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Using high-resolution cosmological simulations, we study hydrogen and helium gravitational cooling radiation from gas accretion by young galaxies. We focus on the He II cooling lines, which arise from gas with a different temperature history (Tmax ~ 105 K) than H I line-emitting gas. We examine whether three major atomic cooling lines, H I λ1216, He II λ1640, and He II λ304, are observable, finding that Lyα and He II λ1640 cooling emission at z = 2-3 are potentially detectable with deep narrowband (R > 100) imaging and/or spectroscopy from the ground. While the expected strength of H I λ1216 cooling emission depends strongly on the treatment of the self-shielded phase of the IGM in the simulations, our predictions for the He II λ1640 line are more robust, because the He II emissivity is negligible below T ~ 104.5 K and less sensitive to the UV background. Although He II λ1640 cooling emission is fainter than Lyα by at least a factor of 10 and, unlike Lyα, might not be resolved spatially with current observational facilities, it is more suitable to study gas accretion in the galaxy formation process because it is optically thin and less contaminated by the recombination lines from star-forming galaxies. The He II λ1640 line can be used to distinguish among mechanisms for powering the so-called Lyα blobs—including gravitational cooling radiation, photoionization by stellar populations, and starburst-driven superwinds—because (1) He II λ1640 emission is limited to very low metallicity [log(Z/Z) -5.3] and Population III stars and (2) the blob's kinematics are probed unambiguously through the He II line width, which for cooling radiation is narrower (σ < 400 km s-1) than typical wind speeds.


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