Polycyclic aromatic hydrocarbon excitation in nearby spiral galaxies
  We have examined polycyclic aromatic hydrocarbon (PAH) excitation in a sample
of 25 nearby face-on spiral galaxies using the ratio of mid-infrared PAH
emission to dust mass. Within 11 of the galaxies, we found that the PAH
excitation was straightforwardly linked to ultraviolet or mid-infrared star
formation tracers, which, along with other results studying the relation of PAH
emission to star formation, indicates that the PAHs are most strongly excited
in dusty shells around the star forming regions. Within another 5 galaxies, the
PAH emission is enhanced around star forming regions only at specific
galactocentric radii. In 6 more galaxies, PAH excitation is more strongly
correlated with the evolved stellar populations as traced by 3.6 micron
emission. The results for the remaining 3 galaxies were ambiguous. The radial
gradients of the PAH/dust ratios were generally not linked to log(O/H)
gradients except when the log(O/H) gradients were relatively steep. Galaxies in
which PAHs were excited by evolved stars had relatively high far-ultraviolet to
mid-infrared ratios, indicating that variations in the link between PAH
excitation and different stellar populations is linked to changes in dust
attenuation within galaxies. Alternately, differences in morphology could make
it more likely that PAHs are excited by evolved stars, as 5 of the 6 galaxies
where this occurs are late-type flocculent spiral galaxies. These heterogeneous
results demonstrate the complexity of describing PAH excitation and have broad
implications for using PAH emission as a star formation tracer as well as for
modelling dust emission and radiative transfer.
