Diffuse Interstellar Bands in z < 0.6 CaII Absorbers

TL;DR

This study searches for diffuse interstellar bands in CaII absorbers at intermediate redshifts, detecting one DIB and providing insights into the physical conditions and dust content of these distant systems.

Contribution

First detection of DIBs in CaII-selected quasar absorbers at z < 0.6, expanding understanding of interstellar molecules at cosmological distances.

Findings

Detected the 5780Å DIB in one absorber at z=0.1556.

DIB-bearing absorbers show weaker 6284Å absorption compared to local sight-lines.

Derived high HI column density and reddening for the detected DIB absorber.

Abstract

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). Since CaII 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 9 CaII-selected absorbers at 0.07 < z_abs < 0.55. We detect the 5780Ang 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 6284Ang absorption compared with the 5780Ang band. This may be indicative of different physical conditions in intermediate redshift QSO absorbers compared with local galaxies. Assuming that local relations between the 5780Ang 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) = 0.23mag and log[N(HI)] >= 20.9, consistent with previous conclusions that CaII systems have high HI column densities and significant reddening. For the remaining 8 CaII-selected absorbers with 5780Ang DIB non-detections, we derive E(B-V) upper limits of 0.1-0.3mag.