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

A primary question is how active galactic nuclei (AGN) feedback modifies the circumgalactic medium (CGM). We present a search for faint H α emission from the cool ionized CGM ( T ∼ 10 4 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 H α excess at >5 σ 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 (<2 σ ), indicating that this excess emission is very localized along the radio jet. We also find that the jet-aligned H α 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 H α 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 H α 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.