THE Z=0.558 ABSORPTION SYSTEM TOWARDS PKS-0118-272 - A CANDIDATE DAMPED LY-ALPHA SYSTEM AT LOW-REDSHIFT

We present a study of the Mg II absorption system at z = 0.558 towards the BL Lac object PKS 0118-272 based on high resolution spectra (lamb da/Delta lambda similar or equal to 2 x 10(4)) obtained at the 3.6m ES O telescope and on direct imaging data obtained at the 3.5m ESO New Te chnology Telescope. At the redshift of the absorber we detect lines of the low ionization species Mg I, Mg II, Ca II, Ti II, Mn II, and Fe I I. Most of the lines are unsaturated and we determine accurate column densities for all the species but Mg II. The derived column densities are typical of the interstellar gas in the disk of the Galaxy. By assu ming dust-free gas with solar abundances these column densities yield N(HI) > 2.5 x 10(19) cm(-2); however, the relative elemental abundance s suggest that some dust is present and that N(HI) similar or equal to 2 x 10(20) cm(-2). The inferred HI column density indicates that the absorber is a Damped Ly alpha system. We find [Ti/Fe] = +0.3, in agree ment with [Ti/Fe] measurements in DLA absorbers, but [Mn/Fe] = +0.4, a t variance with the values [Mn/Fe] < 0 common to DLA systems. The meas ured [Ti/Fe] and [Mn/Fe] ratios match remarkably well the differential depletion pattern of low-density interstellar clouds in the Galaxy. U nlike high-redshift DLA systems (z greater than or equal to 2), the z= 0.558 absorber seems to originate in a galaxy that has already attaine d the abundances and dust content of present-day disk galaxies. The an alysis of our imaging data lends support to the presence of an interve ning galaxy. After a careful subtraction of the BL Lac image, an objec t at 1.6 '' from PKS 0118-272 is detected. At the absorber redshift th e projected distance of this close companion (14 h(50)(-1) kpc) and it s absolute magnitude (M-R approximate to -22.3) are consistent with th ose found for galaxies associated with low-redshift DLA systems.