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An accurate method to measure the abundance of high‐redshift galaxies involves the observation of absorbers along the line of sight towards a background quasar. Here, we present abundance measurements of 13  z ≥ 3 sub‐damped Lyman α (sub‐DLA) systems (quasar absorbers with H  i  column density in the range 19 &lt; log  N (H  i ) &lt; 20.3 cm −2 ) based on high‐resolution observations with the VLT UVES spectrograph. These observations more than double the amount of metallicity information for sub‐DLAs available at  z  &gt; 3. These new data, combined with other sub‐DLA measurements from the literature, confirm the stronger evolution of metallicity with redshift for sub‐DLAs than for the classical damped Lyman α absorbers. In addition, these observations are used to compute for the first time, using photoionization modelling in a sample of sub‐DLAs, the fraction of gas that is ionized. Based on these results, we calculate that sub‐DLAs contribute no more than 6 per cent of the expected amount of metals at  z ∼ 2.5. We therefore conclude that, even if sub‐DLAs are found to be more metal‐rich than classical DLAs, their contribution is insufficient to solve the so‐called ‘missing‐metals’ problem.

A homogeneous sample of sub‐damped Lyman systems – IV. Global metallicity evolution