Cosmic Infrared & X-Ray Backgrounds
The Cosmic Infrared Background (CIB, 8–1000 μm) is the collective thermal emission of interstellar dust grains heated by star formation and AGN across cosmic history, carrying ~27 nW m⁻² sr⁻¹—roughly half of all non-CMB extragalactic photon energy. The Cosmic X-Ray Background (CXB, 0.5–300 keV, peaking near 30 keV) is the integrated emission of millions of obscured and unobscured accreting supermassive black holes.
Physical Mechanism & Dynamics
Predicted by Partridge & Peebles (1967) and first detected by COBE/FIRAS and DIRBE (Puget et al. 1996; Fixsen et al. 1998), the CIB peaks near λ ≈ 150–250 μm because intense ultraviolet light from short-lived O and B stars inside dusty starburst galaxies at Cosmic Noon (z ≈ 1–3) is absorbed by polycyclic aromatic hydrocarbons (PAHs) and silicate/graphite grains (T_dust ≈ 20–45 K) and re-radiated in the far-infrared, then redshifted into the submillimeter band.
Discovered in 1962 by Giacconi, Gursky, Paolini, and Rossi during the rocket flight that also found Scorpius X-1, the Cosmic X-Ray Background puzzled astronomers for decades because its 3–50 keV spectrum is much harder (photon index Γ ≈ 1.4) than typical unobscured quasars (Γ ≈ 1.8–1.9). Population synthesis models (Gilli et al. 2007) and deep Chandra/XMM-Newton/NuSTAR surveys solved this 'spectral paradox': the CXB is dominated by a mixture of unobscured (Type 1), Compton-thin obscured (N_H = 10²²–10²⁴ cm⁻²), and Compton-thick (N_H > 1.5 × 10²⁴ cm⁻²) AGN whose soft X-rays are photoelectrically absorbed, leaving a Compton-reflection hump peaking at 20–30 keV.
Quantitatively, this mechanism is governed by Sołtan Argument: Black Hole Mass Density from Radiation Backgrounds (ρ_BH(z=0) = ((1 - ε) / (ε c^2)) ∫_0^∞ dE ∫_0^∞ dz ((1 + z) / E) (dI_E / dz) ≈ 4.5 × 10⁵ M☉ Mpc⁻³). Andrzej Sołtan's (1982) relation connecting the observed energy density of the CXB/quasar background (corrected for bolometric factor and radiative efficiency ε ≈ 0.1) to the local mass density of relic supermassive black holes.
Sołtan Argument: Black Hole Mass Density from Radiation Backgrounds
ρ_BH(z=0) = ((1 - ε) / (ε c^2)) ∫_0^∞ dE ∫_0^∞ dz ((1 + z) / E) (dI_E / dz) ≈ 4.5 × 10⁵ M☉ Mpc⁻³Andrzej Sołtan's (1982) relation connecting the observed energy density of the CXB/quasar background (corrected for bolometric factor and radiative efficiency ε ≈ 0.1) to the local mass density of relic supermassive black holes.
Quantitative Benchmarks & Parameters
| Parameter / Observable | Measured Value / Scale | Method & Context |
|---|---|---|
| Integrated CIB intensity (8–1000 μm) | 27 ± 4 nW m⁻² sr⁻¹ | COBE/FIRAS, Spitzer, Herschel & Planck sub-mm maps |
| CXB peak energy & intensity | E_peak ≈ 25–30 keV; νI_ν ≈ 4.5 × 10⁻² nW m⁻² sr⁻¹ | HEAO-1, Integral, Swift/BAT & NuSTAR |
| Resolved fraction of 0.5–8 keV CXB | > 90% resolved into discrete AGN & galaxies | 7-Ms Chandra Deep Field-South (Luo et al. 2017) |
| Resolved fraction of 8–24 keV hard CXB | ~35–50% directly resolved by NuSTAR | NuSTAR extragalactic survey (Harrison et al. 2016) |
Observational Evidence & How Scientists Know
Herschel (PACS/SPIRE) and ALMA blank-field and lensing surveys resolved >75–90% of the CIB into dusty star-forming galaxies at z = 1–4, while the 7-megasecond Chandra Deep Field-South resolved >90% of the 0.5–7 keV CXB into discrete point sources (predominantly AGN at z = 0.5–3).
Across reference datasets (Planck 2018 Cosmological Parameters (PR3), JWST / MAST Science Archive & Mission Documentation), quantitative benchmarks for cosmic infrared & x-ray backgrounds are anchored by: Integrated CIB intensity (8–1000 μm) (27 ± 4 nW m⁻² sr⁻¹, via COBE/FIRAS, Spitzer, Herschel & Planck sub-mm maps); CXB peak energy & intensity (E_peak ≈ 25–30 keV; νI_ν ≈ 4.5 × 10⁻² nW m⁻² sr⁻¹, via HEAO-1, Integral, Swift/BAT & NuSTAR); Resolved fraction of 0.5–8 keV CXB (> 90% resolved into discrete AGN & galaxies, via 7-Ms Chandra Deep Field-South (Luo et al. 2017)); Resolved fraction of 8–24 keV hard CXB (~35–50% directly resolved by NuSTAR, via NuSTAR extragalactic survey (Harrison et al. 2016)).
Uncertainties, Systematics & Open Questions
At energies above 20 keV, determining the exact fraction of heavily buried Compton-thick AGN at z > 1.5 requires higher-sensitivity hard X-ray focusing optics beyond NuSTAR.
Real Astronomical Benchmarks & Cross-Domain Links
Chandra Deep Field-South (CDF-S)
Deepest X-ray image ever taken (7 million seconds), resolving over 1,000 faint AGN that build the CXB
Submillimeter Galaxies (SMGs, e.g. HDF 850.1)
Dust-obscured z ≈ 2–5 starburst galaxies forming 500–2,000 M☉/yr that generate the bulk of the submillimeter CIB
NGC 1068 (Messier 77)
Archetypal local Compton-thick Seyfert 2 galaxy whose torus absorbs soft X-rays and reflects 20–30 keV photons
Guided Learning Path
Related Concepts, Methods & Explorers
Scientific Sources & Provenance
- Planck 2018 Cosmological Parameters (PR3) (PR3 (2020 A&A 641, A6)) — Planck Collaboration, Aghanim, N., et al. 2020, A&A, 641, A6. Archive / Reference ↗
- JWST / MAST Science Archive & Mission Documentation (Cycle 1–4 Public Releases (2022–2026)) — Rigby, J., et al. 2023, PASP, 135, 048001 (The Science Performance of JWST as Characterized in Commissioning). Archive / Reference ↗
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