Continuum emission in NGC 1068 and NGC 3147: indications for a turnover in the core spectra

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany e-mail: [krips;eckart;rainer]@ph1.uni-koeln.de
² Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France e-mail: [neri;downes]@iram.fr
³ Instituto de Astrofísica de Andalucía (CSIC), C/ Camino Bajo de Huétor,24, Apartado 3004, 18080 Granada, Spain e-mail: stephane@iaa.es
⁴ Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain e-mail: s.gburillo@oan.es
⁵ Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: Francoise.Combes@obspm.fr

Received: 30 June 2005
Accepted: 20 September 2005

Abstract

We present new interferometric observations of the continuum emission at mm wavelengths in the Seyfert galaxies NGC 1068 and NGC 3147. Three mm continuum peaks are detected in NGC 1068, one centered on the core, one associated with the jet and the third one with the counter-jet. This is the first significant detection of the radio jet and counter-jet at mm wavelengths in NGC 1068. While the fluxes of the jet components agree with a steep spectral index extrapolated from cm-wavelengths, the core fluxes indicate a turnover of the inverted cm- into a steep mm-spectrum at roughly ~50 GHz which is most likely caused by electron-scattered synchrotron emission. As in NGC 1068, the spectrum of the pointlike continuum source in NGC 3147 also shows a turnover between cm and mm-wavelengths at ~25 GHz resulting from synchrotron self-absorption different to NGC 1068. This strongly resembles the spectrum of Sgr A, the weakly active nucleus of our own galaxy, and M 81, a link between Sgr A and Seyfert galaxies in terms of activity sequence, which may display a similar turnover.