Probing galaxy formation with HeII cooling lines

Using high-resolution cosmological simulations, we study hydrogen and helium gravitational cooling radiation from gas accretion by young galaxies. We focus on the He Pi cooling lines, which arise from gas with a different temperature history (T-max similar to 10(5) K) than H Iota line-emitting gas. We examine whether three major atomic cooling lines, H Iota lambda 1216, He Pi lambda 1640, and He Pi lambda 304, are observable, finding that Ly alpha and He Pi lambda 1640 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 Iota lambda 1216 cooling emission depends strongly on the treatment of the self-shielded phase of the IGM in the simulations, our predictions for the He Pi lambda 1640 line are more robust, because the He Pi emissivity is negligible below T similar to 104: 5 K and less sensitive to the UV background. Although He Pi lambda 1640 cooling emission is fainter than Ly alpha by at least a factor of 10 and, unlike Ly alpha, 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 Pi lambda 1640 line can be used to distinguish among mechanisms for powering the so-called Ly alpha blobs-including gravitational cooling radiation, photoionization by stellar populations, and starburst-driven superwinds-because(1)He Pi lambda 1640 emission is limited to very low metallicity [log (Z/Z circle dot)P less than or similar to - 5.3] and Population III stars and (2) the blob's kinematics are probed unambiguously through the He Pi line width, which for cooling radiation is narrower (sigma < 400 km s(-1)) than typical wind speeds.