Detailed analysis of the substructure of Ly \alpha nebulae can put important constraints on the physical mechanisms at work and the properties of galaxies forming within them .
Using high resolution HST imaging of a \lya nebula at z \approx 2.656 , we have taken a census of the compact galaxies in the vicinity , used optical/near-infrared colors to select system members , and put constraints on the morphology of the spatially-extended emission .
The system is characterized by ( a ) a population of compact , low luminosity ( \sim 0.1 ~ { } L ^ { * } ) sources — 17 primarily young , small ( R _ { e } \approx 1 - 2 kpc ) , disky galaxies including an obscured AGN — that are all substantially offset ( \gtrsim 20 kpc ) from the line-emitting nebula ; ( b ) the lack of a central galaxy at or near the peak of the \lya emission ; and ( c ) several nearly coincident , spatially extended emission components — \lya , He ii , and UV continuum — that are extremely smooth .
These morphological findings are difficult to reconcile with theoretical models that invoke outflows , cold flows , or resonant scattering , suggesting that while all of these physical phenomena may be occurring , they are not sufficient to explain the powering and large extent of \lya nebulae .
In addition , although the compact galaxies within the system are irrelevant as power sources , the region is significantly overdense relative to the field galaxy population ( by at least a factor of 4 ) .
These observations provide the first estimate of the luminosity function of galaxies within an individual \lya nebula system , and suggest that large \lya nebulae may be the seeds of galaxy groups or low-mass clusters .