Resolving the Far-IR Line Deficit: Photoelectric Heating and Far-IR Line
  Cooling in NGC 1097 and NGC 4559
  The physical state of interstellar gas and dust is dependent on the processes
which heat and cool this medium. To probe heating and cooling of the ISM over a
large range of infrared surface brightness, on sub-kiloparsec scales, we employ
line maps of [C \ii] 158 $\mu$m, [O \one] 63 $\mu$m, and [N \ii] 122 $\mu$m in
NGC 1097 and NGC 4559, obtained with the PACS spectrometer onboard {\it
Herschel}. We matched new observations to existing Spitzer-IRS data that trace
the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at
small scales in these galaxies that the canonical measure of photoelectric
heating efficiency, ([C \ii] + [O \one])/TIR, decreases as the far-infrared
color, $\nu f_\nu$(70 $\mu$m)/$\nu f_\nu$(100 $\mu$m), increases. In contrast,
the ratio of far-infrared (far-IR) cooling to total PAH emission, ([C \ii] + [O
\one])/PAH, is a near constant $\sim$6% over a wide range of far-infrared
color, 0.5 \textless\ $\nu f_\nu$(70 $\mu$m)/$\nu f_\nu$(100 $\mu$m) $\lesssim$
0.95. In the warmest regions, where $\nu f_\nu$(70 $\mu$m)/$\nu f_\nu$(100
$\mu$m) $\gtrsim$ 0.95, the ratio ([C \ii] + [O \one])/PAH drops rapidly to 4%.
We derived representative values of the local UV radiation density, $G_0$, and
the gas density, $n_H$, by comparing our observations to models of
photodissociation regions. The ratio $G_0/n_H$, derived from fine-structure
lines, is found to correlate with the mean dust-weighted starlight intensity,
$<U>$ derived from models of the IR SED. Emission from regions that exhibit a
line deficit is characterized by an intense radiation field, indicating that
small grains are susceptible to ionization effects. We note that there is a
shift in the 7.7 / 11.3 $\mu$m PAH ratio in regions that exhibit a deficit in
([C \ii] + [O \one])/PAH, suggesting that small grains are ionized in these
environments.
