Illuminating protogalaxies? The discovery of extended Lyman-alpha emission around a QSO at z=4.5

We have discovered extended Lyman-alpha emission around a z = 4.5 QSO in a deep longslit spectrum with Keck/LRIS at moderate spectral resolution (R approximate to 1000). The line emission extends 5 arcsec beyond the continuum of the QSO and is spatially asymmetric. This extended line emission has a spectral extent of 1000km/s, much narrower in velocity spread than the broad Lyman-alpha from the QSO itself and slightly offset in redshift. No evidence of continuum is seen for the extended emission line region, suggesting that this recombination line is powered by reprocessed QSO Lyman continuum flux rather than by local star formation. This phenomenon is rare in QSOs which are not radio loud, and this is the first time it has been observed at z > 4. It seems likely that the QSO is illuminating the surrounding cold gas of the host galaxy, with the ionizing photons producing Lyman-alpha fluorescence. As suggested by Haiman and Rees (2001), this 'fuzz' around a distant quasar may place strong constraints on galaxy formation and the extended distribution of cold, neutral gas.