The Mysterious Absence of Neutral Hydrogen within One Mpc of a Luminous Quasar at Redshift 2.168

Abstract

The intense UV radiation from a highly luminous QSO should excite fluorescent Ly-alpha 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 4.7E-19 erg/cm^2/s/arcsec^2. No fluorescent Ly-alpha 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 Ly-alpha emitting galaxies found by other surveys at this redshift. This is very different fromobservations of high redshift radio galaxies, which seem to be surrounded by clusters of Ly-alpha emitters. We tentatively conclude that there is a deficit of neutral hydrogen close to this quasar, perhaps due to the photo-evaporation of nearby dwarf galaxies.