2008AJ....136..994L

We present equivalent width measurements and limits of six diffuse interstellar bands (DIBs, λ4428, λ5705, λ5780, λ5797, λ6284, and λ6613) in seven damped Lyα absorbers (DLAs) over the redshift range 0.091 ≤ z ≤ 0.524, sampling 20.3 ≤ log N(H I) ≤ 21.7. DIBs were detected in only one of the seven DLAs, that which has the highest reddening and metallicity. Based upon the Galactic DIB-N(H I) relation, the λ6284 DIB equivalent width upper limits in four of the seven DLAs are a factor of 4-10 times below the λ6284 DIB equivalent widths observed in the Galaxy, but are not inconsistent with those present in the Magellanic Clouds. Assuming the Galactic DIB-E(B - V) relation, we determine reddening upper limits for the DLAs in our sample. Based upon the E(B - V) limits, the gas-to-dust ratios, N(H I)/E(B - V), of the four aforementioned DLAs are at least ∼5 times higher than that of the Galactic interstellar medium. The ratios of two other DLAs are at least a factor of a few times higher. The best constraints on reddening derive from the upper limits for the λ5780 and λ6284 DIBs, which yield E(B - V) ≤ 0.08 for four of the seven DLAs. Our results suggest that, in DLAs, quantities related to dust, such as reddening and metallicity, appear to have a greater impact on DIB strengths than does H I gas abundance; the organic molecules, likely responsible for DIBs in DLA-selected sight lines, are underabundant relative to sight lines in the Galaxy of similarly high N(H I). With regard to the study of astrobiology, this could have implications for the abundance of organic molecules in redshifted galaxies. However, since DLAs are observed to have low reddening, selection bias likely plays a role in the apparent underabundance of DIBs in DLAs.