AGN-driven outflows and the formation of Lyα nebulae around high-z quasars

Abstract

ABSTRACT The detection of Lyα nebulae around z ≳ 6 quasars reveals extended gas reservoirs around the first rapidly growing supermassive black holes. Observations of z > 6 quasars can be explained by cosmological models provided that the black holes by which they are powered evolve in rare, massive dark matter haloes. Whether these models also explain the observed extended Lyα emission remains an open question. We post-process a suite of cosmological, radiation-hydrodynamic simulations, targeting a quasar host halo at z > 6 with the Lyα radiative transfer code Rascas. A combination of recombination radiation from photo-ionized hydrogen and emission from collisionally excited gas powers Lyα nebulae with a surface brightness profile in a close agreement with observations. We also find that, even on its own, resonant scattering of the Lyα line associated to the quasar’s broad-line region can also generate Lyα emission on $\sim 100 \, \rm kpc$ scales, resulting in comparable agreement with observed surface brightness profiles. Even if powered by a broad quasar Lyα line, Lyα nebulae have narrow line-widths $\lesssim 1000 \, \rm km \, s^{-1}$, consistent with observational constraints. Even if there is no quasar, we find that halo gas cooling produces a faint, extended Lyα glow. However, to explain the brightest Lyα nebulae, our simulations unambiguously require quasar-powered outflows to clear out the galactic nucleus and allow the Lyα flux to escape and still remain resonant with halo gas. The close match between observations and simulations with quasar outflows suggest that AGN feedback already operates before $z \, = \, 6$ and confirms that high-z quasars reside in massive haloes tracing overdensities.