Linking Characteristics of the Polycyclic Aromatic Hydrocarbon
  Population with Galaxy Properties: A Quantitative Approach Using the NASA
  Ames PAH IR Spectroscopic Database
  Utilizing the data and tools provided through the NASA Ames PAH IR
Spectroscopic Database (PAHdb), we study the PAH component of over 900
Spitzer-IRS galaxy spectra. Employing a database-fitting approach, the average
PAH size, the PAH size distribution, and PAH ionization fraction are deduced.
In turn, we examine their connection with the properties of the host galaxy. We
found that PAH population within galaxies consists of middle-sized PAHs with an
average number of carbon atoms of $\bar{N_{C}}$ = 55, and a charge state
distribution of $\sim$40% ionized - 60% neutral. We describe a correlation
between the 6.2/11.2 $\mu$m PAH ratio with the ionization parameter
($\gamma\equiv(G_{0}/n_{\rm e})(T_{\rm gas} / 1\ \mathrm{K})^{0.5}$), a
moderate correlation between the 8.6/11.2 $\mu$m PAH ratio and specific
star-formation rate, and a weak anti-correlation between $\gamma$ and M$_{*}$.
From the PAHdb decomposition we provide estimates for the 3.3 $\mu$m PAH band,
not covered by Spitzer observations, and establish a correlation between the
3.3/11.2 $\mu$m PAH ratio with N$_{\mathrm{C}}$. We further deliver a library
of mid-IR PAH template spectra parameterized on PAH size and ionization
fraction, which can be used in galaxy spectral energy distribution fitting
codes for the modeling of the mid-IR PAH emission component in galaxies.
