Constraining the nature of active galactic nuclei through circumgalactic Lya emission at z=2-3

TL;DR

This study analyzes circumgalactic Lya nebulae around quasars at z=2-3 to understand AGN nature, revealing differences in nebula symmetry and extent linked to obscuration, supporting an evolutionary AGN model.

Contribution

It provides the first comprehensive comparison of nebulae around obscured and unobscured quasars, challenging the standard orientation-based AGN unified model.

Findings

Unobscured quasars have more extended and less symmetric nebulae.

Velocity dispersion profiles suggest large-scale outflows in unobscured quasars.

Results support an evolutionary scenario where AGN feedback redistributes gas over time.

Abstract

We present a comprehensive analysis of circumgalactic Lya nebulae around 59 unobscured and 26 obscured quasars at z=2-3, observed with the Keck Cosmic Web Imager (KCWI), to constrain the nature of active galactic nuclei (AGN) at cosmic noon. We find that Lya nebulae around unobscured quasars are significantly less symmetric having a symmetry parameter of a_w=0.2-0.6 and more spatially extended having a scale length of r_h=10.7+/-0.5 kpc than those around obscured quasars (a_w=0.6-0.8; r_h=6.6-7.7 kpc).Unobscured quasars also exhibit steeply declining velocity dispersion profiles with the slope of -4.3+/-0.4 km s^-1 kpc^-1, indicative of large-scale outflows, whereas obscured quasars display flat profiles (-0.2+/-0.7 and -0.6+/-0.4 km s^-1 kpc^-1). The degree of quasar obscuration appears to be intrinsically linked to nebular asymmetry and extent, a relationship that could be in tension with the standard orientation-based AGN unified model, as it expects unobscured-quasar nebulae to be more symmetric and compact. These results naturally fit the evolutionary scenario, where AGN feedback drives a transition from an obscured to an unobscured phase-progressively redistributing gas to larger radii, introducing anisotropy, and driving turbulence. Taken together, our findings favor the evolutionary scenario over the purely orientation-based unified model for quasars at cosmic noon.