From the diffuse sky to cosmological galaxy formation
Diffuse-light photons are underutilized in many surveys. In this talk, I will introduce a clustering-based tomographic approach capable of extracting rich information in the extragalactic background light in wide-field imaging datasets. I will show one of our first applications in probing the cosmic UV background at 0<z<2 using GALEX All Sky and Medium Imaging Surveys. We spatially cross-correlate UV photons in two GALEX bands with spectroscopic objects in SDSS as a function of redshift, and use the observed shapes of the K-corrections to constrain the main spectral features of the UV background: the Lyman break, the non-ionizing UV continuum, and the Lya emission. These spectral diagnostics allow us to probe cosmic star-formation and gas absorption in the interstellar medium averaging over all galaxies. Since the technique probes the total background including low surface brightness emission, we can also place constraints on the amount of UV light originating from the diffuse intergalactic medium. This new type of intensity mapping tomography can be generalized to broadband, narrow-band, grism, or integral field unit data in all wavebands. I will discuss future prospects for measuring apanchromatic spectrum of the universe and performing emission-line diagnostics on cosmological scales.