The Effects of Metallicity and AGN Activity on the Mid-Infrared Dust Emission of Galaxies

Using a sample of the $Spitzer$ SWIRE-field galaxies whose optical spectra are taken from the Data Release 4 of the Sloan Digital Sky Survey, we study possible correlations between the Mid-InfraRed (MIR) dust emission from these galaxies and both their metallicities and AGN activities. We find that both metallicity and AGN activity are well correlated with the following ratios: PAH(Polycyclic Aromatic Hydrocarbon)-to-star, VSG(Very Small Grain)-to-star and PAH-to-VSG, which can be characterized by $\nu L_{\nu}[8\mu m({\rm dust})]/\nu L_{\nu}[3.6\mu m]$, $\nu L_{\nu}[24\mu m]/\nu L_{\nu}[3.6\mu m]$, and$\nu L_{\nu}[8\mu m({\rm dust})]/\nu L_{\nu}[24\mu m]$ respectively. We argue that our MIR-metallicity correlation could be explained by either the amount of dust (ongoing dust formation) or dust destruction (PAHs and VSGs could be destroyed by hard and intense radiation fields), and that the MIR-AGN correlation could arise due to either PAH destruction or an enhanced VSG continuum by the central AGN.