ORIGIN OF 12 μm EMISSION ACROSS GALAXY POPULATIONS FROM WISE AND SDSS SURVEYS

We cross-matched Wide-field Infrared Survey Explorer sources brighter than 1 mJy at 12 μm with the Sloan Digital Sky Survey galaxy spectroscopic catalog to produce a sample of ∼105 galaxies at (z) = 0.08, the largest of its kind. This sample is dominated (70%) by star-forming (SF) galaxies from the blue sequence, with total IR luminosities in the range ∼108-1012 L☉. We identify which stellar populations are responsible for most of the 12 μm emission. We find that most (∼80%) of the 12 μm emission in SF galaxies is produced by stellar populations younger than 0.6 Gyr. In contrast, the 12 μm emission in weak active galactic nuclei (AGNs; L[Oiii] < 107 L.) is produced by older stars, with ages of ∼1-3 Gyr. We find that L12μm linearly correlates with stellar mass for SF galaxies. At fixed 12 μm luminosity, weak AGNs deviate toward higher masses since they tend to be hosted by massive, early-type galaxies with older stellar populations. SF galaxies and weak AGNs follow different L12μm-SFR (star formation rate) relations, with weak AGNs showing excess 12 μm emission at low SFR (0.02-1 M☉ yr–1). This is likely due to dust grains heated by older stars. While the specific star formation rate (SSFR) of SF galaxies is nearly constant, the SSFR of weak AGNs decreases by ∼3 orders of magnitude, reflecting the very different star formation efficiencies between SF galaxies and massive, early-type galaxies. Stronger type II AGNs in our sample (L[Oiii] > 107 L.), act as an extension of massive SF galaxies, connecting the SF and weak AGN sequences. This suggests a picture where galaxies form stars normally until an AGN (possibly after a starburst episode) starts to gradually quench the SF activity. We also find that 4.6-12 μm color is a useful first-order indicator of SF activity in a galaxy when no other data are available.