Diffuse interstellar bands and ultraviolet extinction bump: A Milky Way perspective on distant galaxies

LIRA, Observatoire de Paris, PSL University, CNRS, 5 Place Jules Janssen, 92190 Meudon, France

^★ Corresponding author: rosine.lallement@obspm.fr

Received: 22 April 2025
Accepted: 16 June 2025

Abstract

Context. The spectral width and the center wavelength of the ultraviolet (UV) absorption bump measured for at least two z ≃ 7 galaxies were found to differ significantly from Milky Way (MW) values. A decrease in the width by ~45% and a positive shift of the center by ~70-80 Å were measured. Within the MW, the bump amplitude and width do vary; however, such a narrow bump has never been observed and no variability of the peak position has been convincingly found. On the other hand, links have recently been detected between both the amplitude and the width of the bump and the strength of several diffuse interstellar absorption bands (DIBs). The links were found to be limited to the so-called σ-type DIBs and their detection to be strongly favored if the data were limited to monocloud-type lines of sight (LOSs), selected according to 3D maps of dust extinction.

Aims. We aim to extend the study of the links between MW DIBs and UV bump parameters to the center wavelength of the bump and to the ratio between the bump amplitude and the underlying continuum, and to compare the characteristics of the MW variations in the bump parameters to values at a high redshift.

Methods. We used published catalogs of cross-matched measurements of DIBs and reddening law parameters. We assigned monocloud flags to all LOSs. We focused on the strong σ-type DIBs at 5780 and 6284 Å, and ζ-type DIBs at 5797 and 5850 Å, and searched for trends linking the bump parameters to the DIB strength normalized to the reddening.

Results. Similarly to the case of the bump amplitude and width, the center wavelength of the bump is found to react to the abundance of σ-type DIB carriers and to be insensitive to the abundance of the ζ-type DIB carriers, which dominate in dense and UV-shielded cloud cores. A strong abundance of σ carriers induces a shift of the bump peak position to longer wavelengths and a decrease in its width. The variability range for these two parameters in the MW is about half the difference between average MW values and values in the distant galaxies. In the MW, an increase in the abundance of σ carriers also corresponds to an increase in the bump amplitude and in the ratio between the amplitude and the underlying continuum.

Conclusions. In the case of the MW, these results reinforce the hypothesis of the existence of individual types of hydrocarbon molecules that are simultaneously responsible for DIBs and part of the UV bump. They show that the majority of species responsible for narrow and positively shifted bumps in distant galaxies have a link with (or are) those producing the σ DIBs and the long-wavelength part of the bump in the MW, and that, on the contrary, species producing the short-wavelength part of the bump in the MW are of a different nature and are absent along the paths to the regions of those distant galaxies that contribute most to the UV emission. These results are based on a limited number (≃95) of MW LOSs. More measurements of the reddening curve obtained with a space borne UV spectrograph and more massive DIB measurements with ground-based spectrographs would impose better constraints on the relationships between DIBs and UV bumps. Identifications of DIB carriers would shed light on their formation and the origin of the UV bump in the MW and distant galaxies.