Diffuse interstellar bands in z 0.6 Ca II absorbers

The diffuse interstellar bands (DIBs) probably arise from complex organic molecules whose strength in local galaxies correlates with neutral hydrogen column density, N(H i), and dust reddening, E( B− V). Because Ca ii absorbers in quasar (QSO) spectra are posited to have high N(H i) and significant E( B− V), they represent promising sites for the detection of DIBs at cosmological distances. Here we present the results from the first search for DIBs in nine Ca ii-selected absorbers at . We detect the 5780-Å DIB in one line of sight at ; this is only the second QSO absorber in which a DIB has been detected. Unlike the majority of local DIB sight-lines, both QSO absorbers with detected DIBs show weak 6284-Å absorption compared with the 5780-Å band. This may be indicative of different physical conditions in intermediate redshift QSO absorbers compared with local galaxies. Assuming that local relations between the 5780-Å DIB strength and N(H i) and E( B− V) apply in QSO absorbers, DIB detections and limits can be used to derive N(H i) and E( B− V). For the one absorber in this study with a detected DIB, we derive and log N(H i) ≥ 20.9, consistent with previous conclusions that Ca ii systems have high H i column densities and significant reddening. For the remaining eight Ca ii-selected absorbers with 5780-Å DIB non-detections, we derive E( B− V) upper limits of 0.1-0.3 mag.