Letter to the Editor

The infrared jet of M87 observed with JWST

¹ Instituto de Astrofísica de Andalucía, Gta. de la Astronomía, 18008 Granada, Spain
² Department of Physics, Utah Valley University, 800 West University Parkway, Orem, UT 84058, USA
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

^⋆ Corresponding author: contact@janroeder.science

Received: 24 July 2025
Accepted: 6 September 2025

Abstract

Aims. 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 μm. We analysed the large-scale jet structure, identifying prominent components, and we determined the near-infrared spectral index.

Methods. 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.

Results. The total image fluxes measured in the NIRCam filters follow the infrared bump pattern seen near 1.6 μm in the spectrum 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 of S_λ ∝ λ^α with α = 0.7 − 1.0. We identified the most upstream knot, L, at a distance of (320 ± 50) mas from the core. The component HST-1, located at (950 ± 50) mas from the core, is transversely resolved. Both the individual images and the spectral index map clearly indicate its double-component substructure with two elements of a similar size and flux density, with centroids separated by (150 ± 20) mas. In addition, it displays a significantly larger spectral index, α, observed in the downstream component (α_do = 0.30) compared to the upstream one (α_up = −0.15). We also observed the counter-jet component located about 24 arcsec away from the nucleus.