Resolving the optical emission lines of Lya blob 'B1' at z=2.38: another
  hidden quasar
  We have used the SINFONI near-infrared integral field unit on the VLT to
resolve the optical emission line structure of one of the brightest (L~1e44
erg/s) and nearest (z=2.38) of all Lya blobs (LABs). The target, known in the
literature as object 'B1' (Francis et al. 1996), lies at a redshift where the
main optical emission lines are accessible in the observed near-infrared. We
detect luminous [OIII]4959,5007A and Ha emission with a spatial extent of at
least 32x40 kpc (4"x5"). The dominant optical emission line component shows
relatively broad lines (600-800 km/s, FWHM) and line ratios consistent with
AGN-photoionization. The new evidence for AGN photoionization, combined with
previously detected CIV and luminous, warm infrared emission, suggest that B1
is the site of a hidden quasar. This is confirmed by the fact that [OII] is
relatively weak compared to [OIII] (extinction-corrected [OIII]/[OII] of about
3.8), which is indicative of a high, Seyfert-like ionization parameter. From
the [OIII] luminosity we infer a bolometric AGN luminosity of ~3e46 erg/s, and
further conclude that the obscured AGN may be Compton-thick given existing
X-ray limits. The large line widths observed are consistent with clouds moving
within the narrow line region of a luminous QSO. The AGN scenario is capable of
producing sufficient ionizing photons to power the Lya, even in the presence of
dust. By performing a census of similar objects in the literature, we find that
virtually all luminous LABs harbor obscured quasars. Based on simple duty-cycle
arguments, we conclude that AGN are the main drivers of the Lya in LABs rather
than the gravitational heating and subsequent cooling suggested by cold stream
models. We also conclude that the empirical relation between LABs and overdense
environments at high redshift must be due to a more fundamental correlation
between AGN (or massive galaxies) and environment.
