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Register a new userA Cosmic UV/X-ray Background Model Update
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Claude-Andre Faucher-Giguere
Publication date
2019
fields
Physics
and research's language is
English
Authors
C.-A. Faucher-Giguere
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We present an updated model of the cosmic ionizing background from the UV to the X-rays. Relative to our previous model (Faucher-Giguere et al. 2009), the new model provides a better match to a large number of up-to-date empirical constraints, including: 1) new galaxy and AGN luminosity functions; 2) stellar spectra including binary stars; 3) obscured and unobscured AGN; 4) a measurement of the non-ionizing UV background; 5) measurements of the intergalactic HI and HeII photoionization rates at z~0-6; 6) the local X-ray background; and 7) improved measurements of the intergalactic opacity. In this model, AGN dominate the HI ionizing background at z<~3 and star-forming galaxies dominate it at higher redshifts. Combined with the steeply declining AGN luminosity function beyond z~2, the slow evolution of the HI ionization rate inferred from the high-redshift HI Lya forest requires an escape fraction from star-forming galaxies that increases with redshift (a population-averaged escape fraction of ~1% suffices to ionize the intergalactic medium at z=3 when including the contribution from AGN). We provide effective photoionization and photoheating rates calibrated to match the Planck 2018 reionization optical depth and recent constraints from the HeII Lya forest in hydrodynamic simulations.
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Cosmic photons can be efficiently collected by broadband intensity mapping but information on their emission redshift and frequency is largely lost. We introduce a technique to statistically recover these otherwise collapsed dimensions by exploiting information in spatial fluctuations and apply it to the Galaxy Evolution Explorer (GALEX) All Sky and Medium Imaging Surveys. By spatially cross-correlating photons in the GALEX far-UV (1500A) and near-UV (2300A) bands with a million spectroscopic objects in the Sloan Digital Sky Survey as a function of redshift, we robustly detect the redshift-dependent intensity of the UV background (UVB) modulated by its clustering bias up to $zsim2$. These measurements clearly reveal the imprints of UVB spectral features redshifting through the filters. Using a simple parameterization, we simultaneously fit a UVB emissivity and clustering bias factor to these observations and constrain the main spectral features of the UVB spectrum: (i) the Lyman break, (ii) the non-ionizing UV continuum, which agrees with the Haardt & Madau model but does not rely on any assumption regarding the nature of the sources, and (iii) the Ly$alpha$ emission, whose luminosity density is consistent with estimates of the combined galaxy and AGN contributions at $zsim1$. Because the technique probes the total background including low surface brightness emission, we place constraints on the amount of UV light originating from the diffuse intergalactic medium (IGM). Finally, the clustering bias of UV photons is found to be chromatic and evolving. Our frequency- and redshift-dependent UVB measurement delivers a summary statistic of the universes net radiation output from stars, black holes, and the IGM combined.
The cosmic X-ray background (CXB), which peaks at an energy of ~30 keV, is produced primarily by emission from accreting supermassive black holes (SMBHs). The CXB therefore serves as a constraint on the integrated SMBH growth in the Universe and the accretion physics and obscuration in active galactic nuclei (AGNs). This paper gives an overview of recent progress in understanding the high-energy (>~10 keV) X-ray emission from AGNs and the synthesis of the CXB, with an emphasis on results from NASAs NuSTAR hard X-ray mission. We then discuss remaining challenges and open questions regarding the nature of AGN obscuration and AGN physics. Finally, we highlight the exciting opportunities for a next-generation, high-resolution hard X-ray mission to achieve the long-standing goal of resolving and characterizing the vast majority of the accreting SMBHs that produce the CXB.
While the cosmic soft X-ray background is very likely to originate from individual Seyfert galaxies, the origin of the cosmic hard X-ray and MeV gamma-ray background is not fully understood. It is expected that Seyferts including Compton thick population may explain the cosmic hard X-ray background. At MeV energy range, Seyferts having non-thermal electrons in coronae above accretion disks or MeV blazars may explain the background radiation. We propose that future measurements of the angular power spectra of anisotropy of the cosmic X-ray and MeV gamma-ray backgrounds will be key to deciphering these backgrounds and the evolution of active galactic nuclei (AGNs). As AGNs trace the cosmic large-scale structure, spatial clustering of AGNs exists. We show that e-ROSITA will clearly detect the correlation signal of unresolved Seyferts at 0.5-2 keV and 2-10 keV bands and will be able to measure the bias parameter of AGNs at both bands. Once the future hard X-ray all sky satellites achieve the sensitivity better than 10^{-12} erg/cm^2/s at 10-30 keV or 30-50 keV - although this is beyond the sensitivities of current hard X-ray all sky monitors - angular power spectra will allow us to independently investigate the fraction of Compton-thick AGNs in all Seyferts. We also find that the expected angular power spectra of Seyferts and blazars in the MeV range are different by about an order of magnitude, where the Poisson term, so-called shot noise, is dominant. Current and future MeV instruments will clearly disentangle the origin of the MeV gamma-ray background through the angular power spectrum.
The observed relation between the X-ray radiation from AGNs, originating in the corona, and the optical/UV radiation from the disk is usually described by the anticorrelation between the UV to X-ray slope alpha_ox and the UV luminosity. Many factors can affect this relation, including: enhanced X-ray emission associated with the jets of radio-loud AGNs; X-ray absorption associated with the UV Broad Absorption Line (BAL) outflows; other X-ray absorption not associated with BALs; intrinsic X-ray weakness; UV and X-ray variability, and non-simultaneity of UV and X-ray observations. The separation of these effects provides information about the intrinsic alpha_ox-L_UV relation and its dispersion, constraining models of disk-corona coupling. We extract simultaneous data from the second XMM-Newton serendipitous source catalogue and the XMM-Newton Optical Monitor Serendipitous UV Source Survey Catalog, and derive the single-epoch alpha_ox indices. We use ensemble structure functions to analyse multi-epoch data. We confirm the anticorrelation of alpha_ox with L_UV, and do not find any evidence of a dependence of alpha_ox on z. The dispersion in our simultaneous data (0.12) is not significantly smaller than in previous non-simultaneous studies, suggesting that artificial alpha_ox variability introduced by non-simultaneity is not the main cause of dispersion. Intrinsic alpha_ox variability, i.e., the true variability of the X-ray to optical ratio, is instead important, and accounts for ~30% of the total variance, or more. Inter-source dispersion, due to intrinsic differences in the average alpha_ox values from source to source, is also important. The dispersion introduced by variability is mostly caused by the long timescale variations, which are expected to be driven by the optical variations.
We use a sample of 1669 QSOs ($r<20.15$, $3.6<z<4.0$) from the BOSS survey to study the intrinsic shape of their continuum and the Lyman continuum photon escape fraction (f$_{esc}$), estimated as the ratio between the observed flux and the expected intrinsic flux (corrected for the intergalactic medium absorption) in the wavelength range 865-885 AA rest-frame. Modelling the intrinsic QSO continuum shape with a power-law, $F_{lambda}proptolambda^{-gamma}$, we find a median $gamma=1.30$ (with a dispersion of $0.38$, no dependence on the redshift and a mild intrinsic luminosity dependence) and a mean f$_{esc}=0.75$ (independent of the QSO luminosity and/or redshift). The f$_{esc}$ distribution shows a peak around zero and a long tail of higher values, with a resulting dispersion of $0.7$. If we assume for the QSO continuum a double power-law shape (also compatible with the data) with a break located at $lambda_{rm br}=1000$ AA and a softening $Deltagamma=0.72 $ at wavelengths shorter than $lambda_{rm br}$, the mean f$_{esc}$ rises to $=0.82$. Combining our $gamma$ and f$_{esc}$ estimates with the observed evolution of the AGN luminosity function (LF) we compute the AGN contribution to the UV ionizing background (UVB) as a function of redshift. AGN brighter than one tenth of the characteristic luminosity of the LF are able to produce most of it up $zsim 3$, if the present sample is representative of their properties. At higher redshifts a contribution of the galaxy population is required. Assuming an escape fraction of Lyman continuum photons from galaxies between $5.5$ and $7.6%$, independent of the galaxy luminosity and/or redshift, a remarkably good fit to the observational UVB data up to $zsim 6$ is obtained. At lower redshift the extrapolation of our empirical estimate agrees well with recent UVB observations, dispelling the so-called Photon Underproduction Crisis.
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