Constraining the link between the 2175{\AA} dust absorption feature and PAHs in Nearby Star-Forming Galaxies using Swift/UVOT and JWST/MIRI

TL;DR

This study investigates the relationship between the 2175{ extAA} dust absorption feature and PAH molecules in nearby star-forming galaxies, revealing that their correlation is influenced by star formation activity and UV radiation, with no clear direct link.

Contribution

The paper provides spatially-resolved analysis of the 2175{ extAA} feature and PAH abundances in local galaxies, highlighting the influence of star formation and UV radiation on their correlation.

Findings

Moderate positive correlation between 2175{ extAA} feature and PAH abundance.

Most of the correlation is driven by star formation rate surface density.

No significant trend with gas-phase metallicity was observed.

Abstract

The 2175{\AA} bump is a prominent absorption feature at ultraviolet (UV) wavelengths in dust extinction and attenuation curves. Understanding the relative strength of this feature is important for accurate dust corrections at both low- and high-redshift. This feature is postulated to arise from polycyclic aromatic hydrocarbon (PAH) dust grains; however, the carrier has not been definitively established. We present results on the correlation between the 2175{\AA} feature and PAH abundances in a spatially-resolved manner for 15 local galaxies in the PHANGS-JWST survey that have NUV and mid-IR imaging data from Swift/UVOT and JWST/MIRI, respectively. We find a moderate positive correlation between the 2175{\AA} feature strength and PAH abundance, albeit with large intrinsic scatter. However, most of this trend can be attributed to a stronger negative correlation of both quantities with SFR surface density and specific-SFR (proxies of ionising radiation). The latter trends are consistent with previous findings that both the 2175{\AA} carrier and PAHs are small grains that are easily destroyed by UV photons, although the proxy for PAH abundance could also be influenced by dust heating. When controlling for SFR surface density, we find weaker correlations between the 2175{\AA} feature and PAH abundances, disfavouring a direct link. However, analyses based on spectroscopic measurements of the 2175{\AA} feature and PAH features are required to verify our findings. No significant trends with gas-phase metallicity are found for the 2175{\AA} feature and PAHs, however the metallicity range of our sample is limited. We provide prescriptions for the strength of the 2175{\AA} feature and PAHs in local massive (metal-rich) galaxies with SFR surface density and specific-SFR, however the former should be used with caution since bump strengths measured from Swift/UVOT are expected to be underestimated.