Radio spectra of narrow-line Seyfert 1 galaxies observed with Australia Telescope Compact Array and Very Large Array Sky Survey

We present radio spectral analyses for a sample of 29 radio-quiet (RQ) and three radio-loud (RL) narrow-line Seyfert 1 galaxies (NLS1s) detected with the Australia Telescope Compact Array at both 5.5 and 9.0 GHz. The sample is characterized by L-bol/L-Edd > 0.15. The radio slopes in 25 of the 29 RQ NLS1s are steep (alpha(5.5-9.0) < -0.5), as found in earlier studies of RQ high L-bol/L-Edd active galactic nuclei (AGN). This steep radio emission may be related to AGN-driven outflows, which are likely more prevalent in high L-bol/L-Edd AGN. In two of the three RL NLS1s, the radio slopes are flat or inverted (alpha(5.5-9.0) > -0.5), indicating a compact optically thick source, likely a relativistic jet. Archival data at 3.0, 1.4, and 0.843 GHz are also compiled, yielding a sample of 17 NLS1s detected in three bands or more. In nine objects, the radio spectra flatten at lower frequencies, with median slopes of alpha(5.5-9.0) = -1.21 +/- 0.17, flattening to alpha(3.0-5.5) = -0.97 +/- 0.27, and to alpha(1.4-3.0) = -0.63 +/- 0.16. A parabolic fit suggests a median spectral turnover of similar to 1 GHz, which implies synchrotron self-absorption in a source with a size of only a fraction of 1 pc, possibly a compact wind or a weak jet. Two objects show significant spectral steepening to alpha < -2 above 3 or 5 GHz, which may suggest relic emission from past ejection of radio emitting plasma, of the order of a few years to a few decades ago. Finally, two objects present a single spectral slope consistent with star-forming activity.