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

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 $μ$m PAH feature are a promising way to overcome this limitation of far-infrared emission.

Figures (7)

• Figure 1: Top: Difference between the SFD and DESI reddening maps in the shared footprint. The black outline indicates the Ursa Major region where SFD underestimates the extinction compared to DESI. Bottom: A zoom in of the difference the SFD and DESI reddening maps is shown on the left. The $E(B-V)$ (center) and $R_V$ (right) as measured by Gaia for the Ursa Major region. The SFD reddening map underestimates the extinction in the Ursa Major region where the value of $R_V$ is lower.
• Figure 2: Galaxy density per deg$^2$ of the DESI ELG_LOP galaxy sample selected using various reddening maps. The galaxy density is median-subtracted to emphasize the degree of uniformity. The Ursa Major cloud can be seen clearly in the underestimates of the galaxy sample maps obtained using the SFD, Planck-857, and MF15 maps. On the other hand, the galaxy samples created using the DESI and Gaia reddening maps generally show less spatial correlation with the Ursa Major cloud. The WISE-W3, while still showing some spatial correlation, shows comparatively less when compared to the SFD, Planck-857, and MF15 maps.
• Figure 3: Median-subtracted ELG_LOP galaxy sample density vs. $q_{\rm PAH}$ for each reddening map. The Pearson correlation coefficient $r$ is also shown. Both the SFD and Planck-857 reddening maps produce galaxy samples that are strongly correlated with $q_{\rm PAH}$.
• Figure 4: Left: PAH dust grain size from the astrodust+PAH dust grain model Hensley:2023 as a function of effective radius $a$. The solid black line indicates the initial PAH size distribution ($\Delta M_{\rm PAH} / M_{\rm PAH, init}=0$). Distributions with increased PAH mass due to accretion with $\Delta M_{\rm PAH} / M_{\rm PAH, init}=1, 5$, and 10 are plotted as dotted, dashed-dotted, and dashed black lines, respectively. Right: Change in extinction and emission as a function of increasing PAH mass ($\Delta M_{\rm PAH} / M_{\rm PAH, init}$). The change in various extinction properties are shown as the dashed lines while the emission at various wavelengths are shown as the solid lines. As PAH mass grows, the emission at 100 $\mu$m and at 350 $\mu$m (857 GHz) are only weakly sensitive to the resulting increase in $E(B-V)$ while the emission 3.3 $\mu$m and in the WISE W3 band at 12 $\mu$m are highly responsive. The process of PAH growth by accretion results in the loss of the relatively small PAH population that is responsible for the 3.3 $\mu$m emission. This results in a decrease in emission with increasing PAH mass fraction. The WISE W3 band emission at 12 $\mu$m largely originates from the comparatively larger PAHs. Unlike the emission at 3.3 $\mu$m , emission in this band increases with increasing PAH mass fraction as accretion progresses.
• Figure 5: Posterior distributions for the contributions from the mid-infrared and the far-infrared to $A_B$ and $A_V$. $A_V^{\rm MIR}$ and $A_B^{\rm MIR}$ are in units of magnitudes per MJy sr$^{-1}$ while $A_V^{\rm FIR}$ and $A_B^{\rm FIR}$ are in units of magnitudes per $E(B-V)$ magnitude. The median of the marginal distributions and the upper and lower quantiles are shown.