The Background Interstellar Medium as Observed from Off-Order Low-Resolution Spitzer-IRS Spectra

Spitzer 'hidden' observations of the background are used to construct a catalog of 4,090 spectra and examine the signature of polycyclic aromatic hydrocarbon (PAH) molecules and their connection to extinction by dust. A strong positive correlation is recovered between WISE12, E(B-V), and the 11.2 $\mu$m PAH band. For 0.06 $\leq$ E(B-V) $\leq$ 5.0, correlations of the 6.2, 11.2, and 12.7 $\mu$m PAH band are positive with E(B-V). Three dust temperature regimes are revealed. Correlations with WISE12 are well-constrained and that with 12.7/11.2 is flat. Decomposition with the NASA Ames PAH IR Spectroscopic Database reveals a tentative positive correlation between the 6.2/11.2 and the PAH ionization fraction, while that with 12.7/11.2 is slightly negative, suggesting PAH structural changes. The relation with PAH size and 6.2/11.2 is negative, while that with 12.7/11.2 is positive. Averaging spectra into five E(B-V) and three T$_{\rm dust}$ bins shows an evolution in PAH emission and variations in 12.7/11.2. Database-fits show an increase in $f_{\rm i}$ and the PAH ionization parameter $\gamma$, but a more stable large PAH fraction. While the largest $\gamma$s are associated with the highest T$_{\rm dust}$, there is no one-to-one correlation. The analysis is hampered by low-quality data at short wavelengths. There are indications that PAHs in the more-diffuse backgrounds behave differently from those in the general interstellar medium. However, they are often still associated with larger scale filamentary cloud-like structures. The spectra and auxiliary data have been made available through the Ames Background Interstellar Medium Spectral Catalog and may guide JWST programs.