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tokens: faint as 30. They predict for every galaxy a value of L X concordant with the scaling relation with the SFR (approximated by the infrared luminosity) of Basu-Zych et al. ( 2013 ). Without going into details, they estimate L 8–1000μm using photo- z , star-formation rate, UVJ rest-frame colors, and (observed or extrapolated) UV luminosity (). Using their mock catalog, we applied a selection as similar as possible to that of Laigle et al. ( 2016 ) from which we derived our mask. As a result, we find that these galaxies produce a [0.5–2] keV CXB surface brightness of the order of 3.3 × 10 −14 erg cm 2 s −1 deg −2 , which explains about 5% of the nsCXB. By assuming a typical X/O = 0 for AGNs as determined by Civano et al. ( 2012 ) and the i -limiting magnitudes in COSMOS (Scoville et al. 2007b ), we estimate that the undetected AGN [0.5–2] flux is 10 −17 erg cm 2 s −1 . At these low fluxes, star-forming galaxies vastly outnumber AGNs, so we can assume that ordinary AGNs cannot contribute more than galaxies to the soft nsCXB (5%). Being so faint, the sources producing the remaining CXB can be local ( z ∼ 1–3) and of low luminosity. Low-luminosity AGNs are preferentially highly absorbed (Barger et al. 2005 ; Hasinger 2008 ), and therefore we could argue that the hard portion of the nsCXB could be explained by these sources. However, at z ∼ 36, the number density of absorbed sources is still unknown, and we cannot exclude an unpredicted large number of such sources at that redshift. We propose that a large fraction of the remaining emission could arise from still undetected, rapidly accreting black holes at z > 6–7. Assuming the Direct Collapse black hole (DCBH) scenario for the formation of early black hole seeds, Pacucci et al. ( 2015 ) showed that these sources are likely undetected in current deep X-ray/NIR surveys. They compared the emission of DCBHs for two accretion models: radiatively efficient (Standard) and radiatively inefficient (Slim Disk; super-Eddington), in which photon trapping is significant and the outgoing radiation is diminished. In the latter case, the luminosity emitted by these sources is low. These short-lived and fainter black holes are more difficult to detect compared to brighter objects accreting at the Eddington limit (two tentative detections were proposed by Pacucci et al. 2016 ). Indeed, Comastri et al. ( 2015 ) revised the estimate of the local accreted mass density by taking into account that a significant fraction of the local black holes may have grown by radiatively inefficient accretion. From our measurements, the maximum flux produced by accretion onto early black holes is ∼10% of the CXB (see Table 3 ). To place limits on the amount of accretion occurring at , we follow the formalism of Salvaterra et al. ( 2012 ), assuming that the comoving specific emissivity of AGNs can be factorized aswhere is the normalization, , with , is the redshift evolution, and g ( E ) is a (normalized) template spectrum. For these