Spitzer's perspective of polycyclic aromatic hydrocarbons in galaxies
  Polycyclic aromatic hydrocarbon (PAH) molecules, as revealed by the
distinctive set of emission bands at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 $\mu$m
characteristic of their vibrational modes, are abundant and widespread
throughout the Universe. They are ubiquitously seen in a wide variety of
astrophysical regions, ranging from planet-forming disks around young stars to
the interstellar medium (ISM) of the Milky Way and external galaxies out to
high redshifts at z>4. PAHs profoundly influence the thermal budget and
chemistry of the ISM by dominating the photoelectric heating of the gas and
controlling the ionization balance.
  Here, we review the current state of knowledge of the astrophysics of PAHs,
focusing on their observational characteristics obtained from the Spitzer Space
Telescope and their diagnostic power for probing the local physical and
chemical conditions and processes. Special attention is paid to the spectral
properties of PAHs and their variations revealed by the Infrared Spectrograph
(IRS) on board Spitzer across a much broader range of extragalactic
environments (e.g., distant galaxies, early-type galaxies, galactic halos,
active galactic nuclei, and low-metallicity galaxies) than was previously
possible with the Infrared Space Observatory (ISO) or any other telescope
facilities. Also highlighted is the relation between the PAH abundance and the
galaxy metallicity established for the first time by Spitzer.
