Broadband Intensity Tomography: Spectral Tagging of the Cosmic UV
  Background
  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
$z\sim2$. 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
$z\sim1$. 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.
