Spitzer's perspective of polycyclic aromatic hydrocarbons in galaxies

Polycyclic aromatic hydrocarbon (PAH) molecules are abundant and widespread throughout the Universe, as revealed by their distinctive set of emission bands at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 μm, which are characteristic of their vibrational modes. They are ubiquitously seen in a wide variety of astrophysical regions, ranging from planet-forming disks around young stars to the interstellar medium of the Milky Way and other galaxies out to high redshifts at z ≳ 4. PAHs profoundly influence the thermal budget and chemistry of the interstellar medium by dominating the photoelectric heating of the gas and controlling the ionization balance. Here I 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 onboard Spitzer across a much broader range of extragalactic environments (for example, distant galaxies, early-type galaxies, galactic halos, active galactic nuclei and low-metallicity galaxies) than was previously possible with the Infrared Space Observatory 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.