Diffuse Interstellar bands and ultraviolet extinction bump: a Milky Way perspective on distant galaxies

Abstract

The spectral width wB and the center wavelength lB of the UV absorption bump uvB measured for two z=7 galaxies were found to differ from Milky Way (MW) values. A decrease of wB by 45% and a positive shift of lB by 70-80 Ang were measured. In the MW, the uvB amplitude hB and wB do vary; however, such a narrow bump has never been observed and no variability of lB has been convincingly found. Recently, links have been found between both hB and wB and the strength of several Diffuse Interstellar absorption Bands (DIBs). They were found to be limited to the sigma-type DIBs and their detection to be strongly favored if the data were limited to monocloud-type lines-of-sight, selected based on 3D dust maps. We extended the study of the links between MW DIBs and uvB to the lB value and to the hB to continuum ratio, and compared MW variations of the bump parameters to high z values. We used the 5780 and 6284 A sigma, and 5797 and 5850 A zeta DIBs. Similarly to hB and wB, lB reacts to the abundance of sigma carriers and is insensitive to the abundance of zeta DIB carriers, which dominate in dense cloud cores. A strong abundance of sigma carriers induces a shift of lB to longer wavelengths and a decrease of wB. The MW lB and wB variability range is about half the difference between average MW values and values in the distant galaxies. These results reinforce the hypothesis of the existence of individual types of hydrocarbon molecules responsible for both 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 sigma DIBs and the long-wavelength part of the bump in the MW, and that species producing the short-wavelength part of the bump in the MW are of a different nature and absent along the paths to regions of those distant galaxies that contribute most to the UV emission.