Dust and Diffuse Interstellar Bands in the<i>z</i>_<i>a</i>= 0.524 Absorption System toward AO 0235+164

We present new HST STIS NUV-MAMA and STIS CCD observations of the BL Lac object AO 0235+164 and the intervening damped Ly alpha (DLA) line at z_a = 0.524. The line profile gives N(H I) = 5 +/- 1 x 10^{21} cm^{-2} and, combined with the H I 21 cm absorption data leads to a spin temperature of T_s = 220 K +/- 60 K. Those spectra also show a strong, broad feature at the expected position of the 2175 Angstrom graphitic dust feature at z_a = 0.524. Assuming a Galactic type dust extinction curve at z_a = 0.524 gives a dust-to-gas ratio of 0.19 Galactic, but the fit, assuming the underlying, un-reddened spectrum is a single power-law, is poor in the far-UV. A dust-to-gas ratio of 0.19 Galactic is similar to the LMC, but the AO 0235+164 spectrum does not fit the LMC extinction curve, or the SMC extinction curve (which has practically no 2175 Angstrom feature). A possible interpretation includes dust similar to Galactic, but with less of the small particles that produce the far-UV extinction. The metallicity of the z_a = 0.524 absorber, estimated from the observed N(H I) and excess X-ray absorption (beyond Galactic) derived from contemporaneous and archival ASCA and ROSAT X-ray data, is Z = 0.72 +/- 0.28 Z_sun, implying in turn the dust-to-metals ratio of 0.27 Galactic. If the dust mass density is the same in the z_a = 0.524 DLA system as in our Galaxy, only 14% (+/- 6%) of the metals (by mass) are in dust compared with 51%, 36%, and 46% for the Galaxy, LMC, and SMC respectively. Such a dusty z_a = 0.524 AO 0235+164 absorption system is a good example of the kind of DLA system that will be missed due to selection effects, which in turn can bias the measurement of the co-moving density of interstellar gas (in units of the closure density), Omega_g, as a function of z.