tokens: with metallicity as [O/Fe], or other [ α /Fe] ratios do. Relevant discussion in previous comparison studies should therefore be revisited. In particular, Cooke et al. (2015) showed [O/Fe] as a function of [Fe/H] for [Fe/H] ≲−2, and [S/Zn] as a function of [Zn/H] for [Fe/H] ≳−2, assuming that both abundance ratios were tracing [ α /Fe] versus [Fe/H] in DLAs. By doing this, they observed a ‘knee’ in these abundance ratios where they started to decrease at [Fe/H] ≈−2, analogous to that observed in [ α /Fe] as a function of [Fe/H] in dwarf galaxies. However, it is clear from Fig. 8 that this assumption does not hold (see also Sect. 4 ). Therefore, although a ‘knee’ in [ α /Fe] abundance ratios in DLAs cannot be excluded at this point, further confirmation is needed, preferably using the same probe throughout the entire metallicity range. Fig. 8 Schematic view of the mean trends of [O/Fe] (magenta solid lines) and [S/Zn] (green dashed lines) in the Sculptor dSph as a function of [Fe/H] ( top ) and [Zn/H] ( bottom ). References: [O/Fe]: Hill et al. in prep. [S/Zn]: Skúladóttir et al. (2015a , 2017 ). 6 Conclusions For the first time we have made a detailed chemical comparison between DLAs and stars in dwarf galaxies by using the volatile elements S and Zn without the help of any other element as proxy. This was made possible by recent measurements of S and Zn abundances in around 80 RGB stars in the Sculptor dwarf spheroidal galaxy ( Skúladóttir et al. 2015a , 2017 ). Most of the stars in this galaxy formed >10 Gyr ago, which correspond to the redshifts at which the majority of DLAs are observed (i.e., z ≳ 1.8). Our analysis of the Sculptor data shows that S abundances behave as expected of an α -element, primarily created by core-collapse SN. However, there is also a non-negligible contribution to S from the yields of type Ia SN. This makes [S/H] a reasonabletracer of the metallicity evolution in galaxies. In low-metallicity stars, S measurements are challenging, so when not available, other α -elements can be used as proxy, preferably: Ca, Ti or the average of the two (see Appendix A ). The lighter α -elements, O and Mg, should not be used as proxies for S since, unlike S, they have negligible contribution from type Ia SN. In light of the available measurements in the Milky Way and its satellite dwarf galaxies, it is clear that [Zn/Fe] is neither the same at a given metallicity in all galaxies probed here, nor constant in a given galaxy throughout its entire evolutionaryhistory. The nucleosynthetic channels that lead to the formation of Zn remain unclear, but the following statement isin agreement with both theory and observations: Type Ia SN are not significant producers of Zn. The formation channels of Zn are therefore not the same as Fe, and the [Zn/Fe] ratio is highly dependent on the star formation history of each system. Thus, using [Zn/H] as a proxy for [Fe/H] is fraught with potential pitfalls. Furthermore, [S/Zn] should not be used as a
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