2026ApJ...997..239B

Observations by the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx) are combined with counterpart Spitzer spectral map data to study the aromatic, aliphatic, and polycyclic aromatic hydrocarbon (PAH) size evolution across the northwest photodissociation region (PDR) of the Iris Nebula (NGC 7023). The 3.3-3.4 μm complex (I_3.3) and 11.2 μm (I_11.2) PAH band strength are determined through direct integration. In addition, the former is decomposed into a 3.3 (${\rm{I}}{{\prime} }_{3.3}$) and 3.4 μm (${\rm{I}}{{\prime} }_{3.4}$) subfeature by fitting SPHEREx bandpass-integrated photometry using a modeled, highly sampled, multicomponent spectrum. I_3.3, I_11.2, ${\rm{I}}{{\prime} }_{3.3}$, and ${\rm{I}}{{\prime} }_{3.4}$ all peak at the PDR. The NASA Ames PAH IR Spectroscopic Database is used to obtain the average number of carbon atoms ($\bar{{N}_{{\rm{C}}}}$) and small PAH fraction (f_small) by fitting the isolated PAH component of the Spitzer segment: $70\lesssim \bar{{N}_{{\rm{C}}}}\lesssim 76$ and 0.24 ≲ f_small ≲ 0.36, respectively. ${\rm{I}}{{\prime} }_{3.4}$/${\rm{I}}{{\prime} }_{3.3}$, I_11.2/I_3.3, $\bar{{N}_{{\rm{C}}}}$, and f_small all show a demarcation that matches the large-scale morphology of the region. For ${\rm{I}}{{\prime} }_{3.3}$ and ${\rm{I}}{{\prime} }_{3.4}$ this is reflected by two distinct trends when plotted against each other, one associated with the dense medium and the other with the diffuse medium: ${[{N}_{{\rm{H}},{\rm{ali}}}/{N}_{{\rm{H}},{\rm{aro}}}]}_{{\rm{dense}}}$ = 0.42 ± 0.01 and ${[{N}_{{\rm{H}},{\rm{ali}}}/{N}_{{\rm{H}},{\rm{aro}}}]}_{{\rm{diffuse}}}$ = 0.10 ± 0.01, respectively. $\bar{{N}_{{\rm{C}}}}$ and f_small are tentatively correlated with I_11.2/I_3.3 (R = 0.54 ± 0.05 and -0.45 ± 0.05, respectively). A wider variety of large(r) extended interstellar medium objects is required to tighten the correlations, turn them into quantitative calibrators for PAH size, and pin down the discrepancy of correlations with ${\rm{I}}{{\prime} }_{3.3}$ involved.