NOCTURNE. I. The radio spectrum of narrow-line Seyfert 1 galaxies

TL;DR

NOCTURNE's high-frequency radio survey of 50 southern NLS1s reveals that at 15–33 GHz the majority are compact and exhibit steep spectra, consistent with optically thin synchrotron emission from non-relativistic outflows or circumnuclear star formation, rather than dominant jet activity. Spectral fits show occasional curvature due to synchrotron self-absorption, with a notable inverted-spectrum/high-frequency-peaker candidate (J0239-1118) and a jetted-NLS1 candidate (J0452-2953), suggesting a range of jet properties within the NLS1 class. Radio luminosities reach up to $\log L_\nu \sim 40$ erg s$^{-1}$ in some bands, though star formation can mimic jet-like powers, and no significant inter-epoch variability is detected. The results imply that relativistic jets contribute only modestly to the radio output for most NLS1s, highlighting the need for multiwavelength and multiscale studies to fully constrain emission mechanisms and AGN evolution. Overall, the study reinforces the heterogeneous nature of NLS1s and the importance of high-frequency, high-resolution follow-ups to map jet development and its role in AGN feedback.

Abstract

The origin of the radio emission in active galactic nuclei (AGN) is still debated. Multiple physical mechanisms can contribute to the spectrum at these frequencies, including relativistic jets, the jet base, outflows, star formation, and synchrotron emission from the hot corona. Recently, new extreme radio variability has been observed in the class of low-mass/high-Eddington AGN known as narrow-line Seyfert 1 (NLS1) galaxies, suggesting that another, more exotic mechanism may also play a role, especially at frequencies above 10 GHz. To investigate this relatively unexplored area of the radio spectrum, we observed a sample of 50 NLS1s with the Karl G. Jansky Very Large Array (JVLA), and 20 of them were observed twice. In this sample, 24 sources were not detected, while the others are typically characterized by a steep spectrum that can be modeled with a power law. We also identified two new candidate jetted NLS1s, including a high-frequency peaker, which is an extremely young relativistic jet. We found no significant variability in the sources observed twice. We conclude that the radio spectrum of NLS1s is typically dominated by optically thin emission, likely from low-power outflows, or by circumnuclear star formation, with a limited contribution from relativistic jets. Further studies at different spatial scales and at other wavelengths are necessary to fully constrain the origin of the radio emission in this class of active galaxies.

Figures (41)

• Figure 1: Radio map of J0452-2953 at 15 GHz. The map rms is $\sigma = 17 \mu$Jy, the contours are at [-3, 3, 6, 12, 24]$\times\sigma$.
• Figure 2: Radio map of J0622-2317 at 15 GHz. The map rms is $\sigma = 12 \mu$Jy, the contours are at [-3, 3, 6]$\times\sigma$.
• Figure 3: Radio map of J1032-2707 at 15 GHz. The map rms is $\sigma = 22 \mu$Jy, the contours are at [-3, 3, 6, 12, 18]$\times\sigma$.
• Figure 4: In red the contours of J0452-2953 as in Fig. \ref{['fig:J0452_map']}, overlapped with the i-band image of its host galaxy extracted from Pan-STARRS.
• Figure 5: In red the contours of J0622-2317 as in Fig. \ref{['fig:J0622_map']}, overlapped with the i-band image of its host galaxy extracted from Pan-STARRS.