A massive bubble of extremely metal-poor gas around a collapsing Lyα 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 Lyα nebula and a spatially extended Lyα-absorbing structure. From the Lyα nebula, we measure kinematic properties consistent with infall at a rate of ∼10-100 M_⊙ yr^-1 - more than sufficient to power a quasar at the top of the luminosity function.

The absorbing structure lies outside of the Lyα 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 N v λλ1239, 1241, C iv λλ1548, 1551 and Si iv λλ1394, 1403 doublets, with no unambiguous detection of absorption lines from any low-ionization species of metal. The metal column densities, taken together with the H i 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 × 10¹¹ M_⊙, a gas density of ≤18 cm^-3, 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 3σ limit of 12+log(O/H) ≤ 7.3.

To explain these results, we discuss a scenario involving starburst-driven superbubbles 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.