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

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Abstract
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 zabs = 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 H2. 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 3P1 level in the ground state of O I is derived, which provides a lower limit to the physical dimensions of the zabs = 3.3901 system of L > 7 pc.
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