When Jets Don't Quench: Near-Infrared H${}_2$ in Star Forming Low-Excitation Radio Galaxies

Abstract

We present new Gemini/GNIRS near-infrared spectroscopic observations of eight low-redshift ($z<0.1$) blue low-excitation radio galaxies (BLERGs), a rare subset ($\sim 2.5\%$ of low-excitation radio galaxies; LERGs) that complicate the classical jet-mode AGN picture by combining radio activity with star-forming, gas-rich hosts. These star-forming BLERGs exhibit significant warm H${}_2$ emission traced via ro-vibrational transitions at $T\sim 2000$--$4000$ K. We find that BLERGs span a broad range of mass-normalized warm H${}_2$ luminosities ($L_{{\mathrm{H}}_2}/M_{\star }$), comparable to radio-emitting early-type galaxies, yet without a clear positive dependence on radio power. Instead, the strongest H${}_2$ emission preferentially occurs in morphologically disturbed and advanced-merger systems, while compact radio sources ($\lesssim 20$ kpc) remain plausible sites of localized jet-ISM interaction. Together, these results suggest that merger-driven processes, including tidal shocks, gas inflows, and disturbed interstellar medium conditions, are the dominant drivers of warm molecular gas excitation in BLERGs, although localized jet-driven heating may contribute in individual systems. The compact radio morphologies, gas-rich hosts, and rarity of BLERGs are consistent with a short-lived evolutionary phase in which radio AGN activity coexists with an interaction-driven, molecular-rich interstellar medium prior to the onset of large-scale maintenance-mode feedback. Spatially resolved spectroscopy and higher-resolution radio imaging will be essential to disentangle the relative roles of mergers and jets in regulating the molecular gas of jet-mode AGN.