PAH Strength and the Interstellar Radiation Field around the Massive Young Cluster NGC 3603

Abstract

We present spatial distribution of polycyclic aromatic hydrocarbons(PAHs) and ionized gas within the Galactic giant H II region NGC 3603.Using the IRS instrument on board the Spitzer Space Telescope, we studyin particular the PAH emission features at ~5.7, 6.2, 7.7, 8.6, and 11.3{μ}m, and the [Ar II] 6.99 {μ}m, [Ne II] 12.81 {μ}m, [Ar III] 8.99{μ}m, and [S IV] 10.51 {μ}m forbidden emission lines. The observationsprobe both ionized regions and photodissociation regions. Silicateemission is detected close to the central cluster while silicateabsorption is seen further away. We find no significant variation of thePAH ionization fraction across the whole region. The emission of verysmall grains (VSGs) lies closer to the central stellar cluster thanemission of PAHs. The PAH/VSG ratio anticorrelates with the hardness ofthe interstellar radiation field suggesting a destruction mechanism ofthe molecules within the ionized gas, as shown for low-metallicitygalaxies by Madden et al.