HeII emission in Lyman-alpha nebulae: AGN or cooling radiation?

TL;DR

This study investigates the origin of HeII emission in a high-redshift Lyman-alpha nebula, exploring whether it results from active galactic nucleus activity or cooling radiation from accreting gas.

Contribution

It provides multiwavelength observations and proposes that cooling gas accretion, rather than AGN activity, explains the observed emission features in the nebula.

Findings

HeII emission detected in the obscured AGN spectrum.

Lack of CIV and NV emission lines challenges typical AGN signatures.

Cooling gas accretion is a plausible explanation for the emission characteristics.

Abstract

We present a study of an extended Lyman-alpha (Lya) nebula located in a known overdensity at z~2.38. The data include multiwavelength photometry covering the rest-frame spectral range from 0.1 to 250um, and deep optical spectra of the sources associated with the extended emission. Two galaxies are associated with the Lya nebula. One of them is a dust enshrouded AGN, while the other is a powerful starburst, forming stars at >~600 Msol/yr. We detect the HeII emission line at 1640A in the spectrum of the obscured AGN, but detect no emission from other highly ionized metals (CIV or NV) as is expected from an AGN. One scenario that simultaneously reproduces the width of the detected emission lines, the lack of CIV emission, and the geometry of the emitting gas, is that the HeII and the Lya emission are the result of cooling gas that is being accreted on the dark matter halo of the two galaxies, Ly1 and Ly2. Given the complexity of the environment associated with our Lya nebula it is possible that various mechanisms of excitation are at work simultaneously.