The innermost dusty structure in active galactic nuclei as probed by the Keck interferometer

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: mk@mpifr-bonn.mpg.de
² Physics Department, University of California, Santa Barbara, CA 93106, USA
³ National Science Foundation, 4301 Wilson Boulevard, Arlington, VA 22230, USA
⁴ ISAS, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510 Japan
⁵ Silver Spring Observatory, 10712 Meadowhill Rd, Silver Spring, MD 20901, USA

Received: 3 November 2010
Accepted: 23 December 2010

Abstract

We are now exploring the inner region of type 1 active galactic nuclei (AGNs) with the Keck interferometer in the near-infrared. Adding to the four targets previously studied, we report measurements of the K-band (2.2 μm) visibilities for four more targets, namely AKN120, IC 4329A, Mrk6, and the radio-loud QSO 3C 273 at z = 0.158. The observed visibilities are quite high for all the targets, which we interpret as an indication of the partial resolution of the dust sublimation region. The effective ring radii derived from the observed visibilities scale approximately with L^1/2, where L is the AGN luminosity. Comparing the radii with those from independent optical-infrared reverberation measurements, these data support our previous claim that the interferometric ring radius is either roughly equal to or slightly larger than the reverberation radius. We interpret the ratio of these two radii for a given L as an approximate probe of the radial distribution of the inner accreting material. We show tentative evidence that this inner radial structure might be closely related to the radio-loudness of the central engine. Finally, we re-observed the brightest Seyfert 1 galaxy NGC 4151. Its marginally higher visibility at a shorter projected baseline, compared to our previous measurements obtained one year before, further supports the partial resolution of the inner structure. We did not detect any significant change in the implied emission size when the K-band flux was brightened by a factor of 1.5 over a time interval of one year.