The spatially resolved PAH characteristics in the Whirlpool Galaxy
  (M51a)
  We present a detailed study on the spatially resolved polycyclic aromatic
hydrocarbon (PAH) emission properties in the (circum)nuclear region (NR) and
extranuclear regions (ENRs) of M51a using Spitzer-IRS observations.
Correlations among PAH intensity ratios are examined with respect to each
other, local physical parameters, galactocentric distance ($R_{g}$), and very
small grain (VSG) emission. Additional comparison is performed with the
mid-infrared emission features in the H$_{\mathrm{II}}$ regions of M33 and M83.
The NR exhibits the strongest correlation among the PAH intensity ratios,
whereas ENRs are showing increased scatter attributed to ISM emission. Overall,
the radiation field hardness has a higher impact on PAH emission than
metallicity, with the latter regulating PAH variance as a function of $R_{g}$.
Specifically, the variance of PAH emission with respect to the different
physical parameters suggests a higher rate of small/medium PAH processing
compared to large PAHs and a higher ratio of small-to-large PAHs formed with
increasing galactocentric distance. We find similarities between the 7.7 $\mu$m
carriers in M51a's NR and M83's H$_{\mathrm{II}}$ regions, the 8.6 $\mu$m
carriers in M51a's NR and M33 H$_{\mathrm{II}}$ regions, and both types of
carriers between M51a's ENRs, M33's, and M83's H$_{\mathrm{II}}$ regions. We
have identified a positive correlation between PAH/VSG and the PAH intensity
ratios. We conclude that the relative abundance of PAHs and VSG is not solely
driven by the hardness of the radiation field.
