Polycyclic Aromatic Hydrocarbon and the Ultraviolet Extinction Bump at the Cosmic Dawn

TL;DR

This study uses quantum chemistry to show that polycyclic aromatic hydrocarbons (PAHs) likely cause the ultraviolet extinction bump observed at the cosmic dawn, indicating their presence in the early universe before dust formation by evolved stars.

Contribution

It provides the first quantum-chemical modeling evidence linking PAHs to the UV bump at high redshift, suggesting early PAH formation in the universe.

Findings

PAH spectra closely match the observed UV bump at z~6.71

The bump at high redshift peaks at ~2263 Angstrom, wider and shifted from local universe

PAHs were already widespread in the universe 800 million years after the Big Bang

Abstract

First detected in 1965, the mysterious ultraviolet (UV) extinction bump at 2175 Angstrom is the most prominent spectroscopic feature superimposed on the interstellar extinction curve. Its carrier remains unidentified over the past six decades ever since its first detection, although many candidate materials have been proposed. Widely seen in the interstellar medium (ISM) of the Milky Way as well as several nearby galaxies, this bump was recently also detected by the James Webb Space Telescope (JWST) at the cosmic dawn in JADES-GS-z6-0, a distant galaxy at redshift z~6.71, corresponding to a cosmic age of just 800 million years after the Big Bang. Differing from that of the known Galactic and extragalactic interstellar sightlines which always peak at ~2175 Angstrom, the bump seen at z~6.71 in JADES-GS-z6-0 peaks at an appreciably longer wavelength of ~2263 Angstrom and is the narrowest among all known Galactic and extragalactic extinction bumps. Here we show that the combined electronic absorption spectra quantum-chemically computed for a number of polycyclic aromatic hydrocarbon (PAH) molecules closely reproduce the bump detected by JWST in JADES-GS-z6-0. This suggests that PAH molecules have already been pervasive in the Universe at an epoch when asymptotic giant branch stars have not yet evolved to make dust.