ArI as a tracer of ionization evolution

We present a study of Ar abundances in 15 damped Lyalpha systems (DLAs) in the redshift interval 2.3 less than or equal to z less than or equal to 3.4. The sample includes 4 UVES/VLT measurements of Ar I column densities presented here for the first time, together with 6 measurements and 5 upper/lower limits collected from the literature (UVES/VLT and HIRES / Keck observations). The majority of DLAs show significant underabundances of Ar relative to other alpha-capture elements with common nucleosynthetic origin. We show that neither dust depletion nor intervening H II regions inside DLAs offer a viable justification to these underabundances. A natural explanation is found in the framework of photoionization models of H I regions embedded in an ionizing continuum with varying spectral distribution. At z similar to 2.5 the observed Ar deficiencies are large, [Ar/alpha] similar or equal to - 0.6/ - 0.8 dex, suggestive of a hard, QSO-dominated spectrum. At z greater than or similar to 3 the deficiencies are instead small, suggestive of a soft, stellar-type spectrum, though more data are needed to generalize this high-z result. Should the change of Ar abundances with redshift be governed by the evolution of the UV stellar emission internal to DLAs, a synchronization of the star formation in DLAs would be required, with a strong stellar emission at z > 3, but weak at z < 3. This variation seems inconsistent with the weak signal of evolution indicated by abundance studies of DLAs. More likely, the change of Ar abundances is induced by the evolution of the UV metagalactic continuum, in which case the UV emission internal to DLAs must be small (i.e. DLAs should have modest star formation rates) and the external background must become softer at z > 3. The former requirement is consistent with the modest evolution of DLAs abundances and the lack of Ly alpha and H alpha emissions associated with DLAs. The latter requirement is consistent with the observed evolution of Si IV / C IV ratios in the IGM, the claims of high escape fraction of UV photons from Ly-break galaxies at z greater than or similar to 3, and the recent finding that the He II re-ionization seems to occur between z similar to 3.4 and z similar to 3. Comparison with results from local interstellar studies indicates that Ar abundances can be used to trace the evolution of the ionization history of the universe down to z = 0, where [Ar/alpha] similar to - 0.2 dex. We predict a rise of Ar abundances in the redshift range from z similar or equal to 2.3 to z = 0, at the epoch at which the metagalactic field of galaxies overcomes that of quasars.