  We present a study of Ar abundances in 15 damped Ly alpha systems (DLAs) in
the redshift interval from 2.3 to 3.4. The sample includes 4 new UVES/VLT
measurements of Ar I column densities. 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 HII
regions inside DLAs offer a viable justification to these underabundances. A
natural explanation is found in the framework of photoionization models of HI
regions embedded in an ionizing continuum with varying spectral distribution.
At z ~ 2.5 the observed Ar deficiencies are large, [Ar/alpha] ~ -0.6/-0.8 dex,
suggestive of a hard, QSO-dominated spectrum. At z >~ 3 the deficiencies are
instead small, suggestive of a soft, stellar-type spectrum, though more data
are needed to generalize this high-z result. We argue that 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
SiIV/CIV ratios in the IGM, the claims of high escape fraction of UV photons
from Ly-break galaxies at z >~ 3, and the recent finding that the HeII
re-ionization seems to occur between z ~ 3.4 and z ~ 3. From the comparison
with local interstellar studies, we predict a rise of Ar abundances in the
redshift range from z ~ 2.3 to z=0, at the epoch at which the metagalactic
field of galaxies overcomes that of quasars.
