The mysterious absence of neutral hydrogen within 1 Mpc of a luminous quasar at redshift 2.168

The intense ultraviolet (UV) radiation from a highly luminous quasi-stellar object (QSO) should excite fluorescent Lyman α (Lyα) emission from any nearby neutral hydrogen clouds. We present a very deep narrow-band search for such emission near the z = 2.168 quasar PKS 0424-131, obtained with the Taurus Tunable Filter on the Anglo-Australian Telescope. By working in the UV, at high spectral resolution and by using charge shuffling, we have been able to reach surface brightness limits as faint as 4.7 × 10^-19 erg cm^-2 s^-1 arcsec^-2. No fluorescent Lyα emission is seen, whereas QSO absorption-line statistics suggest that we should have seen ≳ 6 clouds, unless the clouds are larger than ∼100 kpc in size. Furthermore, we do not even see the normal population of Lyα-emitting galaxies found by other surveys at this redshift. This is very different from observations of high-redshift radio galaxies, which seem to be surrounded by clusters of Lyα emitters. We tentatively conclude that there is a deficit of neutral hydrogen close to this quasar, perhaps owing to the photoevaporation of nearby dwarf galaxies.