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Published September 20, 2009 | Published
Journal Article Open

Deep Spitzer 24 μm COSMOS Imaging. I. The Evolution of Luminous Dusty Galaxies—Confronting the Models


We present the first results obtained from the identification of ~30,000 sources in the Spitzer/24 μm observations of the COSMOS field at S_(24 μm) ≳ 80 μJy. Using accurate photometric redshifts (σ_ z ~ 0.12 at z ~ 2 for 24 μm sources with i ^+ ≳ 25 mag AB) and simple extrapolations of the number counts at faint fluxes, we resolve with unprecedented detail the buildup of the mid-infrared background across cosmic ages. We find that ~50% and ~80% of the 24 μm background intensity originate from galaxies at z ≳ 1 and z ≳ 2, respectively, supporting the scenario where highly obscured sources at very high redshifts (z ≳ 2) contribute only marginally to the cosmic infrared background. Assuming flux-limited selections at optical wavelengths, we also find that the fraction of i ^+-band sources with 24 μm detection strongly increases up to z ~ 2 as a consequence of the rapid evolution that star-forming galaxies have undergone with look-back time. Nonetheless, this rising trend shows a clear break at z ~ 1.3, probably due to k-correction effects implied by the complexity of spectral energy distributions in the mid-infrared. Finally, we compare our results with the predictions from different models of galaxy formation. We note that semianalytical formalisms currently fail to reproduce the redshift distributions observed at 24 μm. Furthermore, the simulated galaxies at S _(24 μm) > 80 μJy exhibit R–K colors much bluer than observed and the predicted K-band fluxes are systematically underestimated at z ≳ 0.5. Unless these discrepancies mainly result from an incorrect treatment of extinction in the models they may reflect an underestimate of the predicted density of high-redshift massive sources with strong ongoing star formation, which would point to more fundamental processes and/or parameters (e.g., initial mass function, critical density to form stars, feedback,...) that are still not fully controlled in the simulations. The most recent backward evolution scenarios reproduce reasonably well the flux/redshift distribution of 24 μm sources up to z ~ 3, although none of them is able to exactly match our results at all redshifts.

Additional Information

© 2009. The American Astronomical Society. Received 2009 February 25; accepted 2009 July 28; published 2009 August 26. It is a pleasure to acknowledge the contribution from all our colleagues of the COSMOS collaboration. More information on the COSMOS survey is available at http://www.astro.caltech.edu/cosmos. This work is based on observations made with the Spitzer Space Telescope, a facility operated by NASA/JPL. Financial supports were provided by NASA through contracts nos. 1289085, 1310136, 1282612, and 1298231 issued by the Jet Propulsion Laboratory. Our research project was also supplemented with phenomenological model predictions from published papers or future publications and that Damien Le Borgne, Carlotta Gruppioni, Francesca Pozzi, Guilaine Lagache, and Herv´e Dole have been willing to share with us; they are greatly acknowledged for their contribution. We are grateful to David Elbaz and Casey Papovich for insightful comments on our results, and we also appreciated the hospitality of the Aspen Center for Physics where part of our work was prepared. We finally acknowledge the support and the contribution of the Spitzer Science Center to our research program and we warmly thank the referee for a careful reading of the manuscript. Support for ELF's work was provided by NASA/Caltech through the Spitzer Space Telescope Fellowship Program (no. 1080367). Part of this work was also supported by the grant ASI/COFIS/WP3110 I/026/07/0.

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