H i Gas Playing Hide-and-seek around a Powerful FRI-type Quasar at z ∼ 2.1

We present optical spectroscopic and milliarcsecond-scale radio continuum observations of the quasar M1540–1453 (zem = 2.104 ± 0.002) that show associated H i 21 cm absorption at zabs = 2.1139. At subkiloparsec scales, the powerful radio source with 1.4 GHz luminosity of 5.9 × 1027 W Hz−1 shows Fanaroff–Riley class I morphology caused by the interaction with dense gas within 70 pc of the active galactic nucleus (AGN). Interestingly, while there are indications for the presence of absorption from low-ionization species like Fe ii, Si ii, and Si iii in the optical spectrum, the expected strong damped Lyα absorption is not detected at the redshift of the H i 21 cm absorber. In comparison to typical high-z quasars, the Lyα emission line is much narrower. The "ghostly" nature of the H i Lyα absorber partially covering the broad-line region of extent 0.05 pc and the detection of widespread H i 21 cm absorption covering the diffuse radio source (extent >425 pc) imply the presence of a large clumpy H i halo, which may have been blown by the jet–interstellar medium (ISM) interaction. Further observations are needed to confirm the ghostly nature of the Lyα absorber and obtain a better understanding of the role played by the jet–ISM interaction in shaping the radio morphology of this powerful AGN. The study showcases how joint radio and optical analysis can shed light on the gaseous environment and origin of radio morphology in AGNs at high redshifts, when these are still the assembly sites of giant galaxies.