UVES Observations of QSO 0000−2620: Argon and Phosphorus Abundances in the Dust‐free Damped Lyα System atzabs = 3.3901

The UV resonance transitions of neutral argon, Ar I λ1066, and of singly ionized phosphorus, P II λ963, originated in the damped Lyα system (DLA) at z abs = 3.3901 toward QSO 0000-2620, have been detected by means of the Ultraviolet-Visual Echelle Spectrograph (UVES) spectrograph at the 8.2 m ESO KUEYEN telescope. So far, this is the first measurement of Ar I and the second of P II ever performed in damped galaxies and in high-redshift objects. This DLA is well known for having one of the lowest metal abundances and dust content and the lowest fractional abundance of molecular hydrogen H 2 . The measured Ar abundance is [Ar/H] = -1.91 ± 0.09, which is equal to the abundances of the other α-chain elements (O, S, and Si). The similarity of the Ar abundance with the other α-chain elements implies the absence of significant photoionization by either UV background or stellar sources along the sight line throughout the damped Lyα system. Both log(Ar/O) and log(Ar/S) ratios are found close to those measured in the extragalactic H II regions and in blue compact galaxies where O is more abundant by at least 1 order of magnitude. This strengthens the universality of the Ar/O and Ar/S ratios and lends support to the existence of a universal IMF. The abundance of the nonrefractory element phosphorus [P/H] = -2.31 ± 0.10 confirms the low amount of chemical evolution in the DLA. This is the measurement of P in the most metal-poor material and shows a subsolar [P/Fe] = -0.27 value. The measured ratios [P/Si] = -0.40 ± 0.13 and [P/S] = -0.40 ± 0.13 provide evidence for a mild odd-even effect. Finally, a stringent upper limit to the population of the 3 P 1 level in the ground state of O I is derived, which provides a lower limit to the physical dimensions of the z abs = 3.3901 system of L > 7 pc.