"Unveiling the Drivers of the UV 2175 Å Attenuation Bump at Cosmic Noon"

Abstract:

The UV 2175 Å bump is one of the most enigmatic features of galaxy extinction and attenuation curves. Although its physical origin remains uncertain, laboratory and theoretical studies suggest that the carriers of the bump are likely polycyclic aromatic hydrocarbons (PAHs). However, observational evidence directly linking PAHs to the UV attenuation bump is still scarce, especially beyond the local Universe. In this talk, I will present the first statistically significant study of the connection between PAH emission and the UV 2175 Å bump in individual galaxies at cosmic noon (1.5 < z < 2.7). Our sample consists of ~50 galaxies with spectroscopic measurements of the UV bump from VLT/MUSE. PAH emission is traced using JWST/MIRI photometry, while gas-phase metallicities are derived from Keck/MOSFIRE and JWST/NIRSpec spectroscopy. Additional galaxy properties are obtained through SED fitting to multi-wavelength photometry spanning HST, JWST, Herschel, and ALMA data. We find a clear correlation between PAH emission and UV bump strength, providing strong observational support for the idea that PAHs are the primary carriers of the bump. Both PAH emission and bump strength also correlate with stellar mass and gas-phase metallicity, such that lower-mass, lower-metallicity galaxies tend to exhibit weaker PAH emission and weaker UV bumps. These results extend well-known relations observed in the local Universe to galaxies at cosmic noon. At high PAH abundances, however, we observe substantial scatter in the bump strength. We show that this scatter is linked to variations in dust optical depth, suggesting that dust geometry/clumpiness also plays an important role in shaping the observed UV bump.

Finally, we show that the UV bump strength is best described by a combination of PAH abundance, gas-phase metallicity, dust optical depth, and specific star formation rate, pointing toward a complex interplay between dust (and, therefore, PAH) formation and destruction mechanisms in galaxies across cosmic time.