The End of the Road for Far-infrared Reddening Maps? Evidence for Reddening Errors Driven by Changes in PAH Abundance

TL;DR

This study reveals that far-infrared dust reddening maps underestimate extinction in low-extinction regions due to unaccounted variations in PAH abundance, and proposes using mid-infrared data for improved accuracy.

Contribution

It demonstrates that far-infrared reddening maps are insensitive to PAH variations and introduces a method incorporating mid-infrared emission to better trace dust extinction.

Findings

Far-infrared maps under-predict reddening by up to 0.08 mag.

PAH abundance variations significantly affect extinction without altering far-infrared emission.

Mid-infrared observations can improve dust extinction estimates.

Abstract

Accurate correction for extinction by Galactic dust is essential for studying the extragalactic sky. In the low-extinction regions of the Ursa Major molecular cloud complex, we demonstrate that Galactic dust reddening maps constructed from observations of far-infrared emission are insensitive to variations in the abundance of polycyclic aromatic hydrocarbons (PAHs), and, as a result, to PAH-induced variations in reddening. Using galaxy counts to validate various reddening maps, we find evidence that maps based on far-infrared emission erroneously under-predict reddening compared to stellar reddening maps. This underestimation by far-infrared emission based reddening maps -- representing the largest discrepancy between maps of up to $E(B-V)=0.08$ mag -- is correlated with the relative brightness of PAH emission. Furthermore, we demonstrate theoretically that changes in PAH abundance via accretion from the gas phase is capable of altering extinction significantly with only minor changes to far-infrared emission. We show that modeling the extinction of Ursa Major using both far-infrared and mid-infrared emission more accurately traces dust extinction variations due to changes in PAH abundance. Finally, we discuss how SPHEREx observations of the 3.3 $\mu$m PAH feature are a promising way to overcome this limitation of far-infrared emission.