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Abstract

We present the first JWST+NIRCam images of the giant elliptical active galaxy M87 and its jet at 0.90, 1.50, 2.77 and 3.56 $\mu$m. We analysed the large-scale jet structure, identifying prominent components, and determined the near-infrared spectral index. The data were calibrated using the standard JWST pipeline. We subtracted a constant background level and a smooth model of the galaxy surface brightness to isolate the jet. The total image fluxes measured in the NIRCam filters follow the infrared bump pattern seen near 1.6 $\mu$m in the spectral energy distribution of M87, caused by the surrounding stellar population in the galaxy. The residual jet images broadly agree with the radio to optical synchrotron power law $S_\lambda\propto\lambda^\alpha$ with $\alpha=$0.7-1.0. We identified the most upstream knot L at a distance of (320$\pm$50) mas from the core. The component HST-1, at (950$\pm$50) mas from the core, is transversely resolved, and both the individual images and the spectral index map clearly indicate its double-component substructure with two elements of similar size and flux density, with centroids separated by (150$\pm$20) mas and with a significantly larger spectral index $\alpha$ observed in the downstream component ($\alpha_{\rm do}=0.3$) than in the upstream one ($\alpha_{\rm up}= -0.15$). We also observe the counter-jet component located about 24 arcsec away from the nucleus.