Diffuse interstellar bands (DIBs) are common in the spectra of reddened stars in the Milky Way. Although they are numerous (there are now over 100 different known bands), the identification of the DIB carriers is one of the oldest outstanding mysteries in astronomical spectroscopy. Amongst the potential candidates are polycyclic aromatic hydrocarbons and long carbon chains. The DIBs have been detected in only a handful of extra-Galactic sight-lines, including the LMC and SMC (Snow 2001), and in one case, the broad 4428-A feature in a z ~ 0.5 DLA (Junkkarinen et al. 2004). Since the strengths of some DIB lines, such as the 5780-A feature, correlate with N(H i) in the Galaxy (Herbig 1993), selecting high-column-density DLAs may allow us to detect DIBs in high-redshift galaxies. We have been undertaking such a search (Lawton et al. (2008)) and summarise our findings in Figure 22.3. In only one case do we detect DIB absorption: in the z ~ 0.5 DLA towards AO 0235 + 164 (York et al. 2006), where we detect both the 5780-A and the 5705-A DIB. From this detection and upper limits from four other DLAs, we find that the 5780 A line strengths in DLAs are weaker, by up to a factor of 10, for their N(H i) than Galactic sight-lines. Similar deficiencies are seen in Magellanic sight-lines, suggesting that DIB strength probably depends on metallicity and local physical properties as well as on N(H i). On the other hand, the 5780-A and 5705-A DIBs have similar equivalent width ratios in the z ~ 0.5 DLA and the Galaxy, possible evidence that these two bands originate from a similar carrier. Surprisingly, we do not detect the 6284-A DIB, which is usually much stronger than the 5780-A line
Figure 22.3. A search for DIBs in DLAs; only one detection of the 5780-A DIB has been made (York et al. 2006). In general the 5780-A DIB is weaker for its N(H i) by up to a factor of 10 compared with Galactic sight-lines, but is consistent with strengths in the Magellanic Clouds. In contrast, all sight-lines have similar dependences on E(B - V). Figure adapted from York et al. (2006).
which we do detect. This indicates that the physical conditions in the DLA are different from those along the bulk of Galactic and Magellanic sight-lines that have been studied. The only known sight-line with similarly weak 6284-A absorption is towards the sight-line Sk 143 located in the SMC wing. Finally, in contrast with the trends with N(H i), Galactic, Magellanic and DLA sight-lines exhibit a similar trend of E(B — V) with DIB strength. Since DLAs generally have low E(B — V) (Ellison et al. 2005a), this implies that DIBs are unlikely to be commonly detected in these absorbers.
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