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 × 10 5 K ≲ T vir ≲ 2 × 10 6 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.
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A 2001 study studied this question.