The aromatic features in M101 were studied spectroscopically and photometrically using observations from all three instruments on the Spitzer Space Telescope .
The global SED of M101 shows strong aromatic feature ( commonly called PAH feature ) emission .
The spatially resolved spectral and photometric measurements of the aromatic feature emission show strong variations with significantly weaker emission at larger radii .
We compare these variations with changes in the ionization index ( as measured by [ NeIII ] / [ NeII ] and [ SIV/SIII ] , which we probe over the ranges 0.03–20 and 0.044–15 respectively ) and metallicity ( expressed as log ( O/H ) + 12 , which ranges from 8.1 to 8.8 ) .
Over these ranges , the spectroscopic equivalent widths of the aromatic features from 7 HII regions and the nucleus were found to correlate better with ionization index than with metallicity .
This implies that the weakening of the aromatic emission in massive star forming regions is due primarily to processing of the dust grains in these environments , not to differences in how they form ( although formation could still be important on a secondary basis ) .
The behavior of the correlation between the aromatic feature equivalent widths and ionization index can be described as a constant equivalent width until a threshold in ionization index is reached ( [ NeIII ] / [ NeII ] \sim 1 ) , above which the equivalent widths decrease with a power law dependence .
This behavior for M101 HII regions is also seen for the sample of starburst galaxies presented in the companion study of Engelbracht et al .
( 24 ) which expands the range of [ NeIII ] / [ NeII ] ratios to 0.03–25 and log ( O/H ) +12 values to 7.1–8.8 .
The form of the correlation explains seemingly contradictory results present in the literature .
The behavior of the ratios of different aromatic features versus ionization index does not follow the predictions of existing PAH models of the aromatic features implying a more complex origin of the aromatic emission in massive star forming regions .