Spitzer deep and wide legacy mid- and far-infrared number counts and lower limits of cosmic infrared background^*

¹ Institut d'Astrophysique Spatiale (IAS), Bt. 121, 91405 Orsay, France; Université Paris-Sud 11 and CNRS (UMR8617) e-mail: mattieu.bethermin@ias.u-psud.fr
² Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d'Astrophysique, Bt. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France

Received: 11 September 2009
Accepted: 5 January 2010

Abstract

Aims. We aim to place stronger lower limits on the cosmic infrared background (CIB) brightness at 24 μm, 70 μm and 160 μm and measure the extragalactic number counts at these wavelengths in a homogeneous way from various surveys.

Methods. Using Spitzer legacy data over 53.6 deg² of various depths, we build catalogs with the same extraction method at each wavelength. Completeness and photometric accuracy are estimated with Monte-Carlo simulations. Number count uncertainties are estimated with a counts-in-cells moment method to take galaxy clustering into account. Furthermore, we use a stacking analysis to estimate number counts of sources not detected at 70 μm and 160 μm. This method is validated by simulations. The integration of the number counts gives new CIB lower limits.

Results. Number counts reach 35 μJy, 3.5 mJy and 40 mJy at 24 μm, 70 μm, and 160 μm, respectively. We reach deeper flux densities of 0.38 mJy at 70, and 3.1 at 160 μm with a stacking analysis. We confirm the number count turnover at 24 μm and 70 μm, and observe it for the first time at 160 μm at about 20 mJy, together with a power-law behavior below 10 mJy. These mid- and far-infrared counts: 1) are homogeneously built by combining fields of different depths and sizes, providing a legacy over about three orders of magnitude in flux density; 2) are the deepest to date at 70 μm and 160 μm; 3) agree with previously published results in the common measured flux density range; 4) globally agree with the Lagache et al. (2004) model, except at 160 μm, where the model slightly overestimates the counts around 20 and 200 mJy.

Conclusions. These counts are integrated to estimate new CIB firm lower limits of  nW m^-2 sr^-1,  nW m^-2 sr^-1, and  nW m^-2 sr^-1 at 24 μm, 70 μm, and 160 μm, respectively, and extrapolated to give new estimates of the CIB due to galaxies of  nW m^-2 sr^-1,  nW m^-2 sr^-1, and  nW m^-2 sr^-1, respectively. Products (point spread function, counts, CIB contributions, software) are publicly available for download at http://www.ias.u-psud.fr/irgalaxies/