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NGC 7023 is a well-studied reflection nebula, which shows strong emissionfrom polycyclic aromatic hydrocarbon (PAH) molecules in the form of aromaticinfrared bands (AIBs). The spectral variations of the AIBs in this region areconnected to the chemical evolution of the PAH molecules which, in turn,depends on the local physical conditions. We use the capabilities of SOFIA toobserve a 3.2' x 3.4' region of NGC 7023 at wavelengths that we observe withhigh spatial resolution (2.7") at 3.3 and 11.2 um. We compare the SOFIA imageswith existing images of the PAH emission at 8.0 um (Spitzer), emission fromevaporating very small grains (eVSG) extracted from Spitzer-IRS spectral cubes,the ERE (HST and CFHT), and H_2 (2.12 um). We create maps of the 11.2/3.3 umratio to probe the morphology of the PAH size distribution and the 8.0/11.2 umratio to probe the PAH ionization. We make use of an emission model and ofvibrational spectra from the NASA Ames PAHdb to translate the 11.2/3.3 um ratioto PAH sizes. The 11.2/3.3 um map shows the smallest PAH concentrate on the PDRsurface (H_2 and extended red emission) in the NW and South PDR. We estimatedthat PAHs in the NW PDR bear, on average, a number of carbon atoms (N_c) of ~70in the PDR cavity and ~50 at the PDR surface. In the entire nebula, the resultsreveal a factor of 2 variation in the size of the PAH. We relate these sizevariations to several models for the evolution of the PAH families when theytraverse from the molecular cloud to the PDR. The PAH size map enables us tofollow the photochemical evolution of PAHs in NGC 7023. Small PAHs result fromthe photo-evaporation of VSGs as they reach the PDR surface. Inside the PDRcavity, the PAH abundance drops as the smallest PAH are broken down. Theaverage PAH size increases in the cavity where only the largest species surviveor are converted into C_60 by photochemical processing.

Mapping PAH sizes in NGC 7023 with SOFIA