Broadband Intensity Tomography: Spectral Tagging of the Cosmic UV Background

Abstract

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 (1500 Å) and near-UV (2300 Å) 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 z ∼ 2. 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 Lyα emission, whose luminosity density is consistent with estimates of the combined galaxy and AGN contributions at z ∼ 1. 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 universe's net radiation output from stars, black holes, and the IGM combined.

A cosmic UV/X-ray background model update

“…(v) The non-ionizing part of the low-redshift UVB spectrum inferred from cross-correlating GALEX observations with SDSS spectroscopic objects (Chiang et al 2019) is in excellent agreement with the UVB spectrum predicted using the BPASS stellar template.…”

“…In §3, we will also compare our new model to results from Oñorbe et al (2017) and Puchwein et al (2019) on modeling the effects of reionization. Figure 4 compares the frequency-resolved total UV emissivity at z 1.5 inferred by Chiang et al (2019) to the model emissivities for galaxies and AGN based on the assumptions detailed in §2.2-2.3. Interestingly, the spectral shape of the total emissivity is in excellent agreement with the BPASS stellar template used in our UVB model (including an apparent but subtle change in spectral slope around 1216Å), providing support for this stellar template.…”

“…,com ν1500 = (25.62, 25.79, 25.92, 26.04, 26.15, 26.26, (9) 26.32, 26.42, 26.48, 26.54, 26.61, 26.60, 26.60, 26.52, 26.30, 26.11, 25.90, 25.67) at z = (0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.1, 3, 3.8, 4.9, 5.9, 6.8, 7.9), and interpolate linearly between these redshifts (emissivity in erg s −1 Hz −1 Mpc −3 ). At z 2, these values correspond to the total UV emissivity as function of redshift inferred by Chiang et al (2019) by cross-correlating GALEX UV imaging with sources with spectroscopic redshifts in SDSS. The total emissivity at these redshifts is consistent with the emissivity predicted by integrating the galaxy luminosity function down to very faint magnitudes (e.g., Driver et al 2016) and our results below also indicate that AGN contribute only a small fraction of the total 1500 Å emissivity at these redshifts.…”

“…where σi is the photoionization cross section for the species The model stellar (dot-dashes) and AGN (dashes) contributions are compared to an observational inference of the total continuum UV emissivity longward of 912 Å obtained by crosscorrelating GALEX imaging data with spectroscopic objects from the SDSS (solid; Chiang et al 2019). The UV emissivity longward of 912 Å is dominated by stars at all redshifts, even though the ionizing emissivity is dominated by AGN at all redshifts shown here.…”

Testing the SZ-based tomographic approach to the thermal history of the universe with pressure-density cross-correlations: Insights from the Magneticum simulation

“…Following this method, an external sample of reference sources with known z is taken, and cross-correlated with the Compton y parameter as a function of z. This allows correlated intensities to be extracted tomographically [22][23][24], as has been studied in recent papers for the SZ-galaxy cross-correlation with spectroscopic redshifts [10,25,26]. See Refs.…”