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

Accurate correction for extinction by Galactic dust is essential for studying the extragalactic sky. In7 the low-extinction regions of the Ursa Major molecular cloud complex, we demonstrate that Galactic8 dust reddening maps constructed from observations of far-infrared emission are insensitive to variations9 in the abundance of polycyclic aromatic hydrocarbons (PAHs), and, as a result, to PAH-induced10 variations in reddening. Using galaxy counts to validate various reddening maps, we find evidence that11 maps based on far-infrared emission erroneously under-predict reddening compared to stellar reddening12 maps. This underestimation by far-infrared emission based reddening maps—representing the largest13 discrepancy between maps of up to E(B − V ) = 0.08 mag—is correlated with the relative brightness14 of PAH emission. Furthermore, we demonstrate theoretically that changes in PAH abundance via15 accretion is capable of altering extinction significantly with only minor changes to far-infrared emission.16 We show that modeling the extinction of Ursa Major using both far-infrared and mid-infrared emission17 is capable of more accurately tracing dust extinction variations due to changes in PAH abundance.18 Finally, we explore the ability of SPHEREx observations of the 3.3 µm PAH feature to overcome this19 limitation of far-infrared emission.