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 mu m with the Sloan Digital Sky Survey galaxy spectroscopic catalog to produce a sample of similar to 10(5) 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 similar to 10(8)-10(12) L-circle dot. We identify which stellar populations are responsible for most of the 12 mu m emission. We find that most (similar to 80%) of the 12 mu m emission in SF galaxies is produced by stellar populations younger than 0.6 Gyr. In contrast, the 12 mu m emission in weak active galactic nuclei (AGNs; L-[O III] < 10(7) L-circle dot) is produced by older stars, with ages of similar to 1-3 Gyr. We find that L-12 mu m linearly correlates with stellar mass for SF galaxies. At fixed 12 mu m 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 L-12 mu m-SFR (star formation rate) relations, with weak AGNs showing excess 12 mu m emission at low SFR (0.02-1M(circle dot) 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 similar to 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-[O III] > 10(7) L-circle dot), 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 mu m color is a useful first-order indicator of SF activity in a galaxy when no other data are available.