Structure and Colors of Diffuse Emission in the Spitzer Galactic First Look Survey

We investigate the density structure of the interstellar medium using new high-resolution maps of the 8, 24, and 70 μm surface brightness toward a molecular cloud in the Gum Nebula, made as part of the Spitzer Space Telescope Galactic First Look Survey. The maps are correlated with 100 μm images measured with IRAS. At 24 and 70 μm, the spatial power spectrum of surface brightness follows a power law with a spectral index of -3.5. At 24 μm, the power-law behavior is remarkably consistent from the ∼0.°2 size of our maps down to the ∼5″ spatial resolution. Thus, the structure of the 24 μm emission is self-similar even at milliparsec scales. The combined power spectrum produced from Spitzer 24 μm and IRAS 25 μm images is consistent with a change in the power-law exponent from -2.6 at spatial wavenumber k ∼ 2 × 10-3 arcsec-1 to -3.5 at k ∼ 4 × 10-3 arcsec-1. The decrease may be due to the transition from a two-dimensional to three-dimensional structure. Under this hypothesis, we estimate the thickness of the emitting medium to be 0.3 pc.