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R.D. Cohen, E.M. Burbidge, V.T. Junkkarinen, R.W. Lyons (CASS/UCSD), G. Madejski (GSFC)
We have identified unambiguous signatures of dust and diffuse interstellar bands at z=0.524 in the spectrum of AO 0235+164. AO 0235+164 is a BL Lac object at zem=0.94 which is known to have two metal--line absorption systems, at z=0.524 and z=0.851. The lower redshift system has 21 cm absorption, indicative of very high column density. There are two galaxies within 2" of A0235+164 which may be the sites of the z=0.524 absorption system. One of these is itself an unusual object, with a UV spectrum dominated by high-ionization, BAL-like absorption lines.
We have obtained a low resolution spectrum covering from 1600Å\ to beyond 1\mu with STIS and HST. The two dominant features are the damped Ly\alpha line at z=0.524 and a strong, broad feature centered near 3300Å. This feature can only be identified as the \lambda=2175Å\ feature due to interstellar dust extinction at z=0.524. The model that most closely approximates the spectrum is parametrized as a broken power law with Galactic type extinction at z=0 and z=0.524 in addition to the absorption lines. We derive NH \approx 5 \times 1021\ {\rm cm}-2 and E(B-V) \approx 0.25. This indicates that the dust-to-gas ratio is considerably lower than in the Galaxy. The fit to the \lambda=2175Å\ feature suggests there are differences from the average Galactic curve. Acceptable fits with R > 3.1 are also possible, but they imply an extremely high luminosity for AO 0235+164.
From a low--resolution, but very high signal-to-noise spectrum obtained with LRIS at Keck Observatory, we have detected a feature which we identify as the \lambda=4428Å\ diffuse interstellar band at z=0.524, with a rest equivalent width of 0.8Å. The strength of the DIB and E(B-V) \approx 0.25 are consistent with the Galactic relationship.
This work was supported by NASA through a grant from STScI.
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