Overturning the Case for Gravitational Powering in the Prototypical Cooling Lyman-alpha Nebula

TL;DR

This study challenges the gravitational cooling model for the Nilsson et al. (2006) Lyman-alpha nebula, providing new observational evidence that suggests the nebula is primarily illuminated by a nearby obscured AGN rather than gravitational energy.

Contribution

The paper combines new Hubble and Herschel data with recent simulations to reassess the power source of a prototypical Lyman-alpha nebula, arguing against gravitational powering.

Findings

The nebula is associated with 6 nearby galaxies and an offset obscured AGN.

The environment is overdense by a factor of ~10 compared to the field.

Lyman-alpha emission encircles the obscured AGN, indicating illumination rather than gravitational energy.

Abstract

The Nilsson et al. (2006) Lyman-alpha nebula has often been cited as the most plausible example of a Lyman-alpha nebula powered by gravitational cooling. In this paper, we bring together new data from the Hubble Space Telescope and the Herschel Space Observatory as well as comparisons to recent theoretical simulations in order to revisit the questions of the local environment and most likely power source for the Lyman-alpha nebula. In contrast to previous results, we find that this Lyman-alpha nebula is associated with 6 nearby galaxies and an obscured AGN that is offset by $\sim$4"$\approx$30 kpc from the Lyman-alpha peak. The local region is overdense relative to the field, by a factor of $\sim$10, and at low surface brightness levels the Lyman-alpha emission appears to encircle the position of the obscured AGN, highly suggestive of a physical association. At the same time, we confirm that there is no compact continuum source located within $\sim$2-3"$\approx$15-23 kpc of the Lyman-alpha peak. Since the latest cold accretion simulations predict that the brightest Lyman-alpha emission will be coincident with a central growing galaxy, we conclude that this is actually a strong argument against, rather than for, the idea that the nebula is gravitationally-powered. While we may be seeing gas within cosmic filaments, this gas is primarily being lit up, not by gravitational energy, but due to illumination from a nearby buried AGN.