[Paper Review] The Cosmic Infrared Background Resolved by Spitzer. Contributions of Mid-Infrared Galaxies to the Far-Infrared Background

H. Dole, G. Lagache+8 more|CERN Bulletin|Mar 8, 2006

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy1 references277 citations

TL;DR

This study uses stacking analysis of Spitzer MIPS 24 μm sources in deep fields to directly measure the contribution of mid-infrared galaxies to the far-infrared cosmic infrared background (CIB). It finds that 24 μm-selected galaxies resolve over 70% of the CIB at 70 and 160 μm, with luminous infrared galaxies at z ∼ 1 being the dominant contributors, and establishes that the CIB and cosmic optical background are nearly equal in brightness at ~24 nW m⁻² sr⁻¹.

ABSTRACT

Aims. We quantify the contributions of 24um galaxies to the Far-Infrared (FIR) Background at 70 and 160um. 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 160um in three deep fields, we stacked more than 19000 MIPS 24um sources with S24>60uJy at 70 and 160um, 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) 24um selected sources contribute to more than 70% of the CIB at 70 and 160um. 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 z~1 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 160um. By combining the extrapolation of the 24um source counts below 60uJy, with 160/24 and 70/24 colors as measured with the stacking analysis, we obtain lower limits of 7.1+/-1.0 and 13.4+/-1.7 nW/m2/sr for the CIB at 70 and 160um, 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/m2/sr. 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).

Motivation & Objective

• To quantify the contribution of mid-infrared (24 μm) selected galaxies to the far-infrared (FIR) cosmic infrared background (CIB) at 70 and 160 μm.
• To resolve the CIB below the confusion limit of FIR surveys using stacking techniques on deep Spitzer fields.
• To estimate lower bounds for the CIB at 70 and 160 μm by extrapolating 24 μm source counts to fainter fluxes.
• To compare the total CIB with the cosmic optical background (COB) and assess the energy budget of the extragalactic background light (EBL).
• To constrain the spectral energy distribution (SED) of the EBL and determine the ratio of infrared to optical photons produced by galaxy formation and evolution.

Proposed method

• Stacking analysis of 19,181 Spitzer MIPS 24 μm sources with S₂₄ ≥ 60 μJy in three deep fields at 70 and 160 μm to probe fainter FIR fluxes.
• Measuring the average FIR flux density from stacked sources to infer the total contribution to the CIB.
• Accounting for photometric noise, calibration systematics, and large-scale structure in the noise budget to ensure robust signal-to-noise ratios.
• Using constant color ratios (160/24 and 70/24) for faint 24 μm sources to extrapolate and estimate lower limits for the CIB at 70 and 160 μm.
• Combining stacking results with source count extrapolations and upper limits from high-energy experiments to constrain the full EBL SED.
• Integrating the best-estimate SEDs to compute total CIB and COB brightness and determine the ratio of infrared to optical photons.

Experimental results

Research questions

• RQ1 What fraction of the far-infrared cosmic infrared background (CIB) at 70 and 160 μm is contributed by galaxies selected at 24 μm?
• RQ2 To what extent can the CIB be resolved using stacking of 24 μm sources below the confusion limit of FIR surveys?
• RQ3 What is the lower bound estimate for the CIB at 70 and 160 μm, including contributions from galaxies undetected at 24 μm?
• RQ4 How do the total brightness levels of the cosmic infrared background (CIB) and cosmic optical background (COB) compare?
• RQ5 What is the ratio of infrared to optical photons produced by galaxy formation and evolution processes?

Key findings

• The 24 μm-selected galaxies contribute 79%, 92%, and 69% to the CIB at 24, 70, and 160 μm, respectively, with the highest contribution at 70 μm.
• Stacking analysis resolves more than 75% of the CIB at 70 and 160 μm, with the CIB resolved at flux densities of ~0.9 mJy (70 μm) and ~3 mJy (160 μm).
• Lower limit estimates for the CIB are 7.1 ± 1.0 nW m⁻² sr⁻¹ at 70 μm and 13.4 ± 1.7 nW m⁻² sr⁻¹ at 160 μm, consistent with model predictions.
• The total CIB and COB are nearly equal in brightness, both estimated at ~24 nW m⁻² sr⁻¹, with a COB/CIB ratio close to unity.
• Galaxy formation and evolution processes have produced a background brightness equivalent to 5% of the CMB, with equal contributions from direct starlight (COB) and dust-reprocessed light (CIB).
• On average, 115 infrared photons are produced for every one visible photon in the extragalactic background light.