Seminars and Colloquia at ESO Garching and on the campus

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.

Metal-poor galaxies provide a unique window into the physical conditions and chemical enrichment processes that govern star formation in nearly pristine environments. A subset of these systems exhibit spectra with extremely strong high-ionization emission lines that cannot be reproduced by standard stellar population models and, therefore, offer an ideal laboratory for testing the physical mechanisms that produce unusually hard ionizing radiation fields and extreme emission. These extreme emission line galaxies (EELGs) are often modeled under simplified assumptions, such as the low-density limit, and are widely used as benchmarks for interpreting elemental abundances and ionizing spectra across cosmic time. However, growing empirical evidence suggests that more extreme conditions at the heart of these sources are biasing our interpretations.

I will present new empirical methods to constrain the ionizing continua of EELGs from the JWST CLASSYIR Treasury Survey, which combines ultraviolet (UV) through mid-infrared emission lines to map the high-energy ionizing spectrum. These observations reveal radiation fields that are significantly harder and more structured than predicted by standard stellar population models, pointing to additional contributions from very massive stars, ultra-luminous X-ray sources (ULXs), and obscured AGN. At the same time, I will show that nebular conditions in these galaxies are far from uniform. Density stratification, particularly in highly ionized gas, can lead to systematic biases in temperature measurements and subsequent abundance determinations when using traditional low critical-density optical emission lines. As a result, even the long-standing “gold-standard” of metallicity measurements, the direct method, will be significantly biased in extreme environments.

Fortunately, UV diagnostics provide access to the densities and physical conditions of the high-ionization gas, enabling more robust determinations of temperatures and abundances. By combining UV and optical measurements, we can establish a physically consistent framework for interpreting local EELGs and connect them to high-redshift galaxies observed with JWST, which exhibit even more extreme ionization conditions, elevated densities, and enhanced N/O ratios. I will discuss the physical pathways that can drive rapid enrichment in relative abundances, and the implications for interpreting both local and distant galaxy populations.

Together, these results demonstrate that metal-poor EELGs expose the interconnected physics linking ionizing spectra, nebular conditions, and chemical enrichment across cosmic time, but only when interpreted with a self-consistent UV+optical framework.