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(HI), and dust reddening, E(B - V). Because Ca II absorbers in quasar (QSO) spectra are posited to have high N(HI) 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 0.07 < z(abs) < 0.55. We detect the 5780-angstrom DIB in one line of sight at z(abs) = 0.1556; 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-angstrom absorption compared with the 5780-angstrom 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-angstrom DIB strength and N(HI) and E(B - V) apply in QSO absorbers, DIB detections and limits can be used to derive N(HI) and E(B - V). For the one absorber in this study with a detected DIB, we derive E(B - V) = 0.23 mag and log N(HI) >= 20.9, consistent with previous conclusions that Ca II systems have high HI column densities and significant reddening. For the remaining eight Ca II-selected absorbers with 5780-angstrom DIB non-detections, we derive E(B - V) upper limits of 0.1-0.3 mag.