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Published April 1, 2012 | Submitted + Published
Journal Article Open

Origin of the 12 µm Emission Across Galaxy Populations from WISE and SDSS Surveys


We cross-matched Wide-field Infrared Survey Explorer (WISE) sources brighter than 1 mJy at 12µm with the Sloan Digital Sky Survey (SDSS) galaxy spectroscopic catalog to produce a sample of ~10^5 galaxies at =0.08, the largest of its kind. This sample is dominated (70%) by star-forming (SF) galaxies from the blue sequence, with total IR lµminosities in the range ~10^8-10^(12) L_⊙. We identify which stellar populations are responsible for most of the 12µm emission. We find that most (~80%) of the 12µm emission in SF galaxies is produced by stellar populations younger than 0.6 Gyr. In contrast, the 12µm emission in weak AGN (L[OIII]<10^7 L_⊙) is produced by older stars, with ages of ~1-3 Gyr. We find that L_[12µm] linearly correlates with stellar mass for SF galaxies. At fixed 12µm luminosity, weak AGN deviate toward higher masses since they tend to be hosted by massive, early-type galaxies with older stellar populations. Star-forming galaxies and weak AGN follow different L_[12µm]-SFR (star formation rate) relations, with weak AGN showing excess 12µm emission at low SFR (~0.02-1 M_⊙/yr). 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 AGN decreases by ~3 orders of magnitude, reflecting the very different star formation efficiencies between SF galaxies and massive, early-type galaxies. Stronger type II AGN in our sample (L_[OIII]>10^7 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 4.6-12µm color is a useful first-order indicator of SF activity in a galaxy when no other data are available.

Additional Information

© 2012 The American Astronomical Society. Received 2011 September 5. Accepted 2012 January 12. Published 2012 March 9. The authors thank G. Kauffmann, J. Brinchmann, and S. Salim for useful suggestions. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/. R.J.A. was supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities through a contract with NASA.

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Published - 0004-637X_748_2_80.pdf

Submitted - 1201.2943v1.pdf


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