2 Impact on high-redshift surveys Fig. 6 shows the accumulated effect of dust in random sightlines out to cosmological distances. It is seen that up to redshifts of about 3 the CDD() is smaller than the typical uncertainties on individual A V measurements, and the effect can be ignored without too much impact. At redshifts in excess of 4, in the current paradigm, the CDD() continues to rise despite the dropping metallicity, causing the cumulated effect to become significant. It should be recalled that the curve shows the average, but the effect is highly stochastic. Therefore, corrections and predictions should probably only be considered for statistical samples while the effect on individual objects will be stochastic. 4.3 Inclusion of 2175 Å bump The shape of the extinction curve tells us about the dust grains in the ISM (Draine 2003 ). Carbonaceous dust grains are responsible for the 2175 Å bump and silicates induce the steepness in the extinction curve (Weingartner Draine 2001 ; Draine 2003 ). We here aim to see if different extinction curves carrying different dust grain properties could affect our results. Our sample mostly has preference towards featureless Small Magellanic Cloud (SMC)-type extinction curve. However, 21 cases (3 GRBs and 18 QSO-DLAs) are reported to contain a 2175 Å bump, suggesting carbonaceous dust enriched environments. These cases are highlighted by stars drawn around the data points in Figs 1 – 4 . De Cia et al. ( 2013 ) presented a sample of 18 GRBs (11 measurements and 7 limits) which included a single burst with a 2175 Å extinction feature (GRB 070802). That single burst did not fall on the A V and N (Fe) dust relation of their limited sample, and De Cia et al. ( 2013 ) suggested that objects with the 2175 Å bump might follow a different relation and therefore should not be included. Our larger sample includes 19 sightlines with bumps, and is reaching extinctions 1.8 mag higher than the De Cia et al. ( 2013 ) sample, but we see no evidence that the 2175 Å bump cases follow a different relation. We also find that inclusion of the 2175 Å bump cases strengthens the relation especially at higher extinctions. Depending on the amount of metals available to form dust, different grain compositions and sizes could provide a range of optical extinction laws. 4.4 Dust-to-gas ratios and
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