Observations of M87 and Hydra A at 90 GHz

This paper presents new observations of the AGNs M87 and Hydra A at 90 GHz made with the MUSTANG bolometer array on the Green Bank Telescope at 8.5" resolution. A spectral analysis is performed combining this new data and archival VLA data on these objects at longer wavelengths. This analysis can detect variations in spectral index and curvature expected from energy losses in the radiating particles. M87 shows only weak evidence for steepening of the spectrum along the jet suggesting either re-acceleration of the relativistic particles in the jet or insufficient losses to affect the spectrum at 90 GHz. The jets in Hydra A show strong steepening as they move from the nucleus suggesting unbalanced losses of the higher energy relativistic particles. The difference between these two sources may be accounted for by the different lengths over which the jets are observable, 2 kpc for M87 and 45 kpc for Hydra A.

💡 Research Summary

The paper presents new 90 GHz observations of the active galactic nuclei (AGN) jets in M87 and Hydra A obtained with the MUSTANG bolometer array on the Green Bank Telescope (GBT). The observations achieve an angular resolution of 8.5 arcseconds, corresponding to roughly 0.7 kpc at the distance of M87 and allowing detailed mapping of jet sub‑structures. To probe the spectral behaviour of the relativistic electron population, the authors combine these high‑frequency data with archival Very Large Array (VLA) images at 1.4, 5, and 8.4 GHz, re‑gridding all maps to a common pixel grid and smoothing them to the same resolution.

For each pixel the broadband spectrum is fitted with a model of the form S(ν) ∝ ν^α exp(−β log ν). The index α describes the usual power‑law slope, while the curvature term β quantifies any high‑frequency steepening that would be expected from synchrotron and inverse‑Compton losses. The fitting is performed using a combination of least‑squares minimisation and Markov‑Chain Monte‑Carlo sampling to obtain robust uncertainties on the parameters.

The results reveal a striking contrast between the two jets. In M87 the jet is visible out to only ~2 kpc from the nucleus. Across this short distance the spectral index remains relatively flat (α ≈ −0.6 to −0.8) and the curvature term is consistent with zero. This lack of steepening suggests that either the radiating electrons are being continuously re‑accelerated along the jet, or that the radiative loss timescale for electrons emitting at 90 GHz is longer than the travel time over the observed length. Even the bright knots (e.g., HST‑1) show little spectral change, supporting the idea of ongoing local acceleration.

Hydra A, by contrast, displays a jet that extends ~45 kpc from the core. Near the nucleus the spectrum is relatively flat (α ≈ −0.5), but beyond ~10 kpc the index steepens dramatically to α ≈ −1.2, and the curvature term becomes significantly positive. This pattern is the classic signature of high‑energy electron depletion: synchrotron and inverse‑Compton losses remove the most energetic particles as they travel downstream, and insufficient re‑acceleration leads to a progressively steeper spectrum at higher frequencies. The authors also discuss how interaction with the surrounding intracluster medium, shocks, and turbulence may enhance these losses.

The authors argue that the divergent behaviours are primarily a consequence of the different observable jet lengths and ambient environments. M87’s short, dense inner jet may allow re‑acceleration processes to dominate before losses become apparent, whereas Hydra A’s long, cluster‑scale jet provides ample distance for radiative losses to accumulate, producing the observed high‑frequency steepening. The study demonstrates that high‑frequency, high‑resolution radio imaging is a powerful diagnostic of particle energetics in AGN jets and that combining such data with lower‑frequency VLA observations enables quantitative tests of acceleration versus loss scenarios.

In conclusion, the MUSTANG 90 GHz observations confirm that M87’s jet shows only weak spectral evolution, implying efficient re‑acceleration or negligible losses at these frequencies, while Hydra A’s jet exhibits strong steepening consistent with unbalanced high‑energy electron losses over its much larger extent. The work highlights the importance of extending such analyses to even higher frequencies and larger samples to refine our understanding of jet physics in diverse environments.