RESOLVING THE GALAXIES WITHIN A GIANT Lyα NEBULA: WITNESSING THE FORMATION OF A GALAXY GROUP?

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 Hubble Space Telescope (HST) imaging of a Ly alpha nebula at z approximate to 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 (1) a population of compact, low-luminosity (similar to 0.1 L*) sources-17 primarily young, small (R-e approximate to 1-2 kpc), disky galaxies including an obscured active galactic nucleus-that are all substantially offset (greater than or similar to 20 kpc) from the line-emitting nebula; (2) the lack of a central galaxy at or near the peak of the Ly alpha emission; and (3) several nearly coincident, spatially extended emission components-Ly alpha, 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 Ly alpha 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 four). These observations provide the first estimate of the luminosity function of galaxies within an individual Ly alpha nebula system and suggest that large Ly alpha nebulae may be the seeds of galaxy groups or low-mass clusters.