CHANG-ES – VIII. Uncovering hidden AGN activity in radio polarization

Abstract

We report on C-band (5 -7 GHz) observations of the galaxy, NGC 2992, from the CHANG-ES sample. This galaxy displays an embedded nuclear double-lobed radio morphology within its spiral disk, as revealed in linearly polarized emission but not in total intensity emission. The radio lobes are kpc-sized, similar to what has been observed in the past for other Seyfert galaxies, and show ordered magnetic fields. NGC 2992 has shown previous evidence for AGN-related activity, but not the linearly polarized radio features that we present here. We draw attention to this galaxy as the first clear example (and prototype) of bipolar radio outflow that is revealed in linearly polarized emission only. Such polarization observations, which are unobscured by dust, provide a new tool for uncovering hidden weak AGN activity which may otherwise be masked by brighter unpolarized emission within which it is embedded. The radio lobes observed in NGC 2992 are interacting with the surrounding interstellar medium and offer new opportunities to investigate the interactions between nuclear outflows and the ISM in nearby galaxies. We also compare the radio emission with a new CHANDRA X-ray image of this galaxy. A new CHANG-ES image of NGC 3079 is also briefly shown as another example as to how much more obvious radio lobes appear in linear polarization as opposed to total intensity.

The discovery of secondary lobes in the Seyfert galaxy NGC 2639

“…However, the high degree of polarization along with the ideal resolution of our EVLA image has helped us in discovering the second pair of lobes. The case of NGC 2639 is similar to NGC 2992 where an extra pair of lobes which was previously not revealed in total intensity due to contamination from the galaxy disk emission, was discovered in polarized emission by Irwin et al (2017).…”

“…Hence, both the probability estimates and the undisturbed morphology of the host galaxy argues against the feasibility of the binary black hole model. Irwin et al (2017) consider the scenario where the lobe misalignment occurs as a result of interaction with ambient medium, which deflects the radio lobes along the direction of steepest pressure gradient. Given that in NGC 2639 the lobes are well confined within the galaxy itself, interaction with the ISM is inevitable.…”

CHANG-ES. XX. High-resolution Radio Continuum Images of Edge-on Galaxies and Their AGNs: Data Release 3

“…Here, almost 9000 M 82-mean-SNRs would be required to reproduce the observed luminosity in a region that is about the same as the M 82 nuclear starburst. From this result, NGC 2992 clearly has an AGN and, in fact, has been found to have polarized radio jets that have been explored in detail in Irwin et al (2017).…”

CHANG-ES: XVIII—The CHANG-ES Survey and Selected Results

“…The polarization capability of CHANG-ES has revealed previously hidden AGN activity which had either been masked or was not as clear in total intensity. A good example is NGC 2992, whose bipolar radio structure is only revealed in polarization (Irwin et al 2017). NGC 3079 is another galaxy whose radio lobes are more clearly seen in polarization.…”