HeII emission in Lyman-alpha nebulae: AGN or cooling radiation?
  We present a study of an extended Lyman-alpha (Lya) 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 250um, and deep optical spectra of the
sources associated with the extended emission. Two galaxies are associated with
the Lya nebula. One of them is a dust enshrouded AGN, while the other is a
powerful starburst, forming stars at >~600 Msol/yr. We detect the HeII emission
line at 1640A in the spectrum of the obscured AGN, but detect no emission from
other highly ionized metals (CIV or NV) as is expected from an AGN. One
scenario that simultaneously reproduces the width of the detected emission
lines, the lack of CIV emission, and the geometry of the emitting gas, is that
the HeII and the Lya 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 Lya nebula it is possible
that various mechanisms of excitation are at work simultaneously.
