He II emission in Lyα nebulae: Active galactic nucleus or cooling radiation?

We present a study of an extended Lyα nebula located in a known overdensity at z ∼ 2.38. The data include multiwavelength photometry covering the rest-frame spectral range from 0.1 to 250μm, and deep optical spectra of the sources associated with the extended emission. Two galaxies are associated with the Lyα nebula. One of them is a dust enshrouded active galactic nucleus (AGN), while the other is a powerful starburst, forming stars at ≳400 M_· yr^-1. We detect the He II emission line at 1640 Å 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 Lyα 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 Lyα nebula it is possible that various mechanisms of excitation are at work simultaneously.