A Multi-Wavelength Study of the Nature of Type 1.8/1.9 Seyfert Galaxies

TL;DR

This study investigates the physical causes behind the intermediate Seyfert galaxy classifications 1.8 and 1.9, analyzing optical and X-ray data to distinguish between intrinsic faintness and dust reddening effects.

Contribution

It provides a detailed comparison of reddening and intrinsic flux changes to clarify the nature of Seyfert 1.8/1.9 classifications, highlighting their heterogeneity.

Findings

Many 1.8/1.9 Seyferts are faint due to low continuum flux.

Some are reddened by dust near the nucleus.

Classification often does not reflect the true torus properties.

Abstract

We focus on determining the underlying physical cause of a Seyfert galaxy's appearance as type a 1.8 or 1.9. Are these "intermediate" Seyfert types typical Seyfert 1 nuclei with reddened broad-line regions? Or are they objects with intrinsically weak continua and broad emission lines? We compare measurements of the optical reddening of the narrow and broad-line regions with each other and with the X-ray column derived from XMM-Newton 0.5-10 keV spectra to determine the presence and location of dust in the line of sight. We also searched the literature to see if the objects showed evidence for broad-line variability, and determined if the changes were consistent with a change in reddening or a change in the intrinsic ionizing continuum flux. We find that 10 of 19 objects previously classified as Seyfert 1.8/1.9s received this designation due to their low continuum flux. In four objects the classification was due to BLR reddening, either by the torus or dust structures in the vicinity of the NLR; in the remaining five objects there is not sufficient evidence to favor one scenario over the other. These findings imply that, in general, samples of 1.8/1.9s are not suitable for use in studies of the gas and dust in the central torus.