Physical properties of the nuclear region in Seyfert galaxies derived from observations with the European VLBI Network

We report on sensitive dual-frequency (1.7 and 5 GHz) European VLBI Network observations of the central region of nine Seyfert galaxies. These sources are among the faintest and least luminous members of a complete sample of nearby (d<22 Mpc) low luminosity AGNs. We detect radio emission on milliarcsecond scale in the nuclei of 4 galaxies, while for the other five sources we set an upper limit of <~100 microJy. In three sources, namely NGC 3227, NGC 3982, and NGC 4138, radio emission is detected at both 1.7 and 5 GHz and it is resolved in two or more components. We describe the structural and spectral properties of these features; we find that in each of these three nuclei there is one component with high brightness temperature (typically T_B >10^7.5 K) and flat/intermediate spectral index (0.3\leq alpha \leq 0.6, S(nu) \sim nu^(-alpha), accompanied by secondary steep spectrum extended components. In these cases, non-thermal emission from jets or outflows is thus the most natural explanation. A faint feature is detected in NGC 4477 at 5 GHz; keeping in mind the modest significance of this detection (~5sigma), we propose the hot corona as the origin of non-thermal emission, on the basis of the unrealistic magnetic field values required by synchrotron self-absorption. Finally, the five non-detected nuclei remain elusive and further observations on intermediate scales will be necessary to investigate their nature.

💡 Research Summary

The authors present a dual‑frequency (1.7 GHz and 5 GHz) Very Long Baseline Interferometry (VLBI) study of the nuclear regions of nine nearby (distance < 22 Mpc) low‑luminosity Seyfert galaxies, selected from a complete sample of low‑luminosity active galactic nuclei (LLAGN). Using the European VLBI Network (EVN) they achieved sub‑milliarcsecond resolution and sensitivities of order 20–30 µJy beam⁻¹, allowing them to probe radio emission on scales of a few parsecs.

Four of the nine galaxies show compact radio cores at the EVN resolution. In three of these (NGC 3227, NGC 3982, and NGC 4138) emission is detected at both frequencies and resolves into at least two distinct components. One component in each source is compact, with a brightness temperature exceeding 10⁷·⁵ K and a relatively flat to mildly steep spectral index (α ≈ 0.3–0.6, where S ∝ ν⁻ᵅ). The other components are more extended, have steep spectra (α ≈ 1.2–1.8), and are aligned in a direction suggestive of a jet or outflow. The combination of high T_B and flat spectra for the core components strongly favors a non‑thermal synchrotron origin, most naturally interpreted as the base of a low‑power jet. The steep‑spectrum extensions represent downstream jet emission where radiative losses have steepened the electron energy distribution.

A faint (∼5σ) feature is detected only at 5 GHz in NGC 4477. The authors argue that if this emission were due to synchrotron self‑absorption, the required magnetic field would be of order 10⁴ G, which is physically implausible for a galactic nucleus. Instead they propose that the emission originates from a hot corona surrounding the accretion flow, where mildly relativistic electrons produce non‑thermal radiation without requiring extreme magnetic fields. This interpretation is consistent with the X‑ray properties of NGC 4477 and with theoretical models of coronal radio emission in LLAGN.

The remaining five galaxies (including NGC 1058, NGC 3079, etc.) are not detected at the EVN sensitivity limit of ≈100 µJy. The authors suggest that the radio emission in these objects may be resolved out on the longest baselines, implying that the bulk of the flux resides on intermediate scales (tens to hundreds of milliarcseconds). This “missing flux” problem points to the need for observations with arrays that bridge the resolution gap between the EVN and the VLA, such as e‑MERLIN, the ngVLA, or combined EVN‑VLA experiments.

In the discussion the paper places its findings in the broader context of LLAGN studies. The detection of high‑brightness‑temperature, flat‑spectrum cores demonstrates that even the faintest Seyfert nuclei can launch compact jets, supporting the idea that jet production is a ubiquitous feature of accretion onto supermassive black holes, regardless of the overall radiative output. The presence of steep‑spectrum extensions confirms that these jets can propagate to parsec scales, potentially influencing the host galaxy’s interstellar medium. The coronal interpretation for NGC 4477 adds an alternative pathway for radio emission in systems where jet activity is weak or absent, highlighting the role of hot, magnetized coronae in producing observable radio signatures.

Overall, the study provides a valuable high‑resolution view of the radio morphology and spectral properties of low‑luminosity Seyfert galaxies. It demonstrates that EVN observations can disentangle core and jet contributions, quantify brightness temperatures, and test physical models (synchrotron self‑absorption versus coronal emission). The authors conclude that further intermediate‑resolution observations are essential to capture the full radio spectrum of LLAGN, to resolve the “missing flux,” and to better understand the interplay between jets, coronae, and the surrounding interstellar medium in shaping the evolution of low‑luminosity active galaxies.