2026ApJS..282...20F

We introduce J-HERTZ (J-PLUS Heritage Exploration of Radio Targets at z < 5), a new multiwavelength catalog that combines optical narrowband photometry from J-PLUS, infrared observations from the Wide-field Infrared Survey Explorer, and deep low-frequency radio data from LoTSS for nearly 0.5 million sources across 2100 deg² of the northern sky. Key innovations of J-HERTZ include Bayesian neural network classifications for 390,000 galaxies, 31,000 quasars, and 20,000 stars, along with significantly improved photometric redshifts for 235,000 galaxies compared to previous J-PLUS DR3 and LoTSS DR2 estimates. We identify 831 candidate Galactic radio stars, which, if confirmed, would constitute a significant addition to the number of radio-emitting stars identified to date. Among radio-loud galaxies with spectroscopic observations, ≳20% lack Seyfert or LINER signatures, indicating a substantial population of optically quiescent radio galaxies, in agreement with previous works. Spectral energy distribution fitting of their host galaxies using J-PLUS photospectra reveals systematically low specific star formation rates, consistent with quenched stellar populations. J-HERTZ thus provides a powerful data set to exploit radio-optical synergies, enabling studies that span from the origin of stellar radio emission to the active galactic nucleus life cycle and the role of jet activity in shaping host galaxy evolution.