Using high resolution cosmological simulations , we study hydrogen and helium gravitational cooling radiation .
We focus on the He ii cooling lines , which arise from gas with a different temperature history ( T _ { max } \sim 10 ^ { 5 } \mathrm { K } ) than H i line emitting gas .
We examine whether three major atomic cooling lines , H i \lambda 1216 , He ii \lambda 1640 and He ii \lambda 304 , are observable , finding that Ly \alpha and He ii \lambda 1640 cooling emission at z = 2 - 3 are potentially detectable with deep narrow band ( R > 100 ) imaging and/or spectroscopy from the ground .
While the expected strength of H i \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 ii \lambda 1640 line are more robust because the He ii emissivity is negligible below T \sim 10 ^ { 4.5 } \mathrm { K } and less sensitive to the UV background .
Although He ii \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 ii \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 ii \lambda 1640 emission is limited to very low metallicity ( \mathrm { log } ( Z / Z _ { \sun } ) \lesssim - 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 ( \sigma < 400 \mathrm { km s ^ { -1 } } ) than typical wind speeds .