High-frequency excess in the radio continuum spectrum of the type-1 Seyfert galaxy NGC 985

TL;DR

This study confirms a high-frequency excess in the radio spectrum of Seyfert galaxy NGC 985 up to 43 GHz, discusses its potential origins, and constrains the size and position of the emission region.

Contribution

First detailed multi-frequency radio observations of NGC 985's high-frequency excess, analyzing its spectral behavior and possible physical origins.

Findings

High-frequency excess confirmed up to 43 GHz.

Emission region constrained to <16 pc in size.

Spectral behavior above 43 GHz remains uncertain.

Abstract

The Seyfert galaxy NGC 985 is known to show a high-frequency excess in its radio continuum spectrum in a milli-Jansky level on the basis of previous observations at 1.4--15 GHz; a steep spectrum at low frequencies (a spectral index of $\alpha=-1.10 \pm 0.03$) changes at ~10 GHz into an inverted spectrum at higher frequencies ($\alpha=+0.86 \pm 0.09$). We conduct new observations at 15--43 GHz using the Very Large Array and at 100 GHz using the Nobeyama Millimeter Array. As a result, the high-frequency excess continuing at even higher radio frequencies up to 43 GHz has been confirmed. The non-detection at 100 GHz was not so strong constraint, and therefore the spectral behavior above 43 GHz remains unclear. The astrometric position of the high-frequency excess component coincides with the optical position of the Seyfert nucleus and the low-frequency radio position to an accuracy of 0.1 arcsec, corresponding to ~80 pc; the radio source size is constrained to be <0.02 arcsec, corresponding to <16 pc. We discuss the physical origin of the observed high-frequency excess component. Dust emission at the Rayleigh-Jeans regime, free--free emission from X-ray radiating high-temperature plasma, free--free emission from the ensemble of broad-line region clouds, or thermal synchrotron from hot accretion flow cannot be responsible for the observed radio flux. Compact jets under synchrotron self-absorption may be unlikely in terms of observed time scales. Alternatively, we cannot rule out the hypotheses of synchrotron jets free--free absorbed by a circumnuclear photo-ionized region, and self-absorbed nonthermal synchrotron from disk corona, as the origin of the high-frequency excess component.