H I 21-cm absorption in low- and high-excitation radio-loud active galactic nuclei at z < 0.5 from the MeerKAT Absorption Line Survey

We present results from a search of cold neutral gas associated with radio-loud active galactic nuclei (AGNs) at z < 0.5 – corresponding to a lookback time of ∼36% of the age of the Universe – using H I 21-cm absorption line measurements from the MeerKAT Absorption Line Survey (MALS). Crossmatching the MALS 1006 MHz catalog with the SDSS DR18 spectroscopic catalog yields 1908 radio sources at z < 0.5. Of these, 613 are classified as AGNs using Baldwin-Phillips-Terlevich (BPT) diagnostics and radio luminosity criteria. We further classify 426 AGNs into 327 low-excitation radio galaxies (LERGs) and 99 high-excitation radio galaxies (HERGs) using a framework combining multiple optical emission-line diagnostics. We find a significant ( > 3 σ ) difference in k -corrected g − r color, consistent with LERGs residing in older galaxies with quenched star formation. We search a radio-bright subsample of 79 LERGs and 20 HERGs ( S 1.4 GHz > 4 mJy) for associated H I 21-cm absorption using MALS L-band spectra. This subsample spans nearly six decades in radio luminosity (log L 1.4 GHz (W Hz −1 ) ∼21.1–27.0), reaching luminosities an order of magnitude fainter than previous targeted H I surveys. We report five new detections (four LERGs, one HERG) at 0.29 < z < 0.47, yielding an overall detection rate of 3 −2 +3 % at a 3 σ integrated optical depth sensitivity threshold of 10.0 km s −1 . This rate is consistent with sensitivity-matched low- z ( < 0.2) samples, suggesting no significant redshift evolution out to z ∼ 0.5 or dependence on radio luminosity. We quantify absorption-line properties using velocity offset, asymmetry, and width. In three systems, absorption is entirely redshifted relative to the systemic velocity; in the remaining two, it is predominantly blueshifted, indicating complex gas kinematics. H I profiles associated with LERGs show a wide range of asymmetries and velocity offsets exceeding 350 km s −1 , indicative of disturbed cold-gas kinematics likely driven by lobe expansion or jet activity.