http://arxiv.org/abs/1605.07518
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.
A. Doi and Y. Inoue
Wed, 25 May 16
58/62
Comments: 9 pages, 1 figure, 1 table, accepted for publication in PASJ
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