  The UV resonance transitions of neutral argon ArI 1066 A, and of singly
ionized phosphorus PII 963 A, originated in the damped Ly_alpha system (DLA) at
zabs = 3.3901 towards QSO 0000--2620 have been detected by means of the UVES
spectrograph at the 8.2m ESO KUEYEN telescope. So far, this is the first
measurement of ArI, and the second of PII, 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 sightline throughout the damped Ly_alpha system. Both
log(Ar/O) and log(Ar/S) ratios are found close to those measured in the
extragalactic HII regions and in blue compact galaxies where O is more abundant
by at least one 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 non-refractory 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] = - 0.33
(+/- 0.13) provide evidence for a mild odd-even effect. Finally, a stringent
upper limit to the population of the 3P_1 level in the ground state of OI is
derived, which provides a lower limit to the physical dimensions of the zabs =
3.3901 system of L > 7 pc.

Astrophysics