The cosmic infrared background resolved by Spitzer

Contributions of mid-infrared galaxies to the far-infrared background

¹ Institut d'Astrophysique Spatiale (IAS), bât. 121, 91405 Orsay Cedex, France; Université Paris-Sud 11 and CNRS (UMR 8617), France e-mail: Herve.Dole@ias.u-psud.fr
² Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ, 85721, USA
³ Associated with Observatoire de Paris, GEPI, 92195 Meudon, France

Received: 31 October 2005
Accepted: 18 February 2006

Abstract

Aims.We quantify the contributions of 24 μm galaxies to the Far-Infrared (FIR) Background at 70 and 160 μm. We provide new estimates of the Cosmic Infrared Background (CIB), and compare it with the Cosmic Optical Background (COB).

Methods.Using Spitzer data at 24, 70 and 160 μm in three deep fields, we stacked more than 19000 MIPS 24 μm sources with Jy at 70 and 160 μm, and measured the resulting FIR flux densities.

Results.This method allows a gain up to one order of magnitude in depth in the FIR. We find that the Mid-Infrared (MIR) 24 μm selected sources contribute to more than 70% of the Cosmic Infrared Background (CIB) at 70 and 160 μm. This is the first direct measurement of the contribution of MIR-selected galaxies to the FIR CIB. Galaxies contributing the most to the total CIB are thus luminous infrared galaxies, which have intermediate stellar masses. We estimate that the CIB will be resolved at 0.9 mJy at 70 and 3 mJy at 160 μm. By combining the extrapolation of the 24 μm source counts below analysis, we obtain lower limits of and  nW m^-2 sr^-1 for the CIB at 70 and 160 μm, respectively.

Conclusions.The MIPS surveys have resolved more than three quarters of the MIR and FIR CIB. By carefully integrating the Extragalactic Background Light (EBL) SED, we also find that the CIB has the same brightness as the COB, around 24 nW m^-2 sr^-1. The EBL is produced on average by 115 infrared photons for one visible photon. Finally, the galaxy formation and evolution processes emitted a brightness equivalent to 5% of the primordial electromagnetic background (CMB).