Parsec-Scale Radio Structure of the Double Active Nucleus of NGC 6240

Abstract

NGC 6240 is a luminous infrared galaxy that, based on recent MERLIN and Chandra observations, appears to host a pair of active galactic nuclei. We present new multiepoch and multifrequency radio observations of NGC 6240 obtained using the Very Long Baseline Array (VLBA). These observations resolve out all of the diffuse radio continuum emission related to starburst activity to reveal three compact sources, two associated with the infrared and X-ray nuclei: components N1, the northern nucleus, and S, the southern nucleus; and component S1, a weak point source lying northeast of component S. The radio continuum properties of N1 and S both resemble those of compact radio sources in Seyfert nuclei, including an inverted spectrum at low frequencies and high brightness temperatures. Component N1 further resolves into a 9 pc linear source, again resembling compact jets in some well-studied Seyfert nuclei. The inverted radio spectrum is most likely caused by free-free absorption through foreground plasma with emission measures on the order of 107cm-6pc, consistent with the X-ray spectroscopy, which implies the presence of a foreground, Compton-thick absorber. Synchrotron self-absorption may also contribute to the flat spectrum of component S. Component S1 better resembles a luminous radio supernova such as those detected in Arp 220, although we cannot specifically rule out the possibility that S1 may be ejected material from the southern active nucleus. We demonstrate that the light curve of S1 is at least consistent with current radio supernova (RSN) models, and given the high star formation rate in the nuclear region and the (possible) proclivity of infrared merger galaxies to produce luminous RSNe, it is not surprising that one such supernova is detected serendipitously in the VLBA observations.