The Extragalactic Background Light
Earth and Space Sciences
The extragalactic background light (EBL) is the radiation at all wavelengths accumulated throughout the entire age of the Universe. The full spectrum of EBL captures the energy released by all stars and active supermassive blackholes (quasars), as well as the energy absorbed and reprocessed by the dust and gas in between. Direct observations of the EBL are only possible in the local Universe (see figure below for local observations and our model), and impossible at extreme UV wavelengths because of the shielding from the interstellar medium of our Milky-Way. Therefore, to assess the EBL beyond the local Universe, one needs to synthesize models of the EBL. The spectrum of EBL at low energies (optical to infra-red) is essential to study the distant γ-ray sources, and at high energies (UV to soft X-rays; the UV background) it is essential in almost all the studies related to intergalactic and circumgalactic medium (IGM/CGM). For example, UV background is essential for studies involving hydrogen and helium reionization, thermal and ionization state of the IGM, chemical enrichment and characterization of the CGM. It is also an essential input required in cosmological simulations of the structure formation and IGM. We have written a cosmological radiative transfer code to synthesize the EBL spectrum that successfully predicts the EBL from far-IR till TeV γ-rays, over 15 orders of magnitude in wavelength of electromagnetic spectrum. Our predictions for local EBL (at z=0) are shown in the figure below.
Caption: The intensity of local (z = 0) EBL from FIR to TeV γ-rays from our fiducial model (Khaire & Srianand 2019) with various measurements. These measurements are only possible in the local Universe.Shaded region shows the extreme UV part of the EBL known as the UV background (UVB). Although direct measurements of the UVB are not possible, the integrated intensity of UVB over a small wavelength range, quantified by photoionization rates of hydrogen and helium, can be measured at different ages (redshifts) of the Universe (Khaire 2017, Khaire et al. 2019).
Ref: i) Vikram Khaire & Raghunathan Srianand, 2019, MNRAS, 484, 4174, ii) Vikram Khaire, 2017, MNRAS, 471, 255, iii) Vikram Khaire et al, 2019, MNRAS, 486, 769