Probing Galaxy Formation with HeiiCooling Lines

Using high resolution cosmological simulations, we study hydrogen and heliumgravitational cooling radiation. We focus on the HeII cooling lines, whicharise from gas with a different temperature history (T_max ~ 10^5K) than HIline emitting gas. We examine whether three major atomic cooling lines, HI1216A, HeII 1640A and HeII 304A are observable, finding that HI 1216A and HeII1640A cooling emission at z=2-3 are potentially detectable with deep narrowband (R>100) imaging and/or spectroscopy from the ground. While the expectedstrength of HI 1216A cooling emission depends strongly on the treatment of theself-shielded phase of the IGM in the simulations, our predictions for the HeII1640A line are more robust because the HeII 1640A emissivity is negligiblebelow T~10^4.5 K and less sensitive to the UV background. Although HeII 1640Acooling emission is fainter than HI 1216A by at least a factor of 10 and,unlike HI 1216A, might not be resolved spatially with current observationalfacilities, it is more suitable to study gas accretion in the galaxy formationprocess because it is optically thin and less contaminated by the recombinationlines from star-forming galaxies. The HeII 1640A line can be used todistinguish among mechanisms for powering the so-called "Lyman alpha blobs" --including gravitational cooling radiation, photoionization by stellarpopulations, and starburst-driven superwinds -- because (1) HeII 1640A emissionis limited to very low metallicity (log(Z/Z_sun) < -5.3) and Population IIIstars, and (2) the blob's kinematics are probed unambiguously through the HeII1640A line width, which, for cooling radiation, is narrower (sigma < 400 km/s)than typical wind speeds.Comment: 15 pages, 9 figures. Accepted for publication in Ap