UVES observations of QSO 0000-2620:: Argon and phosphorus abundances in the dust-free damped Lyα system at zabs=3.3901

The UV resonance transitions of neutral argon, Ar I lambda 1066, and of singly ionized phosphorus, P II lambda 963, originated in the damped Ly alpha 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 alpha -chain elements (O, S, and Si). The similarity of the Ar abundance with the other alpha -chain elements implies the absence of significant photoionization by either UV background or stellar sources along the sight line throughout the damped Ly alpha 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 P-3(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.