Extended Lyα emission around young quasars: A constraint on galaxy formation

The early stage in the formation of a galaxy inevitably involves a spatially extended distribution of infalling, cold gas. If a central luminous quasar turned on during this phase, it would result in significant extended Lyα emission, possibly accompanied by other lines. For halos condensing at redshifts 3 ≲ z ≲ 8 and having virial temperatures 2 × 105 K ≲ Tvir ≲ 2 × 106 K, this emission results in a "fuzz" of characteristic angular diameter of a few arcseconds and surface brightness ~10-18 to 10-16 ergs s-1 cm-2 arcsec-2. The fuzz around bright, high-redshift quasars could be detected in deep narrowband imaging with current telescopes, providing a direct constraint on galaxy formation models. The absence of detectable fuzz might suggest that most of the protogalaxy's gas settles to a self-gravitating disk before a quasar turns on. However, continued gas infall from large radii, or an on-going merger spreading cold gas over a large solid angle, during the luminous quasar phase could also result in extended Lyα emission, and can be constrained by deep narrowband imaging.