Lighting Up the Circumgalactic Medium: Strong, Jet-aligned Hα Emission around Radio Galaxies

Abstract

A primary question is how active galactic nuclei (AGN) feedback modifies the circumgalactic medium (CGM). We present a search for faint Hα emission from the cool ionized CGM (T ∼ 104 K) around radio galaxies by stacking background-quasar spectra from Dark Energy Spectroscopic Instrument (DESI) sight lines. We take into account the projected distance and position angle of each quasar sight line relative to the radio-jet axis, and test whether jet–CGM coupling is anisotropic. We detect a strong Hα excess at >5σ along the collimated radio-jet axis (θ < 20∘) with a mean integrated flux of 1.19 × 10−17 erg cm−2 s−1. In contrast, the azimuthally averaged stack over all 324 sight-line angles yields no detection (<2σ), indicating that this excess emission is very localized along the radio jet. We also find that the jet-aligned Hα signal is radially structured, where the strongest emission occurs near the host galaxy just outside the optical half-light radius, and rising again near the projected radio-lobe region. The jet-aligned stacks reveal Hα signal that is roughly 100 times brighter than normal halos. In the same sight lines however, Mg ii absorption shows no difference in incidence between jet-aligned and off-axis directions, with broadly similar equivalent widths, column densities, and line widths. This striking contrast shows that while Mg ii traces the ambient, clumpy cool CGM reservoir, the Hα emission directly captures localized, low-covering-fraction clouds whose density, pressure, or ionization level has been dramatically boosted by the propagating jet. These results deliver clear evidence of localized jet–CGM interaction in radio-jetted AGNs.