A massive bubble of extremely metal poor gas around a collapsing
  Ly-alpha blob at z=2.54
  Using long-slit optical spectroscopy obtained at the 10.4 m Gran Telescopio
Canarias, we have examined the gaseous environment of the radio-loud quasar TXS
1436+157 (z=2.54), previously known to be associated with a large Ly-alpha
nebula and a spatially extended Ly-alpha-absorbing structure. From the Ly-alpha
nebula we measure kinematic properties consistent with infall at a rate of
about 10-100 M./yr - more than sufficient to power a quasar at the top of the
luminosity function. The absorbing structure lies outside of the Ly-alpha
nebula, at a radius of >40 kpc from the quasar. Against the bright unresolved
continuum and line emission from the quasar, we detect in absorption the NV
1239,1241, CIV 1548,1551 and SiIV 1394,1403 doublets, with no unambiguous
detection of absorption lines from any low-ionization species of metal. The
metal column densities, taken together with the HI column density measurement
from the literature, indicate that the absorbing gas is predominantly ionized
by the quasar, has a mass of hydrogen of >1.6 x 10E11 M., a gas density of <18
per cubic cm, a line of sight thickness of >18 pc, and a covering factor
approaching unity. While this absorbing structure is clearly not composed of
pristine gas, it has an extremely low metallicity, with ionization models
providing a 3-sigma limit of 12+log(O/H)<7.3. To explain these results, we
discuss a scenario involving starburst-driven super-bubbles and the creation of
infalling filaments of cold gas which fuel/trigger the quasar. We also discuss
the possibility of detecting large-scale absorbers such as this in emission
when illuminated by a powerful quasar.
