Giant Lyman-Alpha Nebulae in the Illustris Simulation

TL;DR

This study uses the Illustris simulation to model giant Lyman-alpha nebulae at redshift 2, demonstrating that existing simulation resolution suffices to explain observed nebulae without additional clumping, and predicting more such objects will be discovered.

Contribution

The paper shows that current high-resolution simulations can reproduce giant Lyman-alpha nebulae without extra sub-resolution clumping, challenging previous assumptions.

Findings

Simulated nebulae match observed extents and luminosities without extra clumping.

Nebula sizes show wide variation at fixed luminosity, with no strong correlation to halo properties.

Detection size depends heavily on surface brightness thresholds.

Abstract

Several `giant' Lyman-$\alpha$ (Ly$\alpha$) nebulae with extent $\gtrsim 300\,$kpc and observed Ly$\alpha$ luminosity of $\gtrsim 10^{44}\,{\rm erg}\,{\rm s}^{-1}\,{\rm cm}^{-2}\,{\rm arcsec}^{-2}$ have recently been detected, and it has been speculated that their presence hints at a substantial cold gas reservoir in small cool clumps not resolved in modern hydro-dynamical simulations. We use the Illustris simulation to predict the Ly$\alpha$ emission emerging from large halos ($M > 10^{11.5}M_{\odot}$) at $z\sim 2$ and thus test this model. We consider both AGN and star driven ionization, and compare the simulated surface brightness maps, profiles and Ly$\alpha$ spectra to a model where most gas is clumped below the simulation resolution scale. We find that with Illustris no additional clumping is necessary to explain the extents, luminosities and surface brightness profiles of the `giant Ly$\alpha$ nebulae' observed. Furthermore, the maximal extents of the objects show a wide spread for a given luminosity and do not correlate significantly with any halo properties. We also show how the detected size depends strongly on the employed surface brightness cutoff, and predict that further such objects will be found in the near future.