Polycyclic aromatic hydrocarbons in spatially resolved extragalactic
  star forming complexes
  The abundance of polycyclic aromatic hydrocarbons (PAHs) in low- and
high-metallicity galaxies has been widely discussed since the time when
detailed infrared data for extragalactic objects were first obtained. On the
scales of entire galaxies, a smaller PAH abundance in lower-metallicity
galaxies is often observed. We study this relationship for star-forming regions
in nearby galaxies, for a sample containing more than 200 HII complexes, using
spatially-resolved observations from the Herschel Space Observatory and Spitzer
Space Telescope. We use a model for the dust emission to estimate the physical
parameters (PAH abundance, metallicity, ultraviolet radiation field, etc.) of
these complexes. The same correlation of PAH abundance with metallicity, as
seen for entire galaxies, is apparently preserved at smaller scales, at least
when the Kobulnicky & Kewley metallicity calibration is used. We discuss
possible reasons for this correlation, noting that traces of less-effective PAH
formation in low-metallicity AGB stars should be smeared out by radial mixing
in galactic disks. Effective destruction by the harder and more intensive
ultraviolet field in low-metallicity environments is qualitatively consistent
with our data, as the ultraviolet field intensity, derived from the infrared
photometry, is indeed smaller in HII complexes with lower metallicity.
