He II EMISSION IN Lyα NEBULAE: ACTIVE GALACTIC NUCLEUS OR COOLING RADIATION?

We present a study of an extended Ly alpha nebula located in a known overdensity at z similar to 2.38. The data include multiwavelength photometry covering the rest-frame spectral range from 0.1 to 250 mu m, and deep optical spectra of the sources associated with the extended emission. Two galaxies are associated with the Ly alpha nebula. One of them is a dust enshrouded active galactic nucleus (AGN), while the other is a powerful starburst, forming stars at greater than or similar to 400 M(circle dot) yr(-1). We detect the He II emission line at 1640 angstrom in the spectrum of the obscured AGN, but detect no emission from other highly ionized metals (C IV or N V) as is expected from an AGN. One scenario that simultaneously reproduces the width of the detected emission lines, the lack of C IV emission, and the geometry of the emitting gas, is that the He II and the Ly alpha emission are the result of cooling gas that is being accreted on the dark matter halo of the two galaxies, Ly1 and Ly2. Given the complexity of the environment associated with our Ly alpha nebula it is possible that various mechanisms of excitation are at work simultaneously.