Dust and Diffuse Interstellar Bands in the z_a = 0.524 Absorption System toward AO 0235+164^{* **}

We present new Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) NUV-MAMA and STIS CCD observations of the BL Lac object AO 0235+164 and the intervening damped Lyα (DLA) line at z_a = 0.524. The line profile gives N(H ) = × 10²¹ cm^-2 and, combined with the H I 21 cm absorption data, leads to a spin temperature of T_s = 220 ± 60 K. Those spectra also show a strong, broad feature at the expected position of the 2175 Å 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 times the Galactic value, but the fit, assuming that the underlying, unreddened spectrum is a single power law, is poor in the far-UV. A dust-to-gas ratio of 0.19 times the Galactic value is similar to the LMC, but the AO 0235+164 spectrum does not fit either the LMC extinction curve or the SMC extinction curve (which has practically no 2175 Å feature). A possible interpretation includes dust similar to that in the Galaxy, but with fewer 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 ) 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_☉, implying in turn a dust-to-metals ratio of 0.27 times the Galactic value. 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 to 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 because of selection effects, which in turn can bias the measurement of the comoving density of interstellar gas (in units of the closure density), Ω_g, as a function of z.