Resolving the Galaxies within a Giant Lya Nebula: Witnessing the Formation of a Galaxy Group?

TL;DR

This study uses high-resolution imaging to analyze the substructure of a giant Lya nebula at z~2.656, revealing a population of offset, low-luminosity galaxies and extended emission components, challenging existing models of nebula powering.

Contribution

It provides the first detailed census of galaxies within a Lya nebula and offers new insights into the nebula's morphology and its potential role in galaxy group formation.

Findings

Presence of offset, low-luminosity galaxies including an obscured AGN.

Lack of a central galaxy at the nebula's peak.

Extended, smooth emission in Lya, HeII, and UV continuum.

Abstract

Detailed analysis of the substructure of Lya 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~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 (~0.1 L*) sources --- 17 primarily young, small (Re~1-2 kpc), disky galaxies including an obscured AGN --- that are all substantially offset (>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, HeII, 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.