Broadband Intensity Tomography: Spectral Tagging the Cosmic UV Background
Most of the photons ever recorded by astronomers are in the form of images—or broadband intensity mapping, but information on their emission redshift and frequency is largely lost. I will introduce a data-driven technique to recover these otherwise collapsed dimensions by exhausting information in the spatial fluctuations. As the first application, we measure the spectrum of the ultraviolet background (UVB) at 0<z<2 in GALEX Imaging surveys. We spatially cross-correlate photons in the Far-UV and Near-UV bands with spectroscopic objects in SDSS as a function of redshift, and use the observed shapes of the K-corrections to constrain the rest-frame spectrum of the UVB. We fit simultaneously a parameterized UVB volume emissivity and a clustering bias factor both as function of frequency and cosmic time. With minimum assumptions, our measured non-ionizing continuum emissivity is broadly consistent with that in the Haardt & Madau model. Cosmic Lya emission is tentatively detected with >95% confidence at z=1, with the luminosity density consistent with being powered by cosmic star-formation with an effective Lya escape fraction of 10%. Our approach probes all sources of radiation without surface brightness thresholding, which includes potential IGM emission. The technique brings some of the future line-intensity mapping science to within the reach of existing broadband data at all wavelengths.